HOLOTHUROIDEA. 249 



happens that the side of the body carrying the three ventral 

 radii and two ventral interradii (trivium) is flattened and modi- 

 fied into a sole-like creeping surface (Fig. 185), while the dorsal 

 side with its two dorsal radii (bivium) and three interradii is 

 arched. In such cases the tube-feet of the trivium have dis- 

 coidal ends and are suctorial in function, while those of the 

 bivium are pointed and probably have a respiratory or tactile 

 function : such non-locomotory pointed tube-feet are called 

 ambulacral papillae. It will be noted that the radii of the 

 bivium are not the same as in Asteroids and Echinoids (Fig. 

 182, cf. with Fig. 83). 



The body may be circular, or pentagonal in section ; when pentagonal 

 the radii occupy the angles. More rarely the body is flask-shaped (Rhopa- 

 lodina) or spherical. The ventral sole may occupy the whole length of 

 the animal (Colochirus, Stichopus, Millleria etc.), or be confined to the 

 middle portion (Psolus, Psolidium). Sometimes the dorsal interradius is 

 much shortened ( Ypsilothuria) and concave, the ventral surface being 

 correspondingly elongated and convex. In such cases the oral and aboral 

 poles are approximated. In Rhopalodina this modification is carried 

 to an extreme in that the dorsal interradius is practically obliterated. In 

 this case the body is flask-shaped, the mouth and anus being closely ap- 

 proximated, with the genital opening between them, at the end of the 

 neck of the flask, and the body appears to have ten radii. In other cases 

 the two ends of the long axis are bent ventralwards so that the mouth and 

 anus appear to be on the ventral surface at each end of the flattened 

 sole-like surface. Finally in certain deep-sea forms processes may be 

 developed from the dorsal surface of the body. In Psychropotes the hinder 

 region of the body projects back over the anus (Fig. 185), and in Peniagone 

 dorsal lobes are developed over the anterior part of the body. In the 

 pelagic form Pelagothuria (Fig. 186) a kind of umbrella is formed round 

 the oral region. 



There are three kinds of ambulacral appendages : the tube- 

 feet and ambulacral papillae already mentioned, and the ten- 

 tacles. 



The tentacles contain a prolongation of the water-vascular 

 system, usually of the radial canals, and are modified tube-feet. 

 They vary in number from 10 to 30. The number is usually 

 a multiple of 5, except in the Synaptidae, in which there are 

 frequently 12, and is usually constant in the same species and 

 even genus ; but there are genera and even species in which the 

 number is variable. In the Dendrochirotae with ten tentacles, 

 the two ventral (adult) tentacles are usually smaller than the 

 others (Fig. 177). They may be pinnate (Molpadiidae, Synap- 

 tidae), shield-shaped (Aspidochirotae), or dentritically branched 



