28 Memoirs of the Indian Museum. [Vol.. V, 



and proportions in the case of both species, but not essentially different from those 

 of many other sponges. The free microscleres, although also variable, are identical 

 or practically identical in the two species. The microscleres of the gemmule of all 

 forms of 5. alba with which I am acquainted differ from those of the typical form of 

 S. lacustris in being relatively more slender and in never having a very strong curva- 

 ture, but both these features are found in some forms of S. lacustris also, e.g. in the 

 common Indian varieties reticulata and proli/erens. 1 The number of free microscleres 

 present is extremely variable. Sometimes they are practically absent from the 

 choanosome. The spicules of a specimen of moderate hardness are figured, on p. 3. 



The number of spicules present in and on the pneumatic coat of the gemmule 

 — in some forms of 5. lacustris both spicules and coat are practically absent — is 

 extremely variable ; their precise arrangement is correlated with their number and 

 with the thickness of the coat, another variable character. 



In the synonymy given above I have included the names of four varieties (the 

 typical form, vars. cerebellata, marina and bengalensis) and of a il species " (S. travan- 

 corica) that I formerly regarded as distinct. Although " typical" (i.e. extreme) 

 specimens of these can be readily distinguished , so many intermediate sponges occur 

 that any attempt to distinguish them consistently is vexatious and unprofitable. 

 The form travancorica is perhaps more strongly differentiated than the others, but 

 the original description of it was founded on a single degenerate specimen and many 

 of those from the Chilka L,ake approach it closely. 



Among the latter are included the types of the var. marina : also many sponges 

 that are even further removed from the typical 5. lacustris than is the type 

 of S. alba, as well as others clearly referable to the typical form of the latter, to 

 bengalensis or, identical, except in the features noted above, with some forms of 

 5. lacustris. Others, again, are much harder than the forms of either species hitherto 

 described. The spicules and skeleton of an average specimen are figured in figs. 1 

 and 2, pp. 27, 30. 



Variation in the structure of the skeleton is definitely correlated, in sponges 

 from the lake, with environment. Generally speaking, those that grow on rocks 

 exposed to the violence of waves in open water are hard, their skeleton-fibres being 

 thick, branching and anastomosing freely and containing much horny matter, while 

 the subsidiary skeleton is well developed ; those that grow among loose filamentous 

 algae have remarkably slender fibres forming a very open network and containing 

 very little horny matter (compare figs. 1, la with fig. 2 on pi. iv). In such sponges 

 the subsidiary skeleton is practically absent. 



But intermediate forms occur. The softest specimens of the species I have seen 

 anywhere were growing among loose weeds in a small pool of practically fresh water 

 on Barkuda Id. ; sponges from rocks in the same pool were much harder, though not 

 as hard as those from similar positions in the lake. 



1 The tubular form of foraminal armature characteristic of 5. proliferans is not constant and the 

 sponge cannot therefore be regarded any longer as a distinct species. 



