170 MANUAL OF ZOOLOGY SECT. 



Asterias, the brittle-star (Fig. 93) has a star-shaped body 

 with a central disc and five radiating arms. But the arms, 

 instead of appearing merely as radiating prolongations of the 

 central disc, are sharply marked off from it, and have rather 

 the appearance of appendages. They are solid, long, slender, 

 and tapering, clothed with plate-like ossicles and beset 

 laterally with spines. They are highly flexible, and instead 

 of creeping along slowly like a starfish the brittle-star moves 

 with comparative activity 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 side of the 

 arm. 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, and there are no 

 pyloric caeca. In certain of the Ophiuroidea the arms are 

 branched. 



3. THE ECHINOIDEA 



The Sea-urchins (Class Echinoidea) differ much more 

 widely from the starfishes than the brittle-stars. The body 

 (Fig. 94) is not star- shaped, but globular. At one pole is 

 the mouth, at the other the anus. The body is enclosed in 

 a shell or corona (Fig. 95), 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. 94). Running 

 over the surface from near the oral to near the aboral poles 

 are five bands of tube-feet which are capable of being 

 extended into long slender tubes (Fig. 94). These have 



