SPOSGES. 



47 



The calcareous sponges are few in number and are repre- 

 sented by a delicate little white sponge called Sycon cilia- 

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

 marks. 



Order 2. Oarneospongim. — In this group the spicules 

 may either be fibrous and horny or silicious. The middle 



Axinella polypoides. 



Fig. Si.—nyalonema boreale, 

 iiauiral eize.— After Lov6n. 



cell-layer is very thick, the endoderm 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 are marine. One of the commonest 



