Freshwater Sponges, Hydroids & Polyzoa Part 6

(b) _From the Gemmule._

The period for which the gemmule lies dormant probably depends to some extent upon environment and to some extent on the species to which it belongs. Carter found that if he cleaned gemmules with a handkerchief and placed them in water exposed to sunlight, they germinated in a few days; but in Calcutta gemmules of _Spongilla alba_ var. _bengalensis_ treated in this way and placed in my aquarium at the beginning of the hot weather, did not germinate until well on in the "rains." Even then, after about five months, only a few of them did so. Zykoff found that in Europe gemmules kept for two years were still alive and able to germinate.

Germination consists in the cellular contents of the gemmule bursting the membrane or membranes in which they are enclosed, and making their way out of the gemmule in the form of a delicate whitish ma.s.s, which sometimes issues through the natural aperture in the outer chitinous coat and sometimes through an actual rent in this coat. In the latter case the development of the young sponge is more advanced than in the former.

The fullest account of development from the gemmule as yet published is by Zykoff, and refers to _Ephydatia_ in Europe (Biol. Centralbl. Berlin, xii, p. 713, 1892).

His investigations show that the bursting of the gemmule is not merely a mechanical effect of moisture or any such agency but is due to development of the cellular contents, which at the time they escape have at least undergone differentiation into two layers. Of the more important soft structures in the sponge the osculum is the first to appear, the ciliated chambers being formed later. This is the opposite of what occurs in the case of the bud, but in both cases the aperture appears to be produced by the pressure of water in the organism. The manner and order in which the different kinds of cells originate in the sponge derived from a gemmule give support to the view that the primitive cell-layers on which morphologists lay great stress are not of any great importance so far as sponges are concerned.

(c) _Development of the Bud._

As the bud of _Spongilla proliferens_ grows it makes its way up the skeleton-fibre to which it was originally attached, pus.h.i.+ng the dermal membrane, which expands with its growth, before it. The skeleton-fibre does not, however, continue to grow in the bud, in which a number of finer fibres make their appearance, radiating from a point approximately at the centre of the ma.s.s. As the bud projects more and more from the surface of the sponge the dermal membrane contracts at its base, so as finally to separate it from its parent. Further details are given on p.

74.

HABITAT.

Mr. Edward Potts[P], writing on the freshwater sponges of North America, says:--"These organisms have occasionally been discovered growing in water unfit for domestic uses; but as a rule they prefer pure water, and in my experience the finest specimens have always been found where they are subjected to the most rapid currents." True as this is of the Spongillidae of temperate climates, it is hardly applicable to those of tropical India, for in this country we find many species growing most luxuriantly and commonly in water that would certainly be considered unfit for domestic purposes in a country in which sanitation was treated as a science. Some species, indeed, are only found in ponds of water polluted by human agency, and such ponds, provided that other conditions are favourable, are perhaps the best collecting grounds. Other favourable conditions consist in a due mixture of light and shade, a lack of disturbance such as that caused by cleaning out the pond, and above all in the presence of objects suitable for the support of sponges.

[Footnote P: P. Ac. Philad. 1887, p. 162.]

I do not know exactly why light and shade must be mixed in a habitat favourable for the growth of sponges, for most species prefer shade, if it be not too dense; but it is certainly the case that, with a few exceptions, Indian Spongillidae flourish best in water shaded at the edges by trees and exposed to sunlight elsewhere. One of the exceptions to this rule is the Indian race of _Spongilla lucustris_, which is found in small pools of water in sand-dunes without a particle of shade.

Several species are only found on the lower surface of stones and roots in circ.u.mstances which do not suggest that their position merely protects them from mud, which, as Mr. Potts points out, is their "great enemy." A notable instance is _Trochospongilla pennsylvanica_, which is found hiding away from light in America and Europe as well as in India.

It is curious that it should be easy to exterminate the sponges in a pond by cleaning it out, for one would have thought that sufficient gemmules would have remained at the edge, or would have been brought rapidly from elsewhere, to restock the water. Mr. Green has, however, noted that _Spongilla carteri_ has disappeared for some years from a small lake at Peradeniya in which it was formerly abundant, owing to the lake having been cleaned out, and I have made similar observations on several occasions in Calcutta.

The question of the objects to which sponges attach themselves is one intimately connected with that of the injury done them by mud. The delta of the Ganges is one of the muddiest districts on earth. There are no stones or rocks in the rivers and ponds, but mud everywhere. If a sponge settles in the mud its ca.n.a.ls are rapidly choked, its vital processes cease, and it dies. In this part of India, therefore, most sponges are found fixed either to floating objects such as logs of wood, to vertical objects such as the stems of bulrushes and other aquatic plants, or to the tips of branches that overhang the water and become submerged during the "rains." In Calcutta man has unwittingly come to the a.s.sistance of the sponges, not only by digging tanks but also by building "bathing-ghats" of brick at the edge, and constructing, with aesthetic intentions if not results, ma.s.ses of artificial concrete rocks in or surrounding the water. There are at least two sponges (the typical form of _Spongilla alba_ and _Ephydatia meyeni_) which in Calcutta are only found attached to such objects. The form of _S. alba_, however, that is found in ponds of brackish water in the Gangetic delta has not derived this artificial a.s.sistance from man, except in the few places where brick bridges have been built, and attaches itself to the stem and roots of a kind of gra.s.s that grows at the edge of brackish water. This sponge seems to have become immune even to mud, the particles of which are swallowed by its cells and finally got rid of without blocking up the ca.n.a.ls.

Several Indian sponges are only found adhering to stones and rocks.

Among these species _Corvospongilla lapidosa_ and our representatives of the subgenus _Stratospongilla_ are noteworthy. Some forms (e. g.

_Spongilla carteri_ and _S. crateriformis_) seem, however, to be just as much at home in muddy as in rocky localities, although they avoid the mud itself.

There is much indirect evidence that the larvae of freshwater sponges exercise a power of selection as regards the objects to which they affix themselves on settling down for life.

Few Spongillidae are found in salt or brackish water, but _Spongilla alba_ var. _bengalensis_ has been found in both, and is abundant in the latter; indeed, it has not been found in pure fresh water. _Spongilla travancorica_ has only been found in slightly brackish water, while _S.

lacustris_ subsp. _reticulata_ and _Dosilia plumosa_ occur in both fresh and brackish water, although rarely in the latter. The Spongillidae are essentially a freshwater family, and those forms that are found in any but pure fresh water must be regarded as aberrant or unusually tolerant in their habits, not as primitive marine forms that still linger halfway to the sea.

ANIMALS AND PLANTS COMMONLY a.s.sOCIATED WITH FRESHWATER SPONGES.

(a) _Enemies._

Freshwater sponges have few living enemies. Indeed, it is difficult to say exactly what is an enemy of a creature so loosely organized as a sponge. There can be little doubt, in any case, that the neuropteroid larva (_Sisyra indica_) which sucks the cells of several species should be cla.s.sed in this category, and it is noteworthy that several species of the same genus also occur in Europe and N. America which also attack sponges. Other animals that may be enemies are a midge larva (_Tanypus_ sp.) and certain worms that bore through the parenchyma (p. 93), but I know of no animal that devours sponges bodily, so long as they are uninjured. If their external membrane is destroyed, they are immediately attacked by various little fish and also by snails of the genera _Limnaea_ and _Planorbis_, and prawns of the genus _Palaemon_.

Their most active and obvious enemy is a plant, not an animal,--to wit, a filamentous alga that blocks up their ca.n.a.ls by its rapid growth (p.

79).

(b) _Beneficial Organisms._

The most abundant and possibly the most important organisms that may be considered as benefactors to the Spongillidae are the green corpuscles that live in the cells of certain species (fig. 2, p. 31), notably _Spongilla lacustris_, _S. proliferens_, and _Dosilia plumosa_. I have already said that these bodies are in all probability algae which live free in the water and move actively at one stage of their existence, but some of them are handed on directly from a sponge to its descendants in the cells of the gemmule. In their quiescent stage they have been studied by several zoologists, notably by Sir Ray Lankester[Q] and Dr.

W. Weltner[R], but the strongest light that has been cast on their origin is given by the researches of Dr. F. W. Gamble and Mr. F. Keeble (Q. J. Microsc. Sci. London, xlvii, p. 363, 1904, and li, p. 167, 1907).

These researches do not refer directly to the Spongillidae but to a little flat-worm that lives in the sea, _Convoluta roscoffiensis_. The green corpuscles of this worm so closely resemble those of _Spongilla_ that we are justified in supposing a similarity of origin. It has been shown by the authors cited that the green corpuscles of the worm are at one stage minute free-living organisms provided at one end with four flagella and at the other with a red pigment spot. The investigators are of the opinion that these organisms exhibit the essential characters of the algae known as Chlamydomonadae, and that after they have entered the worm they play for it the part of an excretory system.

[Footnote Q: Q. J. Microsc. Sci. London, xxii. p. 229 (1882).]

[Footnote R: Arch. Naturg. Berlin, lix (i), p. 260 (1893).]

As they exist in the cells of _Spongilla_ the corpuscles are minute oval bodies of a bright green colour and each containing a highly refractile colourless granule. A considerable number may be present in a single cell. It is found in European sponges that they lose their green colour if the sponge is not exposed to bright sunlight. In India, however, where the light is stronger, this is not always the case. Even when the colour goes, the corpuscles can still be distinguished as pale images of their green embodiment. They are called _Chlorella_ by botanists, who have studied their life-history but have not yet discovered the full cycle. See Beyerinck in the Botan. Zeitung for 1890 (vol. xlviii, p.

730, pl. vii; Leipzig), and for further references West's 'British Freshwater Algae,' p. 230 (1904).

The list of beneficent organisms less commonly present than the green corpuscles includes a _Chironomus_ larva that builds parchment-like tubes in the substance of _Spongilla carteri_ and so a.s.sists in supporting the sponge, and of a peculiar little worm (_Chaetogaster spongillae_[S]) that appears to a.s.sist in cleaning up the skeleton of the same sponge at the approach of the hot weather and in setting free the gemmules (p. 93).

[Footnote S: Journ. As. Soc. Beng. n. s. ii, 1906, p. 189.]

(c) _Organisms that take shelter in the Sponge or adhere to it externally._

There are many animals which take shelter in the cavities of the sponge without apparently a.s.sisting it in any way. Among these are the little fish _Gobius alc.o.c.kii_, which lays its eggs inside the oscula of _S.

carteri_, thus ensuring not only protection but also a proper supply of oxygen for them (p. 94); the molluscs (_Corbula_, spp.) found inside _S.

alba_ var. _bengalensis_ (p. 78); and the Isopod (_Tachaea spongillicola_) that makes its way into the oscula of _Spongilla carteri_ and _S. crateriformis_ (pp. 86, 94).

In Europe a peculiar ciliated Protozoon (_Trichodina spongillae_) is found attached to the external surface of freshwater sponges. I have noticed a similar species at Igatpuri on _Spongilla crateriformis_, but it has not yet been identified. It probably has no effect, good or bad, on the sponge.

FRESHWATER SPONGES IN RELATION TO MAN.

In dealing with _Spongilla carteri_ I have suggested that sponges may be of some hygienic importance in absorbing putrid organic matter from water used both for ablutionary and for drinking purposes, as is so commonly the case with regard to ponds in India. Their bad odour has caused some species of Spongillidae to be regarded as capable of polluting water, but a mere bad odour does not necessarily imply that they are insanitary.

Unless my suggestion that sponges purify water used for drinking purposes by absorbing putrid matter should prove to be supported by fact, the Spongillidae cannot be said to be of any practical benefit to man. The only harm that has been imputed to them is that of polluting water[T], of blocking up water-pipes by their growth--a very rare occurrence,--and of causing irritation to the human skin by means of their spicules--a still rarer one. At least one instance is, however, reported in which men digging in a place where a pond had once been were attacked by a troublesome rash probably due to the presence of sponge-spicules in the earth, and students of the freshwater sponges should be careful not to rub their eyes after handling dried specimens.

[Footnote T: See Potts, Proc. Ac. Philad. 1884, p. 28.]

INDIAN SPONGILLIDae COMPARED WITH THOSE OF OTHER COUNTRIES.

In Weltner's catalogue of the freshwater sponges (1895) seventy-six recent species of Spongillidae (excluding _Lubosmirskia_) are enumerated, and the number now known is well over a hundred. In India we have twenty-nine species, subspecies, and varieties, while from the whole of Europe only about a dozen are known. In the neighbourhood of Calcutta nine species, representing three genera and a subgenus, have been found; all of them occur in the Museum tank. The only other region of similar extent that can compare with India as regards the richness of its freshwater sponge fauna is that of the Amazon, from which about twenty species are known. From the whole of North America, which has probably been better explored than any other continent so far as Spongillidae are concerned, only twenty-seven or twenty-eight species have been recorded.

The Indian species fall into seven genera, one of which (_Spongilla_) consists of three subgenera. With one exception (that of _Pectispongilla_, which has only been found in Southern India) these genera have a wide distribution over the earth's surface, and this is also the case as regards the subgenera of Spongilla. Four genera (_Heteromeyenia_, _Acalle_, _Parmula_, and _Uruguaya_) that have not yet been found in India are known to exist elsewhere.

Five of the Indian species are known to occur in Europe, viz., _Spongilla lacustris_, _S. crateriformis_, _S. carteri_, _S. fragilis_, _Trochospongilla pennsylvanica_; while _Ephydatia meyeni_ is intermediate between the two commonest representatives of its genus in the Holarctic Zone, _Ephydatia fluviatilis_ and _E. mulleri_. Of the species that occur both in India and in Europe, two (_Spongilla lacustris_ and _S. fragilis_) are found in this country in forms sufficiently distinct to be regarded as subspecies or local races.

Perhaps this course should also be taken as regards the Indian forms of _S. carteri_, of which, however, the commonest of the Indian races would be the typical one; but _S. crateriformis_ and _T. pennsylvanica_ seem to preserve their specific characters free from modification, whether they are found in Europe, Asia, or America.

The freshwater sponges of Africa have been comparatively little studied, but two Indian species have been discovered, _S. bombayensis_ in Natal and _S. alba_ var. _cerebellata_ in Egypt. Several of the species from the Malabar Zone are, moreover, closely allied to African forms (p. 11).

FOSSIL SPONGILLIDae.

The Spongillidae are an ancient family. Young described a species (_Spongilla purbeckensis_) from the Upper Jura.s.sic of Dorset (Geol. Mag.