FOEM OF THE BODY OF ECHINODEEMA. 



197 



Fig. 97. Diagram of the body- 

 form of au Ophmrid. o Mont h. 

 s Body disc, r Arms. 



§ 160. 



The arrangements in other Echinoderma are derived from 

 form which obtains in the Astero'ida ; and 

 that in two divergent lines. In Loth, 

 although in different ways, it leads to a 

 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 (v). The arms take but a 

 small share in the formation of the 

 body-cavity, which is 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 Crino'ida, 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 forms, 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 pinnulse 



the 



Fig. 98. 



Rhizocriuus lofoten- 



i s. 4 A young specimen. B Cup 



(k), with seven arms, of a larger 



specimen, p Stalk, w Roots. R 



Anns, t Tentacles (after M. Sars). 



The ambulacra! groove 



