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



1907. Spongilla alba var. marina, id., ibid., p. 389. 



1909. ,, travancorica , id., op. cit., Ill, p. 101, pi. xii, fig. 1. 



191 1. }i alba var. cerebellata and var. bengalensis , Annandale. Faun. Brit, 



Ind., Freshw. Sponges, etc., pp. 76, yy , fig. Sb (p. 71), pi. i, 



figs. 1-3. 

 1911 ,, travancorica, id., ibid., p. 81, fig. 11. 



1913 ,, lacustris var. cerebellata, Susswasserschwänime in Wiss. Ergebn. 



Deutsch Zentralafrika-Exped. 1907-1908, Zool. 11, p. 475. 



The characters usually employed in distinguishing the species of Spongilla com- 

 pletely break down in separating 5. alba from S. lacustris. Nevertheless, I believe 

 them to be distinct, for the following reasons: — 



1 Even when 5. alba is growing side by side with green forms of S. lacustris, as 

 is sometimes the case, its cells never contain chlorophyl- corpuscles (cells of the alga 

 Chlovella) . 



2. In the living sponge, even when it is fully expanded and in full activity, 

 the oscula are not protected by conical dermal collars, but can be partly or com- 

 pletely closed by horizontal or oblique membranous diaphragms, as in 5. (Eunapius) 

 carteri. 



3. The oscula are not surrounded by radiating exhalent canals of small width 

 and running immediately below the dermal membrane; single canals similarly 

 situated but of much greater size often open into them after running along the 

 surface for a considerable distance. 



4. The main exhalent canals in the interior of the sponge are of much greater 

 calibre than in 5. lacustris. 



5. There is a much thicker horny membrane at the base of the sponge. 



6. There is often a subsidiary skeleton in 5. alba, consisting of single macroscleres 

 fastened together to form a dense irregular network by a secretion of chitinoid sub- 

 stance. 



The fact that these characters are for the most part difficult or impossible to 

 recognize in ordinary preserved specimens does not invalidate them from a theoreti- 

 cal point of view, although it renders them inconvenient to the systematist. 



There are other distinguishing characters that can usually be applied to indivi- 

 dual specimens even when these are not in particularly good condition, but they are 

 not constant and both species are of extreme variability in accordance with environ- 

 ment, locality and individual idiosyncracy. The most notable of the usually differ- 

 -ential characters exists in the structure of the skeleton. 



In the typical form of 5. lacustris {i.e. the form usually found in normal cir- 

 cumstances in Northern Europe) the radiating or vertical spicule-fibres are compact 

 though slender, and often run for some distance without branching. The spicules 

 of which they are mainly formed are cemented together by a secretion of horny 

 substance, which does not, however, form a sheath on the surface of the fibre. 

 These fibres are joined together, often at considerable intervals, by more slender 



