38 



SPONGES 



water mark. It is easily obtainable for study, while the 

 simpler sponge is not. 



External features. — The body is about one half an inch 

 in length, cylindrical ia shape, and fixed by one end, while 

 the opposite end is furnished with a small opening, the 

 excurr&nt opening, or the oscvlum (Fig. 13) . The osculum is 



surrounded by a col- 

 lar of long, white, cal- 

 careous, needlelike 

 spines called spicules. 

 Structure of the 

 body. — The body is 

 a hoUow cylinder 

 with rather thick 

 walls. The large cen- 

 tral cavity, or cloaca, 

 extends throughout 

 the length of the 

 body and opens out- 

 ward through the 



Fig. 13. — Sponge (Graniid) : .4, an individual , _,, , , 



spUt lengthwise showing body cavity (6c), in- OSCUlimi. inO DOdy 



halent pore (.it), osculum (o), circle of spic- waUs are perforated 



ules(c) :B, two individuals attached to a stick. -.r j. j. e 



With two sets 01 par- 

 allel canals that rim at right angles to the cloaca. One 

 set, the inhalent canals, runs from the outside of the 

 body almost to the cloaca and ends blindly. The other 

 set, the radial canals, begins at the cloaca and runs par- 

 allel with the inhalent canals, but ends blindly just before 

 reaching the outside of the body. These canals run side by 

 side and communicate with each other by minute pores 

 through their adjoining walls. The water enters the inha- 

 lent canals through the incurrent pores, the mouths by 



