Chapter IX: Introduction: To Part I (5)

_Gemmules_ strongly adherent, arranged in small groups, either single or double; when single spherical, when double oval; each gemmule or pair of gemmules covered by two layers of gemmule-spicules bound together in chitinous substance; the inner layer on the inner coat of the gemmule, the outer one separated from it by a space and in contact with the outer cage of skeleton-spicules; the size of the gemmule-spicules variable in both layers; external to the outer layer a dense cage of skeleton-spicules; foraminal tubule short, cylindrical.

This sponge is closely related to _S. loricata_, Weltner, of which Kirkpatrick regards it as a variety. "The main difference," he writes, "between the typical African form and the Burmese variety consists in the former having much larger microstrongyles (83 × 15.7 µ [0.83 × 0.157 mm.]) with larger and coarser spines;... Judging from Prof. Weltner's sections of gemmules, these bodies lack the definite outer shell of smooth macrostrongyles [blunt skeleton-spicules], though this may not improbably be due to the breaking down and removal of this layer. A further difference consists in the presence, in the African specimen, of slender, finely spined strongyles [amphistrongyli], these being absent in the Burmese form, though perhaps this fact is not of much importance."

TYPE in the British Museum; a piece in the Indian Museum.

HABITAT.--Myitkyo, head of the Pegu-Sittang canal, Lower Burma (_E. W. Oates_).

BIOLOGY.--The sponge had grown over a sheet of the polyzoon _Hislopia lacustris_, Carter (see p. 204), remains of which can be detected on its lower surface.

"Mr. E. W. Oates, who collected and presented the sponge, writes that the specimen was found encrusting the vertical and horizontal surfaces of the bottom beam of a lock gate, where it covered an area of six square feet. The beam had been tarred several times before the sponge was discovered. The portion of the gate on which the sponge was growing was submerged from November to May for eight hours a day at spring tides, but was entirely dry during the six days of neap tides. From May to October it was constantly submerged. The sponge was found in April. Although the canal is subject to the tides, the water at the lock is always fresh. The colour of the sponge during life was the same as in its present condition."

23. Corvospongilla lapidosa* (_Annandale_).

_Spongilla lapidosa_ Annandale, Rec. Ind. Mus. ii, pp. 25,
26, figs. 3, 4, 5 (1908).

The _sponge_ forms a thin but extremely hard and resistant crust the surface of which is either level, slightly concave, or distinctly corrugated; occasional groups of spicules project from it, but their arrangement is neither so regular nor so close as is the case in _C. burmanica_. The dermal membrane adheres closely to the sponge. The oscula are small; some of them are raised above the general surface but not on regular turret-shaped eminences. The colour is grey or black. There is a thick chitinous membrane at the base of the sponge.

The _skeleton_ is extremely dense owing to the large number of spicules it contains, but almost structureless; broad vertical groups of spicules occur but lack spongin and only traverse a small part of the thickness of the sponge; their position is irregular. The firmness of the skeleton is due almost entirely to the interlocking of individual spicules. At the base of the sponge the direction of a large proportion of the spicules is horizontal or nearly horizontal, the number arranged vertically being much greater in the upper part.

_Spicules._ The skeleton-spicules are sausage-shaped and often a little swollen at the ends or constricted in the middle. A large proportion are twisted or bent in various ways, and a few bear irregular projections or swellings. The majority, however, are quite smooth. Among them a few more or less slender, smooth amphioxi occur, but these are probably immature spicules. The length and curvature of the amphistrongyli varies considerably, but the average measurements are about 0.28 × 0.024 mm. The flesh-spicules also vary greatly in length and in the degree to which their shafts are curved. At first sight it seems to be possible to separate them into two categories, one in which the shaft is about 0.159 mm. long, and another in which it is only 0.05 mm. or even less; and groups of birotulates of approximately the same length often occur in the interstices of the skeleton. Spicules of all intermediate lengths can, however, be found. The average diameter of the shaft is 0.0026 mm. and of the rotula 0.0106 mm., and the rotula consists of from 6 to 8 spines. The gemmule-spicules vary greatly in size, the longest measuring about 0.08 × 0.014 and the smallest about 0.034 × 0.007 or even less. There appears to be in their case an even more distinct separation as regards size than there is in that of the flesh-spicules; but here again intermediate forms occur. They are all stout, more or less blunt, and more or less regularly covered with very short spines; most of them are distinctly curved, but some are quite straight.

_Gemmules._ The gemmules are firmly adherent to the support of the sponge, at the base of which they are congregated in groups of four or more. They vary considerably in size and shape, many of them being asymmetrical and some elongate and sausage-shaped. The latter consist of single gemmules and not of a pair in one case. Extreme forms measure 0.38 × 0.29 and 0.55 × 0.25. Each gemmule is covered with a thick chitinous membrane in close contact with its wall and surrounding it completely. This membrane is full of spicules arranged as in a mosaic; most or all of them belong to the smaller type, and as a rule they are fairly uniform in size. Separated from this layer by a considerable interval is another layer of spicules embedded in a chitinous membrane which is in continuity with the basal membrane of the sponge. The spicules in this membrane mostly belong to the larger type and are very variable in size; mingled with them are often a certain number of birotulate flesh-spicules. The membrane is in close contact with a dense cage of skeleton-spicules arranged parallel to it and bound together by chitinous substance. The walls of this cage, when they are in contact with those of the cages of other gemmules, are coterminous with them. There is a single depressed aperture in the gemmules, as a rule situated on one of the longer sides.

This sponge is distinguished from _C. burmanica_ not only by differences in external form, in the proportions of the spicules and the structure of the skeleton, but also by the peculiar nature of the armature of the gemmule. The fact that birotulate spicules are often found in close association with them, is particularly noteworthy.

TYPE in the Indian Museum.

GEOGRAPHICAL DISTRIBUTION.--This sponge has only been found in the Western Ghats of the Bombay Presidency. _Localities_:--Igatpuri Lake and the R. Godaveri at Nasik.

BIOLOGY.--There is a remarkable difference in external form between the specimens taken in Igatpuri and those from Nasik, and this difference is apparently due directly to environment. In the lake, the waters of which are free from mud, the sponges were growing on the lower surface of stones near the edge. They formed small crusts not more than about 5 cm. (2 inches) in diameter and of a pale greyish colour. Their surface was flat or undulated gently, except round the oscula where it was raised into sharply conical eminences with furrowed sides. The specimens from Nasik, which is about 30 miles from Igatpuri, were attached, together with specimens of _Spongilla cinerea_ and _S. indica_, to the sides of a stone conduit full of very muddy running water. They were black in colour, formed broad sheets and were markedly corrugated on the surface. Their oscula were not raised on conical eminences and were altogether most inconspicuous. The skeleton was also harder than that of sponges from the lake.

In the lake _C. lapidosa_ was accompanied by the gemmules of _Spongilla bombayensis_, but it is interesting that whereas the latter sponge was entirely in a resting condition, the former was in full vegetative vigour, a fact which proves, if proof were necessary, that the similar conditions of environment do not invariably have the same effect on different species of Spongillidæ.

APPENDIX TO PART I.

FORM OF UNCERTAIN POSITION.

(Plate I, fig. 4.)

On more than one occasion I have found in my aquarium in Calcutta small sponges of a peculiar type which I am unable to refer with certainty to any of the species described above. Fig. 4, pl. I, represents one of these sponges. They are never more than about a quarter of an inch in diameter and never possess more than one osculum. They are cushion-shaped, colourless and soft. The skeleton-spicules are smooth, sharply pointed, moderately slender and relatively large. They are arranged in definite vertical groups, which project through the dermal membrane, and in irregular transverse formation. Small spherical gemmules are present but have only a thin chitinous covering without spicules or foramen.

These sponges probably represent an abnormal form of some well-known species, possibly of _Spongilla carteri_. I have seen nothing like them in natural conditions.

PART II.

FRESHWATER POLYPS

(HYDRIDA).