584 



1NVERTEBRA TE MORPHOLOG Y. 



primitive, while the Clypeastroidea and Petalosticha are secondarily-de- 

 rived forms. The bilaterality of these latter forms is not to be regarded, 

 therefore, as having any phylogenetic significance. 



CLASS V. HOLOTHUROIDEA. 



The Holothurians (Fig. 268) are characterized so far as 

 their form is concerned by being elongated in the oral-aboral 

 axis, having thus a somewhat wormlike form, the mouth be- 

 ing at or near one extremity and the anus at the other, except 

 in the genus Wiopalodina, in which the two openings are ap- 

 proximated. As a rule the body is cylindrical, but in some 



forms, such as Psolus and the Elasipoda, 

 there is a well-marked flattened ventral 

 surface. Three of the radial hydrocoel- 

 cauals lie upon this ventral surface, the 

 other two being dorsal, and it is usual to 

 apply the term trivium to the ventral radii 

 and bivium to the dorsal. It must be 

 recognized, however, that this use of the 

 terms does not imply a homology with 

 the radii similarly named in the Echin- 

 oidea, since in the latter the radii C, D, 

 and ^'constitute the trivium, whereas in 

 the Holothurians it is the radii A, B, 

 and E. 



The mouth is surrounded by a circle 

 of tentacles varying in number from ten 

 to thirty. There are at first five primary 

 HOLOTHCRIAN. tentacles, interradial in position, which 

 are formed in connection with five ceecal 



outgrowths of the hydrocoal-ring, and the tentacles subse- 

 quently formed receive branches from the five primary caeca. 

 In shape the tentacles vary considerably, being cylindrical 

 in some forms, arborescent or pinnate in others (Fig. 268), 

 and in others peltate, and in some forms they are retractile. 



The exterior of the body is usually covered by an epithe- 

 lium over which a cuticle may be developed, but in some 

 forms the ectodermal cells sink into and become fused with 

 the subjacent connective tissue. The calcareous skeleton is 



PIG. 268. Pentacta fron 



