408 



GOMPARA TIVE ANA TOMY 



CHAP. 



bivium) becomes still more accentuated by the different development of the ambu- 

 lacral feet in the two regions. On the ventral side, these feet are altogether or princi- 

 pally locomotory tube-feet (ending in suckers), on the dorsal side (the bivium) they are 

 exclusively or chiefly non-locomotory papilla (with more or less pointed ends). This 

 difference between the dorsal and ventral tube-feet is found both in those forms in 

 which the ambulacral feet, limited to the radii, are arranged in one or more longi- 

 tudinal rows, and in those in which they are also found in the interradii and arranged 

 irregularly. 



In the genus Psolus, the distinction between dorsal and ventral, and consequently 

 the bilateral symmetry of the body, becomes still more marked by the entire absence 



an 



era.... 



rmv ' 



a go an 



i imd 



aa- 



TO.S-- 



FIG. 351. Derivation of Rhopalodina (A) from an ordinary Holothurian (B) (after Ludwig). 

 C, Ideal intermediate form, rds, rvs, rmv, left dorsal, left ventral, merlioventral railius ; imd, 

 mecliodorsal interradius ; o, mouth ; mi, anus ; go, genital aperture ; CM, water vascular ring. 



of ambulacral appendages on the bivium. The tube-feet of the middle ventral radius 

 may also be wanting in some species of this genus. 



Where ventral and dorsal are sharply distinguished, the mouth and the anus 

 tend to shift on to the ventral side. 



The condition of the genus llhopalodina (Fig. 351) is quite peculiar. 

 The body is pear-shaped, and produced into a long stalk. At the end 

 of this stalk, close to one another, lie the mouth and the anus, and 

 between them the genital aperture. On the swollen portion of the 

 body there are ten double longitudinal rows of ambulacral feet, so that 

 it appears as if Rhopalodina possesses ten radii, whereas it in reality 

 possesses only five. To obtain this condition we have to imagine (1) 

 the body of an ordinary dendrochirote Holothurian bent upward an- 

 teriorly and posteriorly, arid (2) the approximation of the anus and 

 mouth by the great shortening of the dorsal interradius. The accom- 



