SPONGES. 



47 



The calcareous sponges are few in number and are repre- 

 sented by a delicate little white sponge called Sycon cilia- 

 turn Johnston, very common on sea-weeds between tide- 

 marks. 



Order 2. Carneospongiw. In this group the spicules 

 may either be fibrous and horny or silicious. The middle 



Pig. 32Axlnella polypoides. 



Fig. W. JIyalontma boreale, 

 natural size. After Loven. 



cell-layer is very thick, the endoderin being restricted to the 

 numerous digestive cavities or so-called ampullaceous sacs. 



The fresh-water sponge (Spongilla) occurs everywhere 

 on submerged sticks and stones in running or nearly stag- 

 nant water, usually branching. "With the exception of 

 Spongilla and another form, Siphydora echinoides Clark, 

 which grows as large as one's fist in northern ponds and 

 streams, all sponges a v marine. One of the commonest 



