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1911 Encyclopedia Britannica

Sponges

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The Sponges or Porif era form a somewhat isolated phylum (or principal subdivision) of the animal kingdom. This phylum includes an immense number of marine and fresh-water organisms, all of which agree amongst themselves in possessing a combination of important structural characters which is not found in any other animals. Though the phylum is a very large one yet almost the only examples with which the name " sponge " is popularly associated are the common bath sponges (species of the genera Euspongia and Hippospongia ), which are amongst the most highly organized and least typical members of the group.

The history of the group begins with Aristotle, who recognized several different kinds of sponge, some of which were used by the Greek warriors for padding their helmets. Owing, however, to the permanently fixed character, irregular growth and feeble power of movement in the adult organism, it was not until the advent of microscopical research that it was definitely proved that the sponges are animals and not plants. Indeed our scientific knowledge of the group can scarcely be said to begin much before the middle of the 19th century, when the classical researches of R. E. Grant, J. E. Gray, H. J. Carter and J. S. Bowerbank laid the foundations of modern spongology. It very soon became evident that the group is one which illustrates with remarkable clearness and beauty those laws of organic evolution which were beginning to attract so much attention from zoologists, a fact which found abundant recognition in Ernst Haeckel's epoch-making work on the Calcareous Sponges published in 1872. This was followed by a series of remarkable researches by F. E. Schulze on the minute anatomy, histology and embryology of the group, which have served as a pattern to all subsequent investigators. In more recent years our knowledge of the sponges has advanced very rapidly, especially as the result of the great series of scientific exploring expeditions inaugurated by the voyage of H.M.S. " Challenger." The large collection made by the " Challenger " expedition alone, necessitated a complete reorganization of our systematic knowledge of the phylum, and afforded the foundation upon which our present system of classification has been built up. There is perhaps no great group of the animal kingdom in the study of which greater advance has been made in the last twenty years. It is impossible in the space at our disposal to do justice to the numerous valuable memoirs which have appeared during this period, but reference to the more important works of recent investigators will be found in the bibliography at the end of this article, while for a comprehensive account of the whole subject the reader should refer especially to Professor E. A. Minchin's article in Sir E. Ray Lankester's Treatise on Zoology. General Characters of the Phylum. - The sponges are all aquatic organisms, and for the most part marine. They vary in size from minute solitary individuals, scarcely visible to the naked eye, up to great compound masses several feet in circumference, and in form from almost complete shapelessness to the most exquisite and perfect symmetry. The indefiniteness of shape and size which characterizes the vast majority of the group is due to the power of budding, which is almost universal amongst them, whereby extremely complex colonies are built up in which it is usually impossible to determine the limits of the individual zooids or persons, while very frequently, by a process of integration, individuals of a higher order are produced which again form colonies by budding (fig. 2).

The entire body of the sponge is penetrated by a more or less complicated canal-system, beginning with numerous inhalant pores, scattered over the general surface or collected in special pore-areas, and ending in one or several larger apertures, the vents or oscula, situated usually on the uppermost portions of the sponge (fig. 8). If the living animal be kept under observation it will be seen that a stream of water is ejected with considerable force from the vents, carrying with it minute particles in suspension. At the same time numerous smaller streams enter the canal system through the inhalant pores, bringing with them the minute particles of organic matter upon which the sponge feeds and the oxygen which it requires for respiration. This stream of water may be temporarily interrupted by the closure of the pores and vents, to be resumed apparently at will. It is maintained by the activity of certain cells, known as collared cells or choanocytes (fig. 35, g, fig. 36), which line the walls of the canal system either throughout their entire extent or in certain regions only. These cells bear an extraordinarily close resemblance to the choanoflagellate Protozoa or collared Monads. Each is provided with a filmy protoplasmic collar and a long whip-like flagellum, and the movements of the latter drive the water out of the canal-system through the vents and thus keep up the circulation. In all but the simplest sponges the collared cells are confined to certain portions of the canal system known as flagellated chambers (fig. 9), the size, form and arrangement of which vary greatly in different types. That part of the canal-system which is not lined by collared cells is covered with a flattened pavementepithelium (fig. 34, 1), and so also is the outer surface of the sponge. The space between the various branches of the canalsystem is occupied by a gelatinous ground-substance (mesogloea) in which amoeboid and connective-tissue cells are embedded (fig. 34, 3, 4, 5; fig. 35, a ), and in which in most cases a well-developed skeleton is secreted by special cells known. as scleroblasts. This skeleton (figs. 24-3 2, &c.) supports the extremely soft tissues of which the body is composed, and consists either of mineral spicules (carbonate of lime or silica) or of horny fibres (spongin), or of a combination of siliceous spicules with spongin. In many cases the proper skeleton is more or less completely replaced by sand.

The question as to how far the cell-layers of the sponge body correspond to the " germinal layers " usually recognizable in other multicellular animals is an extremely difficult one and not yet by any means settled. It has until recently been generally supposed that the flattened epithelium which covers the outer surface of the sponge, together with part of that which lines the canalsystem, is ectodermal, while the collared cells and the remainder of the flattened epithelium lining the canal-system are endodermal, and the term mesoderm has been frequently applied to the middle gelatinous layer. Recent embryological research, however, makes it extremely doubtful whether this view is justifiable, and whether indeed the germ-layers of typical Metazoa can be identified at all in the Porifera. Embryological research, moreover, tends to show that the primitive gastral epithelium (of collared cells) is in most sponges completely replaced, except in the flagellated chambers, by an invasion of the dermal epithelium (composed of flat pavement, cells).

Sexual reproduction, by means of ova and spermatozoa, is probably universal throughout the group. The segmentation of the ovum gives rise to the free-swimming ciliated larva (figs. 3 8, e, 39) in the form of a hollow " amphiblastula " or of a solid " parenchymula." This larva becomes attached and, by means of a more or less complex metamorphosis, gives rise to the young sponge. During the metamorphosis the outer, ciliated or flagellated cells of the larva take up their position in the interior of the body and give rise to the collared cells of the adult; while the inner cells (of the parenchymula) migrate outwards and form the superficial epithelium, so that the position of the so-called " ectoderm " and " endoderm " is completely reversed in the adult as compared with the larva.

A sexual reproduction is effected by budding, and the buds may either remain attached to the parent and form colonies or become detached and form entirely separate individuals.

1 Types of Structure

2 Euspongia

3 Skeleton

4 Calcarea

5 Euceratosa

6 Porocytes.

7 Scleroblasts

8 Connective-tissue Elements

9 Archaeocytes

10 Germ-Cells

11 Order z. Homocoela

12 Order 2. Heterocoela

13 Order z. Amphidiscophora

14 Order 2. Hexasterophora

15 Order z. Astromonaxonellida

16 Families

17 Families

18 Families

19 Bibliography

Types of Structure

We may illustrate our account of the general characters of the grou p by a brief description of the anatomy of three widely divergent types, selected as being fairly representative of the entire group, viz. Leucosolenia, Plakina and Euspongia. Leucosolenia. - The genus Leucosolenia includes a number of calcareous sponges of very simple structure, and thus forms a suitable starting-point for our studies. Imagine a minute, thinwalled sac (fig. attached at the lower end to some rock or seaweed, and e,nciosing a spacious cavity in its interior. This cavity is the gastral or digestive cavity, and it opens to the exterior through a wide vent or osculum at the upper extremity of the sponge. The thin wall is also pierced by numerous small inhalant pores or prosopyles. The inhalant pores, the gastral cavity and the vent constitute the canal-system, through which a stream of water can be kept flowing by the activity of the collared cells which line practically the whole of the gastral cavity. Each collared cell consists of an oval nucleated body surmounted by a filmy protoplasmic collar, in the middle of which the whiplike flagellum projects into the water. They are placed close together, side by side, and thus form a continuous layer, extending almost up to the vent and interrupted only by the inhalant pores. The outer surface of the sponge is covered by a single layer of flattened pavement-epithelium or epidermis. Some of these cells, distinguished as porocytes, become perforated by the inhalant pores, around which they form contractile diaphragms capable of opening and closing, and thus regulating the supply of water. Between the outer protective, dermal epithelium, and the inner gastral epithelium of collared cells, lies the mesogloea, a layer of gelatinous material containing cells of at least two kinds, amoebocytes and scleroblasts. The former closely resemble the amoeboid white blood corpuscles, or leucocytes, of higher animals, and have the power of wandering about from place to place in the spongewall. They probably serve to distribute food material and carry away waste products, and some of them undoubtedly give rise to the ova and spermatozoa. The scleroblasts are derived from cells of the dermal epithelium which migrate inwards into the gela- (After Hacckel.) tinous ground-substance and there secrete the FIG. I. - Leucoso- spicules of which the skeleton is composed. lenia primordialis These spicules are composed of transparent (Olynthus form). crystalline carbonate of lime (calcite), and may be of three fundamental forms: triradiate, quadriradiate and monaxon. It has been shown by E. A. Minchin, however, that the triradiate and quadriradiate types are not simple spicules but spicule-systems, each formed of three or four primary spicules, originating from as many mother-cells and only secondarily united. In fig. I only triradiate spicules are represented, but very often all three kinds are present in the same sponge (cf. fig. 24). The triradiates lie in the mesogloea with their three rays extended in a plane parallel to the surfaces of the sponge-wall, and form a kind of loose scaffolding upon which the soft tissues are supported. The quadriradiates resemble the triradiates in form and position, but a fourth ray is developed which projects through the layer of collared cells into the gastral cavity, where it serves as a defence against internal parasites. The monaxon spicules have one end embedded in the mesogloea while the other projects outwards and upwards and serves as a defence against external foes.

Although all species of the genus Leucosolenia agree essentially in structure, yet they exhibit very great diversity in external form. This is due to the habit of budding and colony formation. All start life after the metamorphosis of the larva in the simple sac-shaped condition which we have just described, and to which the name " Olynthus-type " is sometimes applied. This is indeed the simplest type of sponge organization known to us and we must look upon the Olynthus as representing a primary spongeindividual or'" person. By a simple process of budding, in which i ilI 1 11 ' ?i? ??i??l 1/, (After Minchin, from Lankes - ter's Treatise on Zoology.) FIG. 2. - Leucosolenia (Clathrina) clathrus, natural size; showing reticulate form of colony, expanded and with open oscula on the left, contracted and with closed oscula on the right.

osc, Osculum. ph, Sphincter of osculum.

cl. osc, Closed osculum. div, Diverticula.

contr. osc, Closed oscula in con- osc. div, Diverticula from which tracted part of colony. new oscula arise.

the buds all remain united together by their bases, we get a branched colony in which the persons or zooids are still easily recognizable, each with its own vent or osculum. Very frequently, however, the zooids become elongated into slender cylindrical tubes which branch in an extremely complex manner and anastomose with one another in many places to form networks, in which it is no longer possible to recognize the component individuals (fig. 2). This is known as the " Clathrina " type of structure, and we may look upon a Clathrina colony as an individual of a higher order, which may assume a definite external form and even acquire a secondary internal cavity (pseudogaster), opening to the exterior through a secondary vent (pseudosculum), while the outer tubes of the colony may give rise to a protective skin (pseudoderm), perforated by secondary inhalant pores (pseudopores) which are obviously quite distinct in nature from the primary inhalant pores or prosopyles of the Olynthus.

Other types of colony-formation in the genus Leucosolenia will be discussed when we come to deal with the canal-system in general. Plakina. - The genus Plakina includes some of the simplest of the siliceous sponges. Just, as in the Calcarea the most primitive " person " or individual is represented by the Olynthus type, so in the non-calcareous sponges we may recognize a primitive or (After Keller.) FIG. 3. - Vertical section of a Rhagon, diagrammatic. o, Osculum; p, Gastral cavity. (X loo.) fundamental form of individual to which the name " Rhagon has been applied. This is the first stage reached after the metamorphosis of the larva in certain species, and the little sponge consists of a cushion-shaped sac, attached below by a broad flattened base and terminating above in a single vent or osculum (fig. 3). There is a large gastral cavity lined by pavement-epithelium and surrounded by a number of more or less spherical " flagellated chambers," lined by collared cells. These chambers open into the gastral cavity by wide mouths (apopyles) and communicate with the exterior by smaller inhalant pores. The entire outer surface of the sponge is covered with pavement-epithelium and there is a well-developed mesogloea which may contain spicules. This Rhagon may be compared to an Olynthus which has become flattened out from above downwards and from which a number of small buds (the flagellated chambers) have been given off all round, except from the attached basal portion; so that the whole forms a small colony, in which the collared cells have become restricted to the buds. We may, therefore, perhaps, look upon the Rhagon as an individual or person of a higher order than the Olynthus. Like the Olynthus the Rhagon occurs as a transient stage in the development of certain sponges, but we do not know any noncalcareous sponge which remains in such a simple condition throughout life. In Plakina monolopha, for example, the entire wall of the Rhagon becomes thrown into folds (fig. 4) so that a system of inhalant and exhalant canals is formed between the folds, through which the water has to pass on its way to and from the chambers. The inhalant canals lead down between the folds from the outer surface of the sponge. In P. monolopha they are wide and ill defined. In another species, Plakina dilopha, they become constricted to form perfectly definite, narrow canals, by the development of a thick layer of mesogloea (and pavement-epithelium) which covers the outer surface of the sponge in such a manner that the folded character is no longer visible externally. The external openings of the inhalant canals now form definite dermal pores. In such a sponge as this the folded chamber-layer of the spongewall is sometimes called the choanosome, while the external layer of mesogloea and pavement-epithelium is called the ectosome. In a third species, Plakina trilopha, further folding of the " choanosomal lamella " takes place and we thus get a still more complex canal-system.

In Plakina the spicules are composed of colloidal silica. The fundamental spicule form is the primitive tetract or calthrops, consisting of four sharp-pointed rays diverging at equal angles from a common centre (fig. 5, a - e). Modifications of this form occur in two directions: in the first place some of the tetracts, by branching of one ray, give rise to " candelabra," while others, by suppression of rays, give rise to forms with three or even two rays only, triacts and diacts, the latter sometimes termed oxeate (fig. 5, f - l). The arrangement of the spicules is very irregular; the candelabra alone are definitely arranged (at the surface of the sponge), the other forms are thickly scattered without any sort of order throughout the mesogloea.

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Euspongia

The genus Euspongia, to which belong all the finer bath sponges, is a typical example of the true " horny " sponges or Euceratosa, characterized especially by the fact that the skeleton is not composed of spicules but of so called horny fibres. A living bath sponge appears as a dark-coloured, irregular or sometimes cup-shaped mass attached by the under surface to the sea-bottom. The outer surface is covered by a skin or dermal membrane, elevated in innumerable minute conuli by the growing apices of the primary ? " ?J ?G o

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o.?.??:'>. ? Fi:: ? ? Z c 1 ? p?;` speak of under the name " sponge." It consists of a very close network of spongin fibres (closely resembling silk in chemical composition), some of which, known as primaries, run towards the surface at fairly regular intervals, while others, known as secondary (After F. E. Schulze. From a coloured plate in Zeits. fur Wissen. Zoologie. by permission of Wilhelm Engelmann.) FIG. 6. - Euspongia officinalis (bath sponge). Part of vertical section showing general arrangement of skeleton and canal-system.

p.f, Primary fibre of skeleton. i.c, Inhalant canals.

s.f, Secondary fibres. e.c, Exhalant canals.

d.p, Dermal pores (inhalant). f.c, Flagellated chambers. fibres, connect the primaries in all directions and themselves branch and anastomose freely. The primary fibres contain particles of sand or foreign spicules which are taken in by their growing (After F. E. Schulze.) FIG. 4. - Plakina monolopha. a, Ciliated embryo (the central e, Rhagon stage, viewed as a part should be shaded). transparent object, show b, Part of section of ciliated ing the inhalant pores on embryo. the surface and the flagel col, Inner cell-mass. lated chambers in the ec, External, columnar cells. interior; the osculum is not fl, Flagella. shown.

c, Attached embryo, viewed f, Part of vertical section from above, with the gasthrough adult sponge, tral cavity appearing in showing the folded choano the interior. somal lamella or spongo d, Vertical section of attached phare.

embryo. ov, Ova. bl, Embryo.

skeleton fibres. This skin is pierced by a vast number of inhalant dermal pores of microscopic size, and by a much smaller number of comparatively large vents or oscula. When the sponge is removed from the water the soft tissues rapidly decay and leave behind only the elastic ' ` horny " skeleton, which is what we usually (After F. E. Schulze. From a plate in Zeitschriftfiir Wissen. Zoologie, by permission of Wilhelm Engelmann.) FIG. 5. - Plakina monolopha. Spicules, a - e, tetracts or calthrops; f - k, triacts or triradiates; l - t, diacts, showing how the monaxon form (I) may be derived from the primitive tetract (a) by suppression of actines.

dZ (After F. E. Schulze. From Lankester's Treatise on Zoology.)

FIG. 7. - Euspongia officinalis (bath sponge). Skeleton. Fibre surrounded by spongoblasts. sp.f, Spongin fibre; sp.bl, Spongoblasts. Coll, Collencytes. apices at the surface of the sponge, and the presence of which may greatly injure the quality of the sponge. The connecting fibres are only about 0.035 mm. in diameter, or even less, and the primaries are a little thicker, while the meshes between the fibres are so narrow as to permit of the soaking up of water by capillary attraction, (After F. E. Schulze.) FIG. 8. - Euspongia officinalis (bath sponge). Diagram of the arrangement of the canal-system as seen in vertical sections of two young individuals.

d.p, Dermal pores; o, Oscula; r, Rock to which the sponges are attached.

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the property upon which the economic value of the bath sponge depends. In the living sponge the fibres are embedded in the mesogloea, where they are secreted by special cells known as spongoblasts, which are often found thickly clustering around them (fig. 7). The canal-system (figs. 6, 8) is very complex and shows but little indication of its origin from a folded rhagon. The inhalant pores lead each into a short, narrow, inhalant canal; these unite in roomy subdermal cavities lying in the ectosome, and from these in turn the main inhalant canals come off. The latter divide and subdivide, and thus ramify through the deeper parts of the sponge amongst the flagellated chambers, to each of which a small number of slender canaliculi are ultimately given off (fig. 9). The chambers themselves, lined by the usual collared cells, are small and approximately spherical, and each one discharges its water through a short and narrow exhalant canaliculus (fig. 9). The openings of the inhalant canaliculi into the chambers, of which there are several, correspond to the prosopyles of an Olynthus, while the single exhalant opening, or apopyle, may possibly correspond to an Olynthus osculum. The exhalant canaliculi unite together to form larger and larger canals which finally lead the stream of water to the vents on the surface of the sponge (fig. 8). The various parts of the canal-system, other than the chambers themselves, are lined by a flat pavement-epithelium, and the mesogloea, occupying all the spaces between the different parts of the canal-system, contains cells of various kinds, embedded in a very granular matrix.

Comparative Anatomy. External Characters. - Amongst the simpler calcareous sponges, which are all of comparatively small size, the external form is usually symmetrical and is evidently a kind of outward expression of the arrangement of the canal-system. This is well seen in the simplest form of all, the sac-shaped Olynthus, and also in its simpler Syconoid and Leuconoid derivatives (described later on), which may be regarded either as individuals of a higher order or as colonies of Olynthus persons grouped around a central individual whose large gastral cavity opens to the exterior through the single osculum. In the more complex Leuconoids, however, the process of colony formation becomes very irregular and may give rise to great compound masses, with many vents. In these masses we may perhaps recognize the presence of individuals of three orders: (I) the primitive Olynthus persons, represented by the individual flagellated chambers; (2) the Leuconoid persons, indicated each by its osculum; and (3) the entire colony formed by the union of many such Leuconoid persons in an irregular manner. It is, however, very doubtful how far the flagellated chambers in such forms as this can be regarded as morphologically equivalent to Olynthus persons.

In the non-calcareous sponges we are always dealing with individuals of a high order, which usually form complex aggregates (colonies) of large size and very various shape. As a general rule the form of those non-calcareous sponges which grow in shallow water is extremely irregular and variable while at great ocean depths the shape is usually definite, constant and often exquisitely symmetrical, a fact which may perhaps be accounted for in part by the absence of disturbing influences such as are met with in shallow water. Perhaps the most extraordinary external form yet discovered is that of Esperiopsis challenchallengeri: a deep-water geri, discovered by the " Challenger " Monaxonellid Sponge. expedition in deep water off the Philip pine Islands (fig. to), a form which reminds one strikingly of a number of flowers arranged in a raceme, except that the largest and oldest member of the compound colony is at the top of the stalk and the smallest at the bottom. In other deepwater species the external form may frequently be explained (Alter Ridley and Dendy. From "Challenger" Reports, xx., by permission of the Controller of H. M. Stationery Office.) FIG. II. - Cladorhiza longipinna: a deep-water Monaxonellid Sponge, showing the "Crinorhiza " form, adapted for support on soft ooze.

as an adaptation to the special exigencies of the environment. Thus, for example, many species are provided with long stalks which lift up the body of the sponge out of the soft in which it would otherwise be smothered, while the bottom of the stalk is frequently extended in root-like processes which serve to attach it to some solid object (e.g. Stylocordyla). In other cases the sponge supports itself on the surface of the ooze by long stiff processes, formed of bundles of spicules which radiate from the central, cap-shaped body; this is known as the " Crinorhiza form," and is met with in several distinct genera (fig. I 1). Amongst the Hexactinellida, which are essentially a deep-water group, many very beautiful external forms are met with, the best known, perhaps, being the so-called Venus's flower basket ( Euplectella, fig. 12).

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Flabellate (orfan-shaped) and cupshaped forms are frequently met with even amongst shallow-water sponges, and in widely separated genera, such as Poterion (the great Neptune's cup sponge) and Reniera testudinaria. In Phyllospongia the flabellate and cup-shaped forms pass insensibly into one another, the cup being apparently merely a folded lamella. Slender branching forms are also not uncommon in shallow water, as seen in the common Chalina oculata of the British coast. Spherical forms, such as Tethya, likewise occur. By far the greater number of shallow-water sponges, however, are quite irregular in shape and either form crusts of varying thickness on the surface of rocks and sea-weed, or large and massive aggregates which may rise to a considerable height above the substratum. In the boring sponges (Family Clionidae) the sponge occupies an elaborate system of chambers and passages which it excavates for itself in the shells of Mollusca and other calcareous organisms. The common British Cliona celata begins (After F. E. Schulze. From a plate in Zeits. fiir Wissen. Zoologie, by permission of Wilhelm Engelmann.) FIG. 9. - Euspongia officinalis (bath sponge). Part of a section such as is shown in fig. 6, more highly magnified, showing three flagellated chambers, with inhalant canaliculi on the left and exhalant canaliculi on the right.

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(After Ridley and Dendy. From a plate in " Challenger " Reports, xx., by permission of the Controller of H.M. Stationery Office.) FIG. Io. - Esperiopsis ooze (After F. E. Schulze. From a plate in " Challenger " Reports, xxi., by permission of the Controller of H.M. Stationery Office.) FIG. 12. - Euplectella aspergillum, " Venus's Flower Basket ": a Hexactinellid Sponge.

life in this way, but soon outgrows the housing capacity of its host, whose shell then serves merely as a base of attachment for the large independent sponge-colony.

One of the most striking features of living sponges is their colour, which is often very brilliant. Yellow, red, orange, purple, brown, black, green and blue are all met with, in varying degrees of purity and intensity, amongst the commoner Non-calcarea; whilst the calcareous sponges are usually white. It appears probable that the colour is more or less constant for each species, and may therefore afford a useful guide to specific identification. As a rule the colour is lost in spirit-preserved or dry specimens, but a noteworthy exception is found in the brilliant purple Suberites wilsoni of Port Phillip, in which the colour, though soluble in water, is permanent in dry specimens and in alcohol. The colouring matter is sometimes lodged in special pigment cells belonging to the sponge itself, and sometimes in symbiotic algae, with which the mesogloea is frequently filled.

Canal-system. - Whether we start with the primitive Olynthus form of the Calcarea or with the more advanced Rhagon of many Non-calcarea, it is evident that further advance in the complication of the canal-system is arrived at either by budding or folding, or by a combination of these processes. As, however, the canalsystems of the calcareous and of the main types of non-calcareous sponges have been evolved along perfectly independent lines it will be well to consider them separately.

In the genus Leucosolenia (Calcarea Homocoela) the primitive Olynthus form may, as we have already seen, give rise, by branching and anastomosing, to complex reticulate colonies of the Clathrina type, in which a pseudoderm, pierced by inhalant pores, may cover over a system of inhalant canals which are simply the interspaces between the branching tubes of which the colony is made up, while at the same time a centrally placed pseudogaster, which is simply a space enclosed by upgrowth of the colony around it, may form the main exhalant canal and open to the exterior through a well-defined vent or pseudosculum. In this direction perhaps the most remarkable modification arrived at is that of Leucosolenia cavata, in which the Clathrina tubes, lined, z by collared cells, widen out into large irregular spaces, while the inhalant interspaces t to spaces become constricted into narrow canals lined by collared cells on the outside. We have here a kind of inversion of the ordinary Clathrina canalsystem, but a perfectly gradual transition from the ordinary to the ! inverted condition is seen as we pass from the older to the younger parts of the colony.

In Leucosolenia (Dendya) tri- r? podifera (fig. 13) we find a totally different type of colony formation, which is of great importance as - p indicating in its canal-system the n possible starting-point of a line, of evolution which culminates in the highest Calcarea. Here a large central individual, whose i spacious gastral cavity is lined }" by collared cells, gives off radial;' t ':

? y ? ') 0 6 0 o ('c? buds from all sides, which branc 000 slightly and terminate in blind 3° ends in contact with one another, -4 > " `, Yrj Y '?,'? so that the entire colony has an o?'oc, approximately even surface. The ooc

?;' inhalant canals are represented by X .

°: .?

? ?Ithe interspaces between the radial

, tubes, between the blind extremi o ?c

i ties of which the water finds its a 6:

way in from the outside. There 'gie' I is only a single vent, situate at (,() the extremity of the central cavity.

o This cavity must be regarded as the original gastral cavity of a parent Olynthus, from which the radial tubes have been produced by budding.

(After Dendy. Simplified from a coloured We have next, amongst the plate in Trans. Roy. Soc. of Victoria, Calcarea Heterocoela, the Sycon Melbourne, vol. iii. pt. r.) Y e FIG. 13. - Leucosolenia tripodi- from o the foregoing in hl that the fern, with part of the spongecollared cells of the central gastral wall cut away to show the cavity are replaced by pavementarrangement of the radial outepithelium. The radial tubes now growths. form definite flagellated chambers, pierced as before by numerous prosopyles through which the water enters from the spaces between the chambers, while the original gastral cavity forms a central exhalant canal terminating in the single vent, a true osculum, corresponding to the osculum of an Olynthus. In the simplest Syconoid forms ( Sycetta ) the radial chambers remain perfectly straight and unbranched. They do not touch one another at all and there is no trace of an ectosome or dermal cortex, and hence there are no true inhalant canals, and the water circulates without interruption between the chambers. In the genus Sycon (fig. 14) the walls of adjacent chambers come into contact with one another and fuse together and thus give rise to more or less well-defined inhalant " inter-canals." The chambers themselves may branch, and in some species of Sycon a thin, pore-bearing dermal membrane connects to gether their distal extremities (From Dendy, in Quart. Journ. Micro. Sci., and covers over the entrances new series, xxxv., by permission of J. and to the inhalant canals. The A. Churchill.) canal-system now exhibits all FIG. 14. - Sycon carteri, part of the different parts found in a transverse (horizontal) section, the most highly-organized showing three radial chambers, the sponges: viz. dermal pores, middle one cut open.

inhalant canals, flagellated fl.ch, Flagellated chamber. chambers, exhalant canal and ex.op, Its exhalant opening osculum. In the genus Grantia apopyle.

and its allies (e.g. Ute, fig. 1 5) pros, Prosopyle.

the thin dermal membrane c.g.c, Central gastral cavity. of Sycon is converted into a i.c, Inhalant canal. well-developed cortex, cover- g,cor, Gastral cortex.

ing the extremities of both g.q, Gastral quadriradiate the inhalant canals and the cule.

radial chambers, and some- s.g.s, Subgastral sagittal trira times containing a system diate spicules, forming the of special cortical inhalant first joint of the articulate canals. We may now distubar skeleton.

tinguish between an ectosome t.ox, Tufts of monaxon spicules at (the dermal cortex), which the ends of the chambers. contains no flagellate cham bers, and a choanosome in which chambers are present. The next stage has probably been arrived at by a kind of fold (After Polejaeff.) FIG. 15. - Ute argentea, part of transverse section, showing the Syconoid canal-system, and thick dermal cortex containing huge longitudinally placed monaxon spicules whose cross-sections are represented by concentric circles.

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radially, not around the central gastral cavity but around diverticula of the latter which form special exhalant canals. This condition, sometimes called the " sylleibid " type, is not characteristic of any particular genus or family, but occurs in a few isolated species, such as Leucilla connexiva (fig. 16). A somewhat similar condition may be arrived at by branching of the radial flagellated chambers, as in Heteropegma (fig. 17). The next stage is marked by great reduction in the size of the chambers, which may become almost spherical, and by further folding of the choanosome, so that in a section of the sponge-wall we see the small chambers scattered irregularly in the mesogloea between the numerous branches of complicated inhalant and exhalant canals. Each ‚pros. or spi ing of the choanosome, for find the chambers arranged we chamber still has several prosopyles, through which it receives water from the ultimate branches of the inhalant canals, while it opens into a relatively large exhalant canal by a wide apopyle. This is the highest type of canal-system met with amongst the Calcarea. It is sometimes known as the Leucon type and is seen in most species of the genus Leucandra, as well as in many others.

(After Polejaeff.) FIG. 16. - Leucilla connexiva, part of transverse section, showing " sylleibid " type of canal system with folded chamber layer and exhalant canals (E) into which the chambers open.

It is almost identical with one of the types commonly found in non-calcareous sponges (e.g. Plakina, fig. 4), but has of course been evolved independently. The various types of canal-system met with in the Calcarea are connected together by numerous intermediate forms, thus forming a very interesting evolutionary series, while both the Sylleibid and Leuconoid types appear to have been independently evolved several times, thus affording excellent examples of the phenomenon of convergence, a phenomenon which is very frequently met with amongst sponges.

(After Polejaeff.) FIG. 17. - Heteropegma nodus-gordii, part of transverse section, showing branching flagellated chambers and huge subdermal quadriradiate spicules, with greatly reduced tubar skeleton.

In describing the anatomy of Plakina as a type of non-calcareous sponge, we have traced the development of a fairly complex canalsystem from the so-called Rhagon form. We can, however, hardly regard the Rhagon as representing a fundamental type of canalsystem common to all the Non-calcarea, for in some of the Myxospongida, which are the most primitive of all, and again in the Hexactinellida, we find a type characterized by the presence of elongated sac-shaped flagellated chambers resembling those of the Sycon type amongst the Calcarea, and these chambers are arranged radially around the exhalant canals ( Halisarca, Hexactinellida). The first recognizable stage in the evolution of the canal-system of the Non-calcarea would thus appear to be a condition not unlike that of Sycon, with a number of elongated chambers arranged radially around a central gastral cavity and having their blind outer extremities covered over by a dermal membrane. This stage is very nearly reproduced in the young form of a Hexactinellid sponge, Lanuginella pupa. From some such form the Rhagon type may perhaps be derived by flattening out of the lower end of the sponge into a broad base of attachment, and by reduction in the size of the flagellated chambers, accompanied by a more irregular arrangement.

Starting from the primitive Myxosponge ancestor, with large sac-shaped chambers, radially arranged, the Non-calcarea have apparently developed along four main lines, giving rise to the existing Myxospongida, the Hexactinellida (Triaxonida), the Tetraxonida osc. (After F. E. Schulze. From Lankester's Treatise on Zoology.) FIG. 18. - Lanuginella pupa. O.S., Vertical section of a young specimen (spicules omitted).

d.m, Dermal membrane. g.m, Gastral membrane. sd.tr, Subdermal trabecular layer. G.C, Gastral cavity.

fl.c, Flagellated chamber. osc, Region of future osculum. sg.tr, Subgastral trabecular layer.

and the Euceratosa. The Myxospongida have retained the large size of the chambers in certain forms (Halisarca, Bajalus ), but have lost this primitive character in the more advanced members of the group (Oscarella). The Hexactinellida have retained the large size and radial arrangement of the flagellated chambers throughout their entire series. The chamber layer, however, tends to become more or less folded (fig. 19), and always lies between two layers of nt. p?((After Schulze. From Lankester's Treatise on Zoology.) FIG. 19. - Section of the Body-wall of Bathydorus fimbriatus, F.E.S. (spicules omitted).

ex.c, Exhalant canals. sg.tr, Subgastral trabecular layer.

d.on, Dermal membrane. g.m, Gastral membrane. sd.tr, Subdermal trabecular layer. G.C, Gastral cavity. fl.c, Flagellated chambers.

loose trabecular tissue in which the canals are represented by irregular spaces. The Tetraxonida appear to have suffered reduction in the size of the flagellated chambers at a very early date, and it is of this group especially that the Rhagon type is characteristic (e.g. Plakina, fig. 4). The Euceratosa exhibit a beautiful series, beginning with forms ( Aplysillidae ) having large sac-shaped chambers like those of Hexactinellids and ending with forms (Spongiidae, Euspongia, figs. 6, 8, 9) having small spherical chambers.

Along all four lines of descent it is probable that folding of the choanosome, or chamber-bearing layer of the sponge-wall, has played a very important part in the evolution of the canal-system. This folding is very clearly seen in the Hexactinellida and in such forms as Oscarella (Myxospongida) and Plakina (Tetraxonida). By this process inhalant and exhalant canal-systems have been formed, and then the ends of the inhalant canals have in most cases been closed in by development of an ectosome, as in Plakina trilopha and Stelletta phrissens (fig. 20). In the majority of cases (e.g.

(After Sollas.) FIG. 20. - Young specimen of Stelletta phrissens (Sollas). Vertical section through the osculum (o), showing the choanosome folded within the ectosome.

Euspongia ) the folding has become so complex that it is no longer recognizable as such, and the origin of the now well-defined inhalant and exhalant canals is completely disguised. In many cases the principal exhalant canals may be surrounded by a layer of tissue of considerable thickness in which there are no flagellated chambers at all, known as the endosome, so that the folded choanosome may be sandwiched in between ectosome on the outside and endosome on the inside.

The manner in which the flagellated chambers communicate with their respective branches of the inhalant and exhalant canal (After Sollas.) FIG. 21. - Transverse section across an exhalant canal and surrounding choanosome of Cydonium eosaster (Sollas), showing the aphodal flagella,ed chambers.

system varies considerably in different forms, and the following types are recognizable, though by no means sharply distinguished from one another. In the more primitive forms (e: g. Hexactinellida, Aplysillidae, Spongeliidae) each chamber is provided with several prosopyles and receives its water supply direct from relatively large inhalant canals or even lacunae, discharging it again through a wide mouth (apopyle) into a relatively large exhalant canal or lacuna which also receives water directly from other chambers.

To this type (fig. 4, f ) the name "eurypylous " has been given, and we may include in it cases where there is only a single prosopyle, and perhaps even a short, narrow inhalant canal. In more advanced forms the water is discharged from each chamber through a narrow exhalant canaliculus (aphodus) peculiar to itself, and thence into wider canals. This is known as the " aphodal " type (e.g. Cydonium, fig. 21). In the " diplodal " type there is a special inhalant canaliculus (prosodus) as well as a special aphodus to each chamber, with usually, at any rate, only a single prosopyle (e.g. Corticium, fig. 22). The progress from the eurypylous to the diplodal condition is accompanied by a corresponding increase in the development of the mesogloea, whereby the canals are greatly restricted in diameter, and at the same time the mesogloea tends to lose its transparent FIG. 22. - Part of a section of gelatinous character and to Corticium candelabrum, O.S., showbecome compact and granular. ing diplodal type of canal-system. With the growth of the ectoThe canal shown on the left is some we necessarily get a inhalant and that on the right ( e) corresponding development of exhalant.

the proximal portion of the inhalant canal-system. At first the ectosome is merely a thin membrane, the dermal membrane, pierced by the inhalant pores, which are usually arranged in groups.

Beneath the groups of pores (pore-areas) lie spacious subdermal cavities which form the commencement of the inhalant canal-system in the choanosome. In more advanced types the ectosome becomes greatly thickened and may be specially strengthened in a variety of ways to form a cortex. The inhalant pores now no longer lead directly into the subdermal cavities, but first into a series of cavities lying in the cortex and known as chones, which may be separated from the underlying subdermal cavities (sub-cortical crypts) by definite sphincters ( Cydonium, fig. 23).

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The arrangement of the oscula and pores on the surface of the sponge varies greatly in different types, and sometimes gives rise to very striking modifications of the external form. The oscula or vents are usually relatively large openings situated on the more prominent parts of the sponge, often on special elevations. Occasionally they are replaced by sieve-like oscular areas (e.g. Geodia perarmata ), a modification which doubtless serves to prevent foreign bodies from entering the wide exhalant canals.

The inhalant pores may be irregularly scattered over the surface of the sponge or collected in more or less well-defined pore-areas. In cup-shaped sponges the pores are usually confined to the outer and the oscula to the inner surface. In flabellate sponges we find pores on one side and oscula on the other. In Tedania actiniiformis, a deepsea form, the pores are restricted to a narrow band surrounding the columnar body of the sponge just beneath the flattened top, which bears the vents; thus they are kept from being choked up by the soft ooze on which the sponge lies. In Xenospongia, a flattened discoid form, they are confined to narrow grooves on the upper surface, the chief of which run round the margin of the disk. In Esperella murrayi the pores are also confined to special grooves on the surface of the sponge, and in both these cases the grooves can apparently be opened and closed by special bands of musclefibres, and the supply of water thus regulated. In some species of Latrunculia we find the surface of the sponge covered with (After F. E. Schulze.) (After Sollas.) FIG. 23. - Section through the cortex and part of the choanosome of Cydonium eosaster (Sollas), showing a pore-sieve and underlying chone in the cortex. The chone communicates below with a subcortical crypt, from which the inhalant canals originate. The cortex contains numerous sterrasters, connected with one another by fibrous bands.

conspicuous projections of two kinds, some conical and bearing each a single vent, others truncated at the top and bearing the inhalant pores.

Skeleton

The original ancestral form (Protolynthus ) from which

all the Porifera are supposed to be descended, probably possessed no proper skeleton at all, and this condition has been retained in the existing Myxospongida, although these sponges have made considerable progress in the evolution of their canalsystem. There appears to be little doubt that the Myxospongida are primitively devoid of skeleton, and in this respect they must becaref ully distinguished from thegenus Chondrosia, in which the skeleton has been secondarily suppressed, as well as from numerous and divers species in which the proper skeleton has been more or less completely replaced by grains of sand or other foreign bodies. The Calcarea, Triaxonida, Tetraxonida and Euceratosa, except in cases of extreme degeneration, all possess a well-developed proper skeleton. As this skeleton has been independently evolved in each of these great groups it is necessary to deal with it separately in each case.

Calcarea

The skeleton in this group is composed of spicules of crystalline carbonate of lime (usually calcite), developed within special mother-cells or scleroblasts. Each spicule is enclosed in a delicate membranous spicule-sheath and contains an axial thread of organic matter. Three main types of calcareous spicule are met with, triradiate, quadriradiate and monaxon (fig. 24). The triradiates and quadriradiates, however, are not simple spicules, but spiculesystems formed of three or four rays each originating independently from its own scleroblast (actinoblast) and all uniting together secondarily. There is reason to believe that this may also sometimes be the case with the monaxon or oxeate spicules. In the most primitive triradiate spicules all three rays lie in the (After E. A. Minchin. From Lankester's Treatise on Zoology.) FIG. 24. - Spicules of Calcareous Sponges.

same plane. Three chief varieties may be distinguished: (I) Regular (fig. 24, li), with all the rays and all the angles equal; (2) Sagittal (fig. 24, c, d, 1, &c. ), with two of the rays or two of the angles forming a pair, differentiated in some respect from the remaining ray or angle, the paired rays being termed " oral " and the odd ray " basal "; (3) Irregular (fig. 24, p), when conforming to neither of the above types. It has been proposed to draw a very sharp distinction between " equi-angular " triradiates and " alate " forms (in which the angle between the oral rays differs from the paired angles), but it may be doubted whether such a distinction has any great value. The quadriradiate (fig. 24, e, f, k, m ) is formed by the addition of an " apical " or "gastral " ray to the three " facial " rays of the triradiate; this ray lies in a plane at right angles to that of the facial rays. The monaxon spicules (fig. 24, h, i, q, r, s ) are straight or curved and the two ends are usually more or less sharply differentiated from one another. In all these spicules the form and arrangement of the rays is usually clearly correlated with their position in the sponge in such a manner that they are specially adapted for the work which they have to do.

The arrangement of the spicules in the case of the genus Leucosolenia has been dealt with above, and we must pass on at once to the Calcarea Heterocoela. In this group the skeleton exhibits an evolutionary series no less remarkable than that of the canal-system. We may take as a convenient starting-point the genus Sycetta, a typical Syconoid form, with the flagellated chambers radiating independently from the central gastral cavity. The wall of the gastral cavity is supported by a gastral skeleton of triradiate or quadriradiate spicules. These may be sagittal, in which case the oral rays are turned towards the osculum while the basal ray is directed downwards. If there is an apical ray it projects into the gastral cavity. The walls of the radial chambers are supported by a special " tubar " skeleton (cf. fig. 14), consisting exclusively of triradiates with their basal rays directed towards the distal end of each chamber. The oral rays are spread out at right angles to the length of the chamber, and as several spicules generally lie at the same level the tubar skeleton forms a series of more or less definite joints and is said to be " articulate." This type of skeleton is almost invariably associated with the Syconoid type of canalsystem. In the genus Sycon itself we find the distal ends of the chambers specially protected by tufts of monaxon spicules (fig. 14), but the next great advance in the evolution of the skeleton is brought about by the development of a dermal cortex, in which a special dermal skeleton is developed. This is well seen in the genus Ute (fig. 15). After this the skeleton of the chamber layer in the spongewall begins to undergo modifications, some of which are obviously correlated with the gradual change of the canal-system from the Syconoid to the Leuconoid condition (cf. figs. 16 and 17). Finally all trace of the articulate tubar skeleton is lost, and we get a " parenchymal " skeleton of scattered radiate spicules in the chamber layer. The skeleton of the chamber layer, no matter what the type of canal-system, may be supplemented by large subdermal sagittal triradiates or subdermal quadriradiates (fig. 17), whose basal or apical rays project inwards from the dermal cortex (Heteropidae and Amphoriscidae). Very generally a special " oscular " skeleton is developed in the form of a fringe of long monaxon spicules around the vent.

Various aberrant types of skeleton are met with in the group. In the genus Lelapia we find a partly fibrous skeleton, in which the fibres are composed of bundles of triradiates shaped like tuningforks (fig. 24, o ), and in Petrostoma the main skeleton is formed of calcareous spicules actually fused together. In Astrosclera (fig. 25) a very anomalous type of calcareous skeletdn is found, consisting of spherical masses of arragonite, each originating in a special scleroblast and having a radiate structure, recalling that of a siliceous (After W. J. Sollas.) FIG. 26. - Typical Siliceous Megascleres.

(oxeate). g, Sterraster (often regarded as a microsclere).

h, Part of section of sterraster, showing two rays united by intervening silica.

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FIG. 25. - A strosclera willeyana (Lister).

A, Entire sponge (x 3): p.s., upper surface with openings system; b, base of attachment.

B, Section of skeleton: sph, spherules of arragonite; c, canals.

of canal-

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A (After J. J. Lister.) Diactinal monaxon Style.

Triact.

Primitive tetraxon (calthrops). Hexact.

Polyaxon desma.

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sterraster. These bodies become closely packed together over large areas, and give the sponge a stony hardness. Hexactinellida.-In this group the skeleton is composed of spicules of colloidal silica deposited in concentric lamellae around slender axes of an organic substance which in life occupies the " axial canal " of the spicule. Although varying greatly in detail and often exhibiting great complication or, it may be, reduction in structure, these spicules are all referable to the same fundamental triaxonid and hexactinellid type, characterized by the possession of three axes intersecting each other at right angles and each thereby divided into two rays or actines (fig. 26, e). According as one, two, three, four or five of these actines are suppressed we distinguish between pentact, tetract, triact, diact and monact spicules, and these may be further subdivided according to special modifications of the rays due to secondary branching, ornamentation by spines, (After F. E. Schulze.) FIG. 27.-Derivatives of the Hexact type of Spicule, found in Hexactinellida.

a, Dagger. d, Amphidisc. f, Tetract (staurus).

b., c, Pinuli. e, Pentact. g, Diact (rhabdus).

knobs, &c., or curvature, or to excessive development of certain rays as compared with the remainder. Some of the most characteristic of these special types are represented in figs. 27 and 28. Two of them require special notice on account of their importance in the classification of the group. These are the hexaster and the ti (After F. E. Schulze.) FIG. 28.-Derivatives of the Hexact type of Spicule, found in Hexactinellida.

a, Uncinaria; b, Clavula; c, Scopula.

amphidisc. A hexaster (=rosette) is a perfectly symmetrical hexact whose actines branch out into secondary or terminal rays, in a star-like manner (fig. 30, t). Various sub-types are distinguished according to the character of the rays ( floricome, plumicome, &c.). An amphidisc (fig. 27, d ) is a diact spicule consisting of two opposite rays each of which terminates in a disk-like or spherical expansion surrounded by marginal teeth.

In some cases the spicules all remain disconnected from one another (Lyssacine condition), in others some of them may be united by siliceous cement into a continuous framework (Dictyonine condition), and the distinction between these two types of arrangement was for a long time regarded as indicating a primary subdivision of the Hexactinellida into Lyssacina and Dictyonina, but this subdivision has now been abandoned. The term prostalia is applied to spicules which project freely from the surface of the sponge, and these are further distinguished as basalia, pleuralia and marginalia, according to their position at the base of the sponge, on the sides, or round the margin of the osculum. The basalia frequently form a root-tuft for attaching the sponge to the substratum ( Hyalonema, Euplectella ) and commonly have anchorlike distal extremities. They may be extremely long, as in the wellknown " glass-rope " of Hyalonema. In the remarkable genus Monorhaphis we find a single gigantic diact spicule, which may attain a length of two or three feet and the thickness of a lead pencil, transfixing the body of the sponge like a skewer from above downwards. A special dermal skeleton is usually formed by a number of spicules distinguished as dermalia, and a gastral skeleton may be similarly formed by special gastralia surrounding the central gastral cavity. Between the dermal and gastral skeletons another set of spicules, known as parenchymalia, form the most important part of the skeleton, supporting the chamber-layer and adjacent tissues. The distinction into large megascleres and small microscleres is perhaps less well marked in this group than in the Tetraxonida.

Tetraxonida.-Here, again, the spicules are composed of colloidal silica deposited around organic axial threads. The starting-point in the evolution of the very complex series of tetraxonid spicules is the primitive tetract or calthrops, characteristic of the most primitive members of th


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Bibliography Information
Chisholm, Hugh, General Editor. Entry for 'Sponges'. 1911 Encyclopedia Britanica. http://www.studylight.org/encyclopedias/bri/view.cgi?number=31174. 1910.

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