OPHIUROIDEA 153 



support the various arms and are named thereafter (Fig. 87) ; 

 the body-rod continues backwards as the end rod, which is, as a 

 rule, tripartite at the end. Some distance from the end proceed 

 a pair of transverse rods, directed inwards and joining those from 

 the opposite side, thus forming a ring beneath the stomach. 

 The transverse rods may have a shorter or longer process, the 

 median rod. The body skeleton is of special importance for 

 distinguishing the various larval species. The arms of the 

 larvae may be narrow or broad, of various length, often very long. 

 The larval body may have peculiar widenings, ciliated lobes. 

 Often the posterior end carries a tuft of cilia, sometimes a ciliated 

 ring. Generally the tip of the arms, especially the postero- 

 lateral arms, is strongly pigmented. As a rule the stomach is 

 greenish. Some Ophiuroid larvae are more or less rudimentary, 

 with feebly developed arms and indistinct ciliated band. During 

 the metamorphosis the larval body, with the skeleton, is absorbed, 

 either completely, or the postero-lateral arms remain unaltered 

 and serve as floating apparatus for the young brittle-star, until 

 the latter is ready to assume life on the bottom, when these arms 

 are thrown off ; they still remain in connection and may continue 

 swimming for a little while, and are not rarely met with in 

 plankton samples ; but, of course, they soon perish. In a 

 tropical form it appears that a new larval body may regenerate 

 from these postero-lateral arms which have been thrown off. 

 The young Ophiuroids may keep floating for some time, the long- 

 extended tube-feet serving as a floating apparatus. The larval 

 stage as a rule has a duration of a few weeks. The larvae feed 

 on minute pelagic organisms. Some Ophiuroids have direct 

 development, without a pelagic larval stage ; this probably holds 

 good of those which have large, yolky eggs. 



Key to the larvce of Ophiuroids {Ophiopluteus) Tcnown from or 

 likely to occur in the British seas. (Fig. 88.) 



1. Fully formed larva, with arms well developed and ciliated 



band distinct ........ 2 



Larva more or less rudimentary, ciliated band indistinct . 12 



2. A ciliated ring around the posterior end of the body (skeleton 



unknown) .... Ophiopluteus coronatus Mrtsn. 

 No ciliated ring around the jjosterior end of the body . 3 



3. Body rod simple ........ 4 



Body rod forms two meshes . . . . . .11 



4. Postero -dorsal arm lacking ...... 5 



Postero -dorsal arm present ....,,() 



