HOLOTHURIOIDEA 353 



organs. They are entirely lacking in the Synaptids and some 



deep-sea forms. When most developed (the Aspidochirotes) the 



left respiratory tree is spun into a net of fine blood-vessels [rete 



mirabile), which are in direct connection with the blood-vessels 



of the intestinal canal. On the cloacal wall are found in various 



Aspidochirotes the so-called Cuvierian organs, a sort of protective 



organ, worm-shaped bodies which are pushed out through the 



cloacal wall ; they swell in the water and become very sticky, 



entangling animals Avith which they come into contact (the 



" cotton-spinner " of the British seas is a good example of this). 



In most Synaptids there are found on the inside of the body 



wall, or on the mesentery, some peculiar small cup-shaped, 



strongly ciHated organs, the ciliated funnels or urns, which have 



probably some excretory function ; they are of rather diversified 



shape and of no small systematic value. In Lahidoplax BusJci, 



and probably other Synaptids, a pair of large pores are found in 



the posterior end of the body ; they may open and close, and 



probably also have some excretory function. Some Synaptids 



have peculiar '" contractile rosettes ", of a rather complicated 



structure, on the inside of the body wall near the calcareous ring ; 



their function is unknown. 



From the watervascular ring or ring canal, which surrounds 

 the oesophagus behind the calcareous ring, the radial water- 

 vessels issue (Fig. 212), running at first forwards on the inside of 

 the calcareous ring, between the latter and the oesophagus, then 

 backwards, to the posterior end of the body, lying in the body 

 wall, covered by the longitudinal muscles. They give off canals 

 to the tentacles ; in the Synaptids, where the radial water 

 vessels are usually absent or at least rudimentary, the tentacle 

 canals issue from the ring canal. From the tentacle canals large 

 ampullce may issue, hanging free in the body cavity, as do the 

 Polian vesicles, of which there may be one or more. The stone 

 canal, which issues from the watervascular ring in the mid- dorsal 

 line, and is usually fastened to the mesentery, usually opens into 

 the body cavity ; only in some deep-sea forms, a few Synaptids 

 and Pelagothuria, it opens outwards through the skin, which 

 is, no doubt, the more primitive arrangement. The stone canal 

 usually ends in a small swelling, which corresponds to the madre- 

 porite of other echinoderms. Some forms have several stone canals 

 (e.g. Fig. 212). The axial organ has not been found with certainty 

 in the Holothurians. The nerve system, which accompanies the 

 watervascular system, lies wholly inside the epidermis ; the 

 dorsal nervous system is not represented. Sensory organs are 



2a 



