THE STELLEROIDEA 239 



branch ends in a podion, which may be pointed, or may end in a 

 sucker. Connected with each podion there may be a globular 

 ampulla (absent from some Asteroids, e.g. Brisinga and from all 

 Ophiuroids). The madreporite in either group is dorsal, marginal, 

 or ventral. The nervous system comprises a circumoesophageal 

 ring and a radial branch along each arm or ray. 



The reproductive organs consist of strands connected with the 

 axial coelomic system ; there is a central ring whence a pair of 

 strands pass to each ray. A number of small gonads occur on 

 each strand ; the gonads of each strand may be grouped into one 

 bundle, with a common aperture at the margin of the disc (as in the 

 Asteroid Asterias and the Ophiuroid Ophiothrix) ; or they may occur 

 only at the bases of the arms, as in Freyella ; or they may occur as 

 a series of distinct gonads with separate apertures, as in Brisinga and 

 Ophiactis. In most Ophiuroids the gonads discharge into a bursa. 



As both Ophiuroids and Asteroids are therefore constructed 

 upon the same fundamental plan, as they contain the same varia- 

 tions from the typical arrangement, and as there is not a single 

 constant difference between them, it seems indispensable that they 

 should be united into one class, the STELLEROIDP;A, which may be 

 diagnosed as follows : 



Eleutherozoic, actinogonidial, and lysactinic Echinoderma in 

 which ambulacral plates lie internal to the radial ambulacral vessels. 

 The madreporite is not connected with an apical system of plates. 

 The body is more or less depressed, and is markedly stellate. 



In spite of the fact that the separate treatment of the Asteroids 

 and Ophiuroids has led to many unfortunate errors, and still 

 hampers the classification of the group, it seems advisable here 

 to consider the sub-classes separately. 



SUB-CLASS 1. ASTEROIDEA. 



The sub-class Asteroidea includes the Echinoderms known as 

 Starfish. The animals consist of a central body marked on the 

 ventral side by a series of radial furrows which are usually continued 

 outward along prolongations of the body known as arms. They 

 live on the sea-floor and creep about by means of suckers or podia in 

 the radial furrows, the side containing which is always placed down- 

 wards. The general aspect of starfish is therefore very different 

 from that of any of the previously described groups of Echinoderms. 

 They are closely related to the Ophiuroids, and no very satisfactory 

 line of demarcation can be drawn between the two sub-classes ; but 

 the Asteroids usually have diverticula from the alimentary canal 

 extending along the arms, which pass gradually into the disc. 



Before 1841 the Asteroids and Ophiuroids were always considered 

 members of one group. Thus J. H. Linck (23), who in 1733 began 



