Chap. HI. GENUS IDYIA. 277 



iiiterambulacral expansion is somewhat wider, in consequence of which the outline 

 of the animal appears oval, the sides being more or less flattened. 



The cellular structure of the spherosome is easily seen, and, by following the 

 fibre-like outlines of their angles, the arrangement of the cells may be traced without 

 much difficulty. It is much more simple and uniform than in either Pleuro- 

 brachia or Bolina, and, owing to the total absence of tentacles and tentacular tubes, 

 no trace of the complicated arrangement of motory cells, which in Pleurobrachia 

 constitute the lateral system, is \4sible. The radiating system is most prominent, 

 and, in fact, forms the chief bulk of the body. Its general arrangement is that 

 described in Pleurobrachia, with this difference however, that, owing to the much 

 greater elongation of the vertical diameter of the body, the cells of which it is 

 composed form long rows parallel to one another, converging only toward the 

 abactinal pole, but remaining straight on the actinal side. The interambulacral 

 system is the least developed, and, far from forming the doubly convex vertical bands 

 of transverse cells which Ave have seen in Pleurobrachia, it consists only of a few 

 cells extending between the ambulacra. Hence the marked difference from Pleuro- 

 brachia in the outline of the body when seen from either the actinal or abactinal 

 side, the surface of the interambulacra appearing concave {Fig. 99, p. 283), unless 

 the body be fully distended, while the ambulacra are raised above the general level 

 of the surface. In Pleurobrachia the reverse is the case, in consequence of the great 

 development of the interambulacral system. But this contrast between the inter- 

 ambulacra and the ambulacra is only striking along the sides, for toward the 

 abactinal pole, and especially beyond the extent of the rows of locomotive flappers, 

 the peripheric system is interwoven with the radiating system, very much as in 

 Pleurobrachia, and the surface of that region of the body is more even. The 

 lateral interambulacra, however, are here somewhat prominent, bulging above the 

 level of the ambulacra about as much as in Pleurobrachia. On the actinal side and 

 beyond the extent of the rows of locomotive flappers, the peripheric system is again 

 interwoven with the radiating system and powerfully developed ; and, as the sphero- 

 meres do not converge and are not arched on the actinal pole, but extend in 

 a straight course to the edge of the mouth, this part of the body is capable of 

 the most varied and extensive motions. Notwithstanding the prominence of the 

 ambulacra, the ambulacral system of motory cells is not more developed than in 

 Pleurobrachia. 



These structural details may explain the characteristic movements of Idyia. The 

 weakness of the interambulacral system forbids a close approximation of the ambu- 

 lacra, so that the vertical diameter is not reduced by its contractions, but by the 

 contractions of the radiating system, which may go so far as to bend the actinal 

 side of the body inward and reduce the length of the animal in a most remarkable 



