ARBACIA PUNCTULATA 



195 



Five minute sphceridia are situated on the peristomial edge 



of the test. They are modified spines and function as sense 



organs. To be made visible the clusters of ambulacral feet have 



to be removed when each spheridium appears as a little bean- 



3. IV. 



1 



c.a ~ ~ 



an .n. 



m 



v. 



sin.rd 



cn.rd 



''lac.ann. 



FIG. 39. Longitudinal section through the lantern of a sea urchin show- 

 ing the relative position of various organs. After Delage and Herouard. 

 ann, epineural nervous ring; sin. or, oral sinus; py, pyramid (lantern); 

 dnt, tooth; ph, pharynx; mcl. t, transverse muscles; sp, sponge-organ; cnhy, 

 hydrophone canal; lac. ann, oral lacuna; en. ami, ring canal; y, compas; 

 o, opening from oral sinus to Stewart's pouch; b, rotula; PS, Stewart's 

 pouch; mcl. i. py, interpyramidal muscles; lac. rd. ph, pharyngeal radial la- 

 cuna; en. rd, radial canal; sin. rd, radial sinus; n. rd, epineural radial nerve; 

 sq, ossicles of test. 



shaped body in a small depression in the median line of the 

 radius. 



The entire test including spines, pedicellaria, sphaeridiae, etc., 

 is covered with a ciliated epidermis. The inside surface of the 

 test forming the wall of the body cavity or codome is lined with 

 a ciliated peritoneal epithelium. 



