viii ECHINODERM ATA MORPHOLOGY OF HOLOTHURIOIDEA 407 



The application of the terms "ventral" and "dorsal," in very 

 many Holothurioidea, is fully justifiable, since there arises, parallel 

 to the principal (longitudinal) axis, a flattened creeping sole, along 

 the middle of which the above-named ventral radius runs, while the 

 middle of the vaulted dorsal surface opposed to this creeping sole is 

 occupied by the middle dorsal interradius. 



The radii and interradii now become arranged in such a way that 

 three radii (one middle and two lateral) belong with their ambulacral 



rdd 



\ 



6ef 



idd 



Trivium 



z'c/b 



FIG. 350. Diagrammatic section illustrating the symmetry of the Holothurioidea. De- 

 velopment of the bivium and the trivium (mainly after Ludwig). imd, Medio-dorsal interradius ; 

 ids, left dorsal ditto ; isv, left ventral ; idv, right ventral ; idd, right dorsal interradius ; rds, left 

 dorsal radius ; rsv, left ventral ditto ; rmv, medio-ventral ; rdv, right ventral ; rdd, right dorsal 

 radius ; mi, anterior or dorsal mesentery ; mo, middle or left ; m^, posterior or right mesentery ; 

 *l> *2> *3i first, second, and third, or anterior, middle, and posterior limbs of the intestine ; vd and 

 vv, dorsal and ventral intestinal vessels ; bd and bs, right and left branchial tree (aquatic lung) ; 

 go, gonad ; dg, genital duct ; a, body cavity. 



feet to the creeping sole, and form the trivium ; while on the dorsal 

 surface two radii (one right and the other left) form the bivium. Two 

 interradii, on the other hand, belong to the creeping sole, and three 

 to the dorsal surface (cf. the diagram, Fig. 350). 



The creeping sole usually runs along over the whole length of the body, less fre- 

 quently (Psolus, Psolidium) it is limited to a circumscribed region between the 

 anterior and posterior ends. 



The difference between the ventral and the dorsal side (the trivium and the 



