J. G. WALLER ON FRESH-WATER SPONGES. 43 



growth, overlaying and ojDpressing the sponge, constrained its de- 

 velopment in the usual way, and the lobular growth was in such 

 places as were free from this parasitic obstruction. As soon, how- 

 ever, as the sponge escapes this its tendency to spread laterally is 

 manifested, and the terminations of the lobes widen. I believe 

 this is a weakly condition of the sponge, only seen in young 

 examples, and not under healthier circumstances. 



The external character, then, of the two is entirely dissimilar 

 when mature and fully developed. Colour is a very variable cha- 

 racter, dependant so much upon light, but, in general, S. lacustris 

 is of a dark green, whilst that of S. Jluviatilis varies from a dull 

 yellow to an emerald tint. More intimate examination makes the 

 distinction between them still wider. The membranes of S. lacustris 

 are covered with minute spiniferous spicula, whilst those of the other 

 have none. The structure of the skeletons are similar, but coarser 

 in the former than in the latter. Their spicula have but slight 

 differences ; and whilst on this part of my subject, I must call at- 

 tention to what is somewhat remarkable — that in both these sponges 

 a form of spiculum has entirely been overlooked by those eminent 

 observers who have done so much to instruct us on this interest- 

 ing group of organisms. 



Dr. Bowerbank, in his work on the British Spongiadse, which 

 must now be regarded as the text-book on Sponges, gives only one 

 form of spicula as belonging to the skeleton of either of the 

 British Spongilla. That of S. fluviatilis he calls " Acerate," that 

 of S. lacustris " Subfusiform Acerate." As regards the first, if 

 you take a mature example of the sponge and make a vertical 

 section, mount it in balsam and examine it under the microscope, 

 you will find that nearly the whole of the spicula are spiniferous , 

 the plain '' Acerate " form being very few in number. But now 

 make a horizontal section, and examine it in a similar manner, and 

 then the spiniferous form is found to be few in number, the greater 

 part being that which Dr. Bowerbank has described. This at once 

 shows us the true position each occupies in the skeleton, and that 

 there are two forms is without doubt. The question now arises how 

 this form could possibly be overlooked if it be constant. I am not 

 sure if I have always seen it myself — and I have seen a slide sold as 

 " Spicules of S. Jluviatilis,^^ in which it certainly did not apjoear ; 

 nevertheless, I am inclined to think it a constant characteristic in 

 mature examples. In *S'. lacustris the spiniferous spicule does not 



