_Tubella_, _id._ (_partim_), _ibid._, p. 248.

_Meyenia_, Carter (_partim_), Ann. Nat. Hist. (5) vii, p. 90 (1881).

_Trochospongilla_, Weltner, in Zacharias's Tier- und Pflanzenwelt, i, p. 215 (1891).

_Trochospongilla_, _id._, Arch. Naturg. lxi (i), p. 120 (1895).

_Tubella_, _id._ (_partim_), _ibid._ p. 128.

TYPE, _Spongilla erinaceus_, Ehrenberg.

The characteristic feature of this genus is that the rotulae of the gemmule-spicules, which are h.o.m.ogeneous, have smooth instead of serrated edges. Their stem is always short and they are usually embedded in a granular pneumatic coat. The sponge is small in most of the species as yet known; in some species microscleres without rotulae are a.s.sociated with the gemmules.

[Ill.u.s.tration: Fig. 23.--A=skeleton-spicule of _Trochospongilla latouchiana_; A'=gemmule-spicule of the same species; B=gemmule of _T.

phillottiana_ as seen in optical section from above; B'=skeleton-spicule of same species: A, A', B' 240; B 75. All specimens from Calcutta.]

I think it best to include in this genus, as the original diagnosis would suggest, all those species in which all the gemmule-spicules are definitely birotulate and have smooth edges to their disks, confining the name _Tubella_ to those in which the upper rotula is reduced to a mere k.n.o.b. Even in those species in which the two disks are normally equal, individual spicules may be found in which the equality is only approximate, while, on the other hand, it is by no means uncommon for individual spicules in such species as _"Tubella" pennsylvanica_, which is here included in _Trochospongilla_, to have the two disks nearly equal, although normally the upper one is much smaller than the lower.

There is very rarely any difficulty, however, in seeing at a glance whether the edge of the disk is smooth or serrated, the only species in which this difficulty would arise being, so far as I am aware, the Australian _Ephydatia capewelli_* (Haswell), the disks of which are undulated and nodulose rather than serrated.

GEOGRAPHICAL DISTRIBUTION.--The genus includes so large a proportion of small, inconspicuous species that its distribution is probably known but imperfectly. It would seem to have its headquarters in N. America but also occurs in Europe and Asia. In India three species have been found, one of which (_T. pennsylvanica_) has an extraordinarily wide and apparently discontinuous range, being common in N. America, and having been found in the west of Ireland, the Inner Hebrides, and near the west coast of S. India. The other two Indian species are apparently of not uncommon occurrence in eastern India and Burma.

_Key to the Indian Species of_ Trochospongilla.

I. Rotules of the gemmule-spicules equal or nearly so.

A. Skeleton-spicules smooth, usually pointed _latouchiana_, p. 115.

B. Skeleton-spicules spiny, blunt _phillottiana_, p. 117.

II. Upper rotule of the gemmule-spicules distinctly smaller than the lower.

Skeleton-spicules spiny, pointed _pennsylvanica_, p. 118.

18. Trochospongilla latouchiana*, _Annandale_.

_Trochospongilla latouchiana_, Annandale, J. Asiat. Soc.

Bengal, 1907, p. 21, fig. 5.

_Trochospongilla latouchiana_, _id._, Rec. Ind. Mus. ii, p.

157 (1908).

_Trochospongilla leidyi_, _id._ (_nec_ Bowerbank), _ibid._ iii, p. 103 (1909).

[Ill.u.s.tration: Fig. 24.--_Trochospongilla latouchiana._

Vertical section of part of skeleton with gemmules _in situ_, 30; also a single gemmule, 70. (From Calcutta).]

_Sponge_ forming cushion-shaped ma.s.ses rarely more than a few centimetres in diameter or thickness and of a brown or yellow colour, hard but rather brittle; surface evenly rounded, minutely hispid; oscula inconspicuous, small, circular, depressed, very few in number; external membrane adhering closely to the parenchyma; a chitinous membrane at the base of the sponge. Larger sponges divided into several layers by similar membranes.

_Skeleton_ dense, forming a close reticulation; radiating fibres slender but quite distinct, running up right through the sponge, crossed at frequent intervals by single spicules or groups of spicules.

_Spicules._ Skeleton-spicules smooth, about twenty times as long as the greatest transverse diameter, as a rule sharply pointed; smooth amphistrongyli, which are often inflated in the middle, sometimes mixed with them but never in large numbers. No flesh-spicules.

Gemmule-spicules with the rotulae circular or slightly asymmetrical, flat or nearly flat, marked with a distinct double circle as seen from above, sometimes not quite equal; the shaft not projecting beyond them; the diameter of the rotule 4-1/2 to 5 times that of the shaft, which is about 2-2/3 times as long as broad.

_Gemmules_ small (0.2 0.18 mm.), as a rule very numerous and scattered throughout the sponge, flask-shaped, clothed when mature with a thin microcell coat in which the birotulates are arranged with overlapping rotulae, their outer rotulae level with the surface; foraminal aperture circular, situated on an eminence.

_Average Measurements._

Diameter of gemmule 0.2 0.18 mm.

Length of skeleton-spicule 0.28 "

Length of birotulate-spicule 0.175 "

Diameter of rotula 0.02 "

_T. latouchiana_ is closely related to _T. leidyi_ (Bowerbank) from N.

America, but is distinguished by its much more slender skeleton-spicules, by the fact that the gemmules are not enclosed in cages of megascleres or confined to the base of the sponge, and by differences in the structure of the skeleton.

TYPE in the Indian Museum.

GEOGRAPHICAL DISTRIBUTION.--Lower Bengal and Lower Burma.

_Localities_:--BENGAL, Calcutta and neighbourhood (_Annandale_): BURMA, k.a.w.kareik, Amherst district, Tena.s.serim (_Annandale_).

BIOLOGY.--This species, which is common in the Museum tank, Calcutta, is apparently one of those that can grow at any time of year, provided that it is well covered with water. Like _T. leidyi_ it is capable of producing fresh layers of living sponge on the top of old ones, from which they are separated by a chitinous membrane. These layers are not, however, necessarily produced in different seasons, for it is often clear from the nature of the object to which the sponge is attached that they must all have been produced in a short s.p.a.ce of time. What appears to happen in most cases is this:--A young sponge grows on a brick, the stem of a reed or some other object at or near the edge of a pond, the water in which commences to dry up. As the sponge becomes desiccated its cells perish. Its gemmules are, however, retained in the close-meshed skeleton, which persists without change of form. A heavy shower of rain then falls, and the water rises again over the dried sponge. The gemmules germinate immediately and their contents spread out over the old skeleton, secrete a chitinous membrane and begin to build up a new sponge. The process may be repeated several times at the change of the seasons or even during the hot weather, or after a "break in the rains."

If, however, the dried sponge remains exposed to wind and rain for more than a few months, it begins to disintegrate and its gemmules are carried away to other places. Owing to their thin pneumatic coat and relatively heavy spicules they are not very buoyant. Even in the most favourable circ.u.mstances the sponge of _T. latouchiana_ never forms sheets of great area. In spite of its rapid growth it is frequently overgrown by _Spongilla carteri_.

19. Trochospongilla phillottiana*, _Annandale_.

_Trochospongilla phillottiana_, Annandale, J. Asiat. Soc.

Bengal, 1907, p. 22, fig. 6.

_Trochospongilla phillottiana_, _id._, Rec. Ind. Mus. i, p.

269 (1907).

_Trochospongilla phillottiana_, _id._, _ibid._ ii, p. 157 (1908).

_Sponge_ hard but friable, forming sheets or patches often of great extent but never more than about 5 mm. thick; the surface minutely hispid, flat; colour pale yellow, the golden-yellow gemmules shining through the sponge in a very conspicuous manner; oscula inconspicuous; external membrane adherent; no basal chitinous membrane.

_Skeleton_ dense but by no means strong; the reticulation close but produced mainly by single spicules, which form triangular meshes; radiating fibres never very distinct, only persisting for a short distance in a vertical direction; each gemmule enclosed in an open, irregular cage of skeleton-spicules.

_Spicules._ Skeleton-spicules short, slender, blunt, more or less regularly and strongly spiny, straight or feebly curved. No flesh-spicules. Gemmule-spicules with the rotulae circular, very wide as compared with the shaft, concave on the surface, with the shaft projecting as an umbo on the surface; the lower rotula often a little larger than the upper.

_Gemmules_ numerous, situated at the base of the sponge in irregular, one-layered patches, small (0.32 0.264 mm.), of a brilliant golden colour, distinctly wider than high, with a single aperture situated on an eminence on the apex, each clothed (when mature) with a pneumatic coat that contains relatively large but irregular air-s.p.a.ces among which the spicules stand with the rotulae overlapping alternately, a funnel-shaped pit in the coat descending from the surface to the upper rotula of each of them; the surface of the gemmule covered with irregular projections.

Diameter of gemmule 0.32 0.264 mm.

Length of skeleton-spicule 0.177 "

Length of gemmule-spicule 0.015 "

Diameter of rotule 0.022 "

This species appears to be related to _T. pennsylvanica_, from which it differs mainly in the form of its gemmule-spicules and the structure of its gemmule. My original description was based on specimens in which the gemmule-spicules were not quite mature.