BITITIM TRITIUM. 



269 



while, in Spatangidcs, both poles, or only the oral pole, are shifted 

 in the direction of the unpaired radius, and become eccentric 

 (fig. 208). 



Only a few of the regular Echinodermata have the means of loco- 

 motion on all the five rays, and seldom then along the whole length 

 of their meridians ; far more frequently the area surrounding the 

 oral pole becomes with regard to the position during movement the 

 ventral surface; it is flattened and mainly or entirely possesses the 

 organs of locomotion (am- 

 bulacral surface). These re- 

 lations always obtain among 

 the irregular EcJdnoder- 

 mata which do not move 

 indifferently in the direction 

 of all five rays, but princi- 

 pally in that of the unpaired 

 one. In these animals the 

 mouth, and therewith the 

 oral pole, being pushed to- 

 wards the anterior edge, 

 the two posterior radii 

 (bivium) seem principally 

 concerned in the formation 

 of the ventral surface (Spa- 

 tangidce). It is otherwise 

 in the case of the cylindrical 

 Hololhurians. Their mouth 

 and anus preserve the nor- 

 mal position at the poles 

 of the elongated axis, and 

 the body is not unfrequently 

 compressed in the direction 

 of the axis in such a 



manner that three radii (triviuni) with their organs of locomotion 

 are placed on the foot-like ventral surface. On the body of these 

 Holothurians one unpaired and two paired radii can be distinguished, 

 only in this case the unpaired radius with its inter-radius marks, not 

 the anterior and posterior, but the dorsal and ventral surfaces. 



In many Echinoderms \Echinoidea] the oblate spheroidal form is 

 the prevalent one. The principal axis appears shortened, the apical 

 pole may be either pointed or flattened, and the ventral half is 



FIG. 209. Cucumaria with extended dendritically 

 branched tentacles (Tj. Af, ambulacral feet. 



