38 



SPONGES 



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

 simpler spunge is not. 



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

 in length, (.■ylin(hical in shape, and fixed by one end, while 

 the opposite end is furnished with a small opening, the 

 excurrent opening, or the oscidum (Fig. 13). The osculum is 



surroiuK.led by a col- 

 lar of long, white, cal- 

 careous, needlelike 

 spines called spicules. 

 Structure of the 

 body. — The body is 

 a hollow cylmcler 

 with rather thick 

 \\'alls. The large cen- 

 tral cavity, or cloaca, 

 extends throughout 

 the length of the 

 body and opens out- 

 ward through the 

 osculum. The body 

 walls are perforated 

 with two sets of par- 

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

 set, the inJialent canals, nuis from the outside of the 

 body almost to tlie cloaca and ends blindly. The other 

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

 allel with the inlialent canals, but ends l)lindty just before 

 reaching the outside of the body. These canals run side bj' 

 side and connnuiiicate with each other by minute pores 

 thiougli theii- adjoining walls. The water enters the inha- 

 lent canals through the incurrent pores, the uiouths by 



Fig. 13. — Sitnnp:e (O'rnniia) : A, an indi\-i(Uial 

 split lcngt!i\\ise slHi\\iii<j; 1 loily oa\'ity (he) , in- 

 halent pore (If), (isctiIuui ("). cirflo of .spic- 

 ules (f) ; B,tv:o indi\-i(li.lals attach ft I to a stick. 



