PHYLUM ECHINODERMA 179 



anastomoses on the alimentary canal. The nervous system is 

 of the type previously described for other echinoderms, but the 

 quick responses given by the holothurians seem to indicate that 

 the nervous and sensory organs are more highly developed than 

 in other members of the phylum. 



The digestive canal is held in definite position by mesenteries. 

 The esophagus passes into a stomach which is followed by a tubu- 

 lar intestine. The main course of this tube is, in most species, 

 posterior in the median dorsal interradius, then anterior in the 

 left ventral interradius, and finally posterior in the right dorsal 

 interradius to the cloaca. From the walls of the cloaca, there 

 are usually a pair of minutely branched respiratory trees which, 

 by the muscular action of the cloaca, are filled with water and 

 serve as respiratory organs. 



A genital pore occurs in the anterior region of the body. 

 This is the opening of the single, much-branched gonad. In 

 development of the embryo, a larval form known as an auricularia 

 is produced. Sea cucumbers have marked powers of regenera- 

 tion. Individuals may automatically eject much of the internal 

 organs and yet be able to regenerate them. 



SUBPHYLUM PELMATOZOA 



The Pelmatozoa are echinoderms which, during the whole or 

 at least the early stages of their existence, are fixed by a jointed, 

 flexible stalk or are attached by the dorsal or aboral surface of 

 the body. The principle organs are enclosed in a cup-shaped or 

 spherical test, called the calyx, the walls of which contain a 

 system of calcareous plates and ambulacral or food grooves 

 leading to the mouth. The crinoids are the only living examples 

 of this group, which also includes the extinct Cystoidea and 

 Blastoidea. 



Class Crinoidea 



The crinoids, or sea lilies, are usually provided with a long stalk 

 or column at one end of which is attached the calyx with its 

 movable arms. In numerous forms, lateral projections called 

 cirri are borne along the stalk. In those instances where the stalk 

 is lacking, the cirri are frequently attached directly to the base of 

 the calyx. Occasionally, there are free-swimming crinoids, but in 

 their development these pass through a fixed stage (Fig. 88), thus 

 giving evidence that the free condition is not primitive in mem- 



