SPONGES. 47 



The calcareous sponges are few in number and are repre- 

 sented by a delicate little white sponge called Sycon cilia- 

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

 marks. 



Order 2. CarncospongicB. — In this group the spicules 

 may either be fibrous and horny or silicioiis. The middle 



Yig. 3i —Axinella polypoides. Fig. Z'i.—Styloeordi/lahorenle, 



luuiiral fizc. -After Lovcn. 



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

 numerous digestive cavities or so-called ampullaceous sacs. 



The fresh-water sponge {Sponfjilla) occurs everywhere 

 on submerged sticks and stones in running or nearly stag- 

 nant water, usually branching. "With the exception of 

 ^pongilla and another form, Sipliydora echinoides Clark, 

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

 streams, all sponges are marine. One of the commonest 



