PLATE LXXXVI. 301 



Plate LXXXVI. There is but a single large oscular 

 orifice, beneath which is a cavity of considerable size, 

 into which other orifices apparently discharge the fa?cal 

 streams of the sponge previous to their ultimate ejec- 

 tion from the large external one. Very little of the 

 dermal membrane remains on the sponge, but on a 

 small fragment of it mounted in Canada balsam the 

 characteristic sceptriform tension spicula were rather 

 abundant. They are so slender and minute that with- 

 out the aid of a microscopic power of five or six hun- 

 dred linear they would most probably elude observa- 

 tion. The minute acerate tension spicula are the most 

 abundant of the two forms, and they are very much 

 more slender and minute than the sceptriform ones. 

 Both forms of spicula are mixed together and irregu- 

 larly distributed over the inner surface of the dermal 

 membrane. The structure of the skeleton is more com- 

 pact than that of Spongilla lacustris, but the spicula of 

 which it is composed are quite as large as those of the 

 last-named species, so that the two may be readily mis- 

 taken for one species by a hasty observer. 



A few of the skeleton spicula have a slight central 

 inflation, but this is of such rare occurrence as scarcely 

 to be considered as a specific character. 



