1 70 MANUAL OF ZOOLOGY SECT. 



clothed with plate-like ossicles, and beset laterally with 

 spines. They are, moreover, highly flexible, and, instead 

 of creeping along slowly like a Starfish, the Brittle-star 

 moves comparatively actively by means of lateral movements 

 of the arms. As in the Starfish there are distinct dorsal 

 and ventral surfaces, the former having the mouth in its 

 centre. An anus is absent, and the madreporite is on the 

 ventral surface instead of the dorsal. There are no ambula- 

 cral grooves, and the tube-feet project at the sides of the 

 arms. The internal structure is similar in most respects to 

 that of the Starfish, but the radial prolongations of the 

 body-cavity into the arms are absent. In certain of the 

 Ophiuroidea the arms are branched. 



3. THE ECHINOIDEA 



The Sea-urchins differ much more widely from the 

 Star-fishes than do the Brittle-stars. The body (Fig. 88) 

 is not star-shaped, but globular. At one pole (oral) is 

 the mouth, at the other (aboral or apical] the anus. 

 The body is enclosed in a shell or corona (Fig. 89), 

 formed of firmly-united plate-like ossicles, arranged in 

 rows which run from oral to aboral poles. Supported 

 on these are numbers of long, slender, sharp-pointed, 

 freely movable spines (Fig. 88). Running over the sur- 

 face from near the oral to near the aboral pole are five 

 bands of tube-feet which are capable of being exteaded 

 into long slender tubes (Fig. 88). These have sucker- 

 like extremities, and, like the tube-feet of the Starfish, are 

 the organs of locomotion. A remarkable and characteristic 

 feature of the internal structure is the presence of a com- 

 plicated apparatus for mastication known as Aristotle's 

 lantern, consisting of five jaw-like parts, each bearing a 



