TYPE ECHINODERMA. 531 



OHAPTEE XVI. 



TYPE ECHmODERMA. 



The Echinoderms are exclusively marine organisms and 

 vary considerably in shape, some forms being elongated and 

 vermiform, others stellate, and others again almost spherical. 

 Whatever may be the shape, however, a well-marked radial 

 symmetry can be distinguished, which suggested to the older 

 zoologists the association of the members of this group with 

 the Ccelentera in a type Kadiata. The radii in the Echino- 

 derma are, however, almost invariably five, instead of four or 

 six or some multiple of these numbers as in the Ccelentera ; 

 and, furthermore, while in the Ccelentera the radial symmetry 

 represents a primitive condition and any departure from it 

 towards bilaterality, as in the Anthozoa, is secondary, the 

 reverse is the case with the Echinoderma. The larval forms 

 of this group are strictly bilateral, and even in the adults 

 certain organs or parts of organs interfere with the regularity 

 of the pentamerous arrangement and bring about a more or 

 less pronounced bilaterality. 



This may be clearly seen if one of the stellate forms, such 

 for instance as the ordinary five-rayed Starfish (Fig. 246), 

 be examined. This animal consists of a central disk, at the 

 centre of one surface of which, the oral surface, the mouth is 

 found, while the anus occupies a somewhat excentric position 

 on the other surface, which may be termed the aboral or 

 apical surface. From the edge of the disk the five arms or 

 rays project outwards, and along the median line of the oral 

 surface of each arm there extend outwards from rings around 

 the mouth a nerve-cord and a hydroccel canal, this latter 

 forming a part of a peculiar system of vessels characteristic 

 of the Echinoderms. In consequence of this radiation of 

 these structures out along the arms, and the arrangement of 



