298 



ECIIIXODERMATA. 



rows from the oral to the anal pole, but may be principally or 

 altogether confined to the three rays of the so-called trivium. In 

 this latter case the Holothurid moves upon a more or less foot-like 

 ventral surface. The ainbulacral feet may also be distributed uni- 

 formly over the surface of the body, especially on the ventral side. 

 As a rule, the tube-feet have a cylindrical shape, and terminate with 

 a suctorial disc : in other cases they are conical, and the suctorial 

 disc is absent. The tentacles, which are in communication with the 

 water-vascular system, and represent specially modified ainbulacral 

 appendages, are simple or pennate, or even dendritic (Dendrochirota) 

 or shield-shaped (Aspidochirota), that is, provided with a disc, which 



is often divided into many parts. 

 In certain genera (Synapta), the 

 ambulacra! feet are altogether 

 wanting, and the tentacles re- 

 main as the sole appendages of 

 the ambulacral system (fig. 240). 

 Locomotion is effected by the 

 strongly developed dermal mus- 

 cular system, the longitudinal 

 fibres of which are attached to. 

 the calcareous ring surrounding 

 the oesophagus. It is charac- 

 teristic of the water- vascular 

 system that the stone canal, 

 which is usually simple, hangs 

 freely in the body cavity, ending 

 in a calcareous framework com- 

 parable to the madreporic plate. 

 The respiratory trees at the end 

 of the intestine perform the func- 

 tion of respiration, while certain 

 glandular appendages (organs of Cuvier), which open into the rectum, 

 may be regarded as excretory organs: these, as well as the respiratory 

 trees, may be wanting. The generative organs consist of a bundle of 

 branched tubes, the duct of which opens on the dorsal surface in the 

 region of the mouth. The genus Synapta is hermaphrodite. The 

 development is in many HolotJiurians direct (as e.g. in Ilolothuria 

 tremula according to Koren and Danielssen) ; where there is a com- 

 plicated metamorphosis, the larvse have the Auricularia form, and 

 pass through a barrel-shaped pupa stage. 



FIG. 240,Synapta inJiasrens (after Quatre- 

 fages). 0, Mouth ; A, amis : the intestine 

 can be seen through the skin. 



