FORM OF THE BODY OF ECHINODEEMA. 



197 



§ 160. 



The ai-ningements in other Echinoderma are derived from the 

 form which obtains in the Asteroida ; and 

 that in two divergent hnes. In both, 



in different ways. 



it leads to a 



although 



greater centralisation of the organism. 

 Along one line the arms are more largely 

 developed, and at the same time gradually 

 lose their relations to the internal organs. 

 Along the other the arms pass completely 

 into the common body. The number of 

 rays appears to be always five. The first 

 arrangement is seen in Brisinga and the 

 Ophiurida, where the body is separated 

 into a discoid central portion (Fig. 97, s), 

 and the projecting, but sharply sepa- 

 rated, arms (y). The arms take but a 

 small share in the formation of the 

 body-cavity, whichis almost exclusively 

 limited to the body disc. The ambu- 

 lacral groove is wanting in the Ophi- 

 urida, but the ambulacra still extend 

 along the arms. 



The arms of the Euryalida are 

 greatly developed, being repeatedly 

 divided in a dichotomous fashion. A 

 shallow groove is continued into the 

 divisions. The Crinoida, which in 

 earlier periods were widely distributed, 

 and very rich in forms, but which are 

 now represented by few genera, have 

 lost the power of free locomotion, and 

 passed into the fixed condition. In 

 the division Brachiata, which includes 

 the extant foi-ms, a stalk, often of 

 great size, jointed, and complicated 

 by branchings and appended struc- 

 tures (Fig. 98), is developed on the 

 antambulacral portion of the cup-like 

 body. This serves as an organ of 

 attachment. The arms are not always 

 limited to five, but there is often a 

 larger number of them ; they are con- 

 siderably developed, being divided, or 

 provided with secondary appendages. 

 These appendages, which are attached 

 to the arms and have the form of 

 feathery plates, are known as pinnulee 



Fig. 97. Diagram of the body- 



form of an Ophiurid. o Month. 



s Body disc, r Arms. 



Fig.98. Ehizocrinus lofofcen- 

 sis. A A young specimen. B Cup 

 (fc), with seven arms, of a larger 

 specimen, p Stalk, w Eoots. R 

 Arms, t Tentacles (after M. Sars). 



The ambulacral groove 



