igi5-] Fauna of the Chilka Lake : Sponges. 45 



Laxosuberites lacustris, sp. nov. 

 (Plate v, figs. 2, 3). 



The sponge forms thin and fragile films, sometimes a little less so than those of 

 L. aquae-dulcioris , on stones and rocks. Its colour varies in the same manner and for 

 the same reasons as that of the latter species. In spirit any that may be present 

 (except the yellow of the gemmules, which is remarkably permanent) disappears and 

 the whole specimen assumes a milky opacity. The external surface, except imme- 

 diately round the gemmules and on the roofs of the superficial canals, is level and 

 minutely hispid These areas are smooth and, in the living sponge, convex. 



Probably each sponge possesses only one osculum, but many frequently grow so 

 close together as to form an apparently uniform layer of considerable area. The 

 osculum is slightly raised on a crater-like eminence with gently sloping sides. In the 

 living sponge it is protected by an oscular collar capable of expansion to a consider- 

 able length. This structure is a hollow cone formed of dermal membrane without 

 skeletal support. The actual exhalent orifice is situated at its free extremity and is con- 

 siderably narrower than the base of the cone. When fully expanded the latter is 

 much longer than it is broad at the base, where it is almost equal in width to the 

 main exhalent channel from the roof of which it arises. The external (i.e. im- 

 mediately subdermal) horizontal exhalent channels form a very conspicuous feature 

 in the external appearance of the sponge. Each system of the kind consists of a 

 main channel which runs along the surface of the parenchyma in a straight or sinuous 

 course for a distance of some 5 to 10 mm. The oscular collar arises from its roof at 

 or near the middle. Running into it at fairly regular intervals on either side are 

 lateral channels like itself but narrower; these, in their turn, receive yet other, still 

 narrower channels, so that an entirely horizontal ramification is formed. In the 

 living sponge the roofs of all those channels, that is to say those parts of the dermal 

 membrane that cover them, are markedly convex and quite hyaline. The inhalent 

 dermal pores lie scattered in the intervals between the lateral channels. They are 

 somewhat variable in size, but always minute ; the largest I have seen were not more 

 than 0'o8 mm. in diameter. In the preserved sponge apertures of both kinds are as 

 a rule obliterated, the oscular collars disappear and the roofs of the exhalent channels 

 collapse. In both living and preserved specimens ridges may frequently be observed 

 on the surface, sometimes marking off enclosed areas. These are, however, due not 

 to any peculiarity in the structure of the sponge, but to the growth in its substance 

 or below its base of algae, of the stolons of a Hydroid (Bimiria) or of a Polyzoon 

 (Loxosomatoides) , or else to the tubes made by a minute Polychaete worm. 



The dermal pores open directly, as is so often the case in thin encrusting 

 Monaxon sponges of different families, into cylindrical channels of considerably 

 greater diameter than themselves and running in a vertical direction. The upper 

 part of these channels, which is wider than the lower, represents the subdermal cavity, 

 but the lower part extends nearly to the base of the sponge. Finer afferent chan- 

 nels are given off radially from the lower part of the main ones, run in a horizontal 



