408 



COMPARATIVE ANATOMY 



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 papillae (with more or less pointed ends). This 

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

 which the ainbulacral 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 



TVS ' 



rmv 



Fio 351 -Derivation of Bhopalodina (A) from an ordinary Holothurian (B) (after Ludwig). 

 n.'elio? In 1 ter ? iedi ? te form ' ** " , left dorsal, left ventral, medioventral radius; imd, 

 .1 interradms ; o, mouth ; an, anus ; go, genital aperture ; , water vascular ring. 



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



o 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. 



P 



genUS Rh P alodi ( F ig- 351) is quite peculiar. 



th ' and produced into a lon stalk - At ^ end 



betwee tti ,l! e '"i 0t ^ He the m Uth and the anus > and 

 odv UP f ^ al apertUre ' n the swolle11 P rtion ^ the 



I' ^ f te ^ dou ^. lon S itudinal ^ws of ambulacral feet, so that 



onlvfiv P T M T - P SSeSSeS ten radli ' Whereas {i ^ realit y 

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



n ? ^ de j ldrochirot Holothurian bent upward an- 

 b t Tl y ' and (2) the W^imation of the anus and 



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



