CHAPTER XIII. 



ELEUTHEROZOA THE HOLOTHURIOIDEA. 



GRADE B. ELEUTHEROZOA. 



CLASS I. HOLOTHURIOIDEA. 

 II. STELLEROIDEA. 

 III. ECHINOIDEA. 



ECHINODERMA in which the theca, which may be but slightly or 

 not at all calcified, is not attached by any portion of its surface, 

 but is usually placed with the oral surface downwards or in the 

 direction of forward locomotion. Food is not conveyed by a 

 subvective system of ciliated grooves, but is taken in directly by 

 the mouth. The anus when present is typically aboral and 

 approaches the mouth only in a few specialised forms. The aboral 

 nervous system, if indeed it be present at all, is very slightly 

 developed. The circumoesophageal water-ring may lose its connec- 

 tion with the exterior medium ; the podia (absent only in some 

 exceptional forms) may be locomotor, respiratory, or sensory in 

 function, but usually are locomotor tube-feet. 



As explained on p. 33, the genetic affinity between the three 

 classes now to be dealt with is not so obvious as that between the 

 classes of Pelmatozoa. Some writers have taken the Holothurioidea 

 to be the most primitive class of Echinoderms, or at least to be 

 widely separated from the Stelleroidea and Echinoidea. Without 

 denying a large measure of truth to such statements, it is here 

 maintained that all these three classes bear the impress of a Pelma- 

 tozoic ancestry. And, though they arose from the Pelmatozoan 

 stem, probably at different periods, and possibly from different 

 branches thereof, yet the trend of the evolution of each was in the 

 same direction a direction opposed to that of the Pelmatozoa. 



At any rate the plain facts set down in the above definition 

 are conveniently connoted and emphasised by the adoption of the 

 term Eleutherozoa, whatever be the precise systematic value attached 

 to it. In discussing the Eleutherozoan classes, the order followed 

 is from the more primitive to the more specialised. 



