646 ECHINODERMATA 



Class 5. HOLOTHURIOIDEA.* 



Sea cucumbers. Holothurians. Ecbinoderms in which the main body 

 axis is very long (Fig. 965, D) so that the form is often quite worm-like. 

 The oral surface is not directed towards the ground, as in most ecbino- 

 derms, but the animal rests on the side of the cylindrical body, with the body 

 axis parallel with the ground, the oral surface forming the anterior and 

 the aboral surface with the anus the posterior end. This arrangement may 

 be sometimes disturbed, however, and in Rhopalodina lageniformis of the 

 west African coast the mouth and anus are side by side on the upper 

 surface. The side on which the holothurian rests and which is thus its 

 ventral side is often flattened and consists of three radii and two inter- 

 radii, the remaining two radii and three interradii forming the dorsal 

 surface. 



The calcareous plates are mostly minute and the body wall thus 

 lacks the rigidity of other echinoderms. The external surface is not cili- 

 ated and is without spines or pedicellariae. The ambulacral appendages 

 exist in a variety of forms: around the mouth are ten or more long, 

 branched, oral tentacles; ambulacral feet may occur on all sides of the 

 body, or only on the ventral side of it, and ambulacral tentacles without 

 suckers (papillae) may occur on the dorsal side. The oral tentacles are 

 alone present in several groups. The ambulacral feet and papillae may 

 occur in rows in the radii or may be scattered irregularly over both radii 

 and interradii. 



Internal Structure.— The alimentary canal is a long tube which runs 

 from the mouth at the forward end of the body to the anus at the hinder 

 end, generally turning on itself twice. The hinder portion is enlarged and 

 from it in several families long tubular sacs extend into the body cavity. 

 These are of two kinds, which are called the respiratory tree and Cuvier's 

 organs, respectively. The former are a pair of extensively branched 

 organs in and out of which water is pumped through the anus; they are 

 consequently respiratory in function. The latter are unbranched, glan- 

 dular tubes connected with the terminal portion of the respiratory tree 

 and are found only in certain species. They are exceedingly extensile 

 and sticky and can be thrust out through the anus and be used to entangle 

 an enemy, having thus a defensive function. The body wall is well pro- 

 vided with muscles, powerful longitudinal muscles running along the 

 radii and transverse muscles being in the interradii. 



The ambulacral system consists of a ring canal, well within the body 

 cavity around the oesophagus with one or more Polian vesicles, five radial 



• "Holothurians of the Challenger," by H. Th^el, 188G. "Holothurians," In 

 Bronns Tierreich, by H. Ludwig, 1889-92. "Holothurioidea," by H. L. Clark, 1901, 

 Amer. Nat, Vol. 35, pp. 479-496. "The Apodous Holothurians," by H. L. Clark, 1908. 



