298 



ECHINODERMATA. 



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 ambulacral 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 ambulacral 

 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 (Synaptct), the 

 ambulacral 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 Xt/itapta is hermaphrodite. The 

 development is in many Holothurians direct (as e.g. in Holothuria 

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

 plicated metamorphosis, the larva? have the Auricular ia form, and 

 pass through a barrel-shaped pupa stage. 



FIB. 240. Syiiiijitu inhcerens (after Quatre- 

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

 can be seen through the skin. 



