Arthur Dendy

“ The segmentation of the ovum appears to be in all cases complete or holoblastic, and the young sponge usually leaves the parent in the form of a free-swimming ciliated larva, which, after fixing itself to some object, undergoes a metamorphosis and then grows into the adult form. The details of development appear to differ widely in different species and various interpretations have been placed upon somewhat limited and discrepant observations. ”

“ 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 skeleton fibres. ”

“ In Leucosolenia (Dendya) tripodifera (fig. 13) we find a totally different type of colony formation, which is of great importance as indicating in its canal-system the possible starting-point of a line of evolution which culminates in the highest Calcarea. Here a large central individual, whose spacious gastral cavity is lined by collared cells, gives off radial buds from all sides, which branch slightly and terminate in blind ends in contact with one another, so that the entire colony has an approximately even surface. ”

“ 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. ”

“ The most obvious expression of the vital activity of the organism is the stream of water which flows in through the dermal pores or ostia and out through the vents or oscula. That this stream is maintained by the undulatory movements of the flagella of the collared cells there can be no doubt, but the fact that the movements of the flagella of different cells are not co-ordinated, so that they do not act in unison, indicates that the mechanical problem involved is not so simple as is usually supposed. ”

“ There is reason to believe also that amoebocytes (in this case therefore phagocytes) may capture minute organisms on their way through the canal system, and even porocytes are sometimes credited with this power. Digestion, no doubt, is, at any rate chiefly, intracellular. The amoebocytes probably serve not only to ingest food themselves but also to receive surplus food from the collared cells and distribute it through the sponge (fig. 34, 5) . ”

“ 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. ”

“ 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. ”

“ With the growth of the ectosome we necessarily get a corresponding development of 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 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. ”

“ 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. ”

“ The space between the various branches of the canal-system 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. 24-32, &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. ”

“ The Hexactinellida are essentially a deep-water group and are therefore much more rarely met with than other forms. The Tetraxonida and Euceratosa abound in shallow and in moderately deep water, and a comparatively small number of species of Tetraxonida occur at great depths. Both are dominant groups at the present day, represented by very large numbers of species and individuals. The Myxospongida are comparatively rare and represented by very few species. The Calcarea are common, in the littoral region, especially in sheltered situations amongst rocks and seaweed. ”

“ There appears to be a well-marked relation between temperature and the power of spongin-secretion, and as a result we find that sponges with a really well-developed horny skeleton (whether Euceratosa or Pseudoceratosa) are usually only met with in comparatively warm waters. This fact brings about a striking contrast between the sponge-faunas of different latitudes. ”

“ Presently the gastral cavity appears (or reappears) in the middle, around which the gastral cells arrange themselves in a single layer. The young sponge elongates upwards, some of the dermal cells form porocytes which become perforated by prosopyles, others migrate into the gelatinous mesogloea and form scleroblasts, from which spicules are developed. The cells of the gastral layer acquire collars in addition to their flagella, an osculum is formed by perforation at the apex, and the young sponge begins to feed. ”

“ In the genus Lelapia we find a partly fibrous skeleton, in which the fibres are composed of bundles of triradiates shaped like tuning-forks (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 skeleton is found, consisting of spherical masses of arragonite, each originating in a special scleroblast and having a radiate structure, recalling that of a siliceous sterraster. These bodies become closely packed together over large areas, and give the sponge a stony hardness. ”

“ 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. ”

“ In spite of the undoubted rapidity with which sponges grow, as shown by the fact that on the coast of Florida marketable sponges are found commonly in places that had been stripped of saleable specimens in the preceding year, there appears to be considerable danger of injury to the sponge industry by over-fishing and by the reckless destruction of young specimens, and it has been found necessary to introduce special legislation in America to counteract these evil tendencies. ”

“ The cladi of these spicules are commonly extended in or beneath the ectosome and form a very efficient dermal skeleton, while the shafts are directed centripetally through the choanosome. In the genus Discodermia the discotriaenes form a continuous dermal armour of siliceous plates. When anatriaenes and protriaenes are developed their cladi commonly project beyond the surface of the sponge and render it more or less strongly hispid, thus forming a protection from the attacks of enemies. ”

“ The ripe gemmule is very resistant to adverse conditions and is capable of remaining dormant for a lengthened period, and of developing into a new sponge on the return of favourable conditions. In temperate climates the gemmules remain dormant throughout the winter and develop in the spring, the development being very similar to that of an ordinary fertilized ovum except that it begins at the “morula” stage, with the numerous statocytes representing the blastomeres. ”

“ 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. ”

“ No reticulation is formed in the simplest cases (Aplysilla, Dendrilla) , but in Megalopastas secondary connecting fibres are established (in relation, doubtless, to the increase in size and massive form of the sponge) , and the skeleton thus simulates the pseudoceratose reticulate type of the Sigmatomonaxonellida. In Darwinella we have, in addition to the dendritic skeleton, isolated “spicules” of spongin scattered irregularly through the mesogloea. ”

“ The eight-celled stage (fig. 38, b, c,) has the form of a somewhat flattened cushion, with an axial cavity which is the beginning of the blastocoel or segmentation cavity. A horizontal cleft now divides each blastomere into a somewhat smaller upper and a somewhat larger lower portion, and the sixteen blastomeres arrange themselves in the form of a hollow sphere surrounding the blastocoel. The smaller cells multiply rapidly and become columnar, while still remaining as a single layer. Each one presently acquires a flagellum (“cilium”) at its outer end. ”

“ At first, however, the gastral cavity is lined by the porocytes, which presently separate and migrate outwards. [2] Scleroblasts migrate inwards from the dermal layer and secrete spicules. An osculum and prosopyles are formed as in Sycon and the Olynthus stage is reached.
The development of sponges in general appears to be characterized by a remarkable want of uniformity in the arrangement of the different kinds of cells of which the larva is composed. Two, or possibly three, primary groups of cells are universally present ”

“ In the Hexactinellida the form of the collared cells appears to be somewhat unusual (fig. 36) .
Archaeocytes.—The term “archaeocytes” has been applied to certain undifferentiated amoeboid cells which make their appearance at an extremely early stage in the ontogeny, and some of which persist throughout life, with little, if any, modification, as the amoebocytes of the adult sponge, while others become germ-cells, differentiated into ova and spermatozoa. ”

“ 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. ”

“ Marine sponges occur everywhere, from low-water mark to the greatest depths, but certain localities, such as the Gulf of Manaar, Port Phillip and Port Jackson, appear to be much richer than others both in individuals and species. ”

“ Most families and even genera of sponges enjoy a very wide geographical range, very many being cosmopolitan. Species are usually much more restricted in distribution, but even here there are some noteworthy exceptions, and future researches will probably show that many species from different localities which are at present regarded as distinct are connected by intermediate forms living in intermediate situations. ”

“ Many sponges probably have the power of contracting as a whole, which may in some cases be due, in part at any rate, to the presence of bands of muscular fibres, and Sollas observes that in Pachymatisma irritation of the oscular margin is invariably followed after a short interval by a slow closure of the sphincter. The power of movement in adult sponges is, however, chiefly confined to individual cells acting independently. ”

“ 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 together their distal extremities and covers over the entrances to the inhalant canals. ”