THE FRESHWATER Sl'ONGES. 175 



ciliated appearance like that of vegetable cell structure. Next 

 comes a comparatively thick, chitinous membrane, of an amber 

 colour, which, when viewed in the whole Statoblast, has a deeper 

 colour than when separated. 



Then comes another coating or crust which, in two instances, is 

 composed of cell structure, hexagonal in section, but in the rest of 

 a white granular or micro-cellular substance, which can only be seen 

 by a very high power object-glass. It appears to afford a floating 

 property, like cork, to the Statoblast, and varies much in thickness 

 according to the species. Its composition is still, I believe, a dis- 

 puted point, Meyen thinking it was lime, having a cellular forma- 

 tion, but in no case has it been known to effervesce when brought 

 into contact with hot or cold acids. 



This crust is charged or accompanied by spicules of different 

 forms, variously arranged according to the species, and on which 

 the classification of the Freshwater Sponges is now founded. 



Although the Statoblasts have been known so many years, John- 

 ston, in his description of the British species, does not mention the 

 presence of spicules except in a foot-note, stating that Meyen, in 

 1839, discovered bi-rotulate spicules, and others with minute spines 

 on their surface, evidently believing, at that time, that the two be- 

 longed to the one species of sponge ; perhaps a natural conclusion 

 to have come to then, as the two were, and are often, found growing 

 together in the same locality, and the microscopical appliances for 

 seeing them were not then anything like so perfect as they are now. 



In some species, as in Tubella reticulata, the Statoblast is en- 

 closed in a distinct layer of spicules, which partake more of the 

 character of the skeleton spicules of the Sponge, forming a cap- 

 sular covering, in which it was probably developed. 



We will now pass on to the classification of the Freshwater 

 Sponges, as founded by Mr. Carter, on the form and structure of 

 the Statoblasts, as far as present known, omitting for brevity the 

 general structure of the Sponge, as I mentioned at the commence- 

 ment of this paper. 



The first genus comprises the Spongilla, containing ten species, 

 whose Statoblasts are globular, crust thick, thin, and in some cases 

 absent altogether, accompanied by minute acerate spicules, smoothed 

 or spined according to species. 



In Spongilla Carteri the spicules are smoothly acerate, and the 

 crust is composed of pyramidal columns of dodecahedral or poly- 



