i40 Echinoderma. 



of varied and often beautiful form, ancliors, wheels, etc. ; they 

 are sometimes large, scale-like, and projecting. The anterior region 

 of the digestive tract, is, however, surrounded by a number (usually 

 ten) of large calcareous plates, forming a calcareous ring, from 

 "which various muscles take their origin. 



The tube-feet of some forms are arranged in five radial longi- 

 tudinal rows (Fig. 75, 4), just as in the Sea-urchins; in others they 

 are more irregularly distributed over the whole surface. Sometimes 

 the dorsal tube-feet do not possess suckers, and herein differ from 

 those on the ventral surface ; or they may be wanting on the 

 dorsal side ; or again they may be altogether absent. 



The mouth is surrounded by a circle of tentacles (10-30), 

 which are usually branched (plumose or arboriform). They are 

 hollow, and like the tube-feet, are connected with the water vascular- 

 system ; each is traversed by a large vessel, which arises from a radial 

 canal, or occasionally direct from the ring cana], and is usually 

 provided with an ampulla. Probably the tentacles are to be regarded 

 as specially modified tube-feet. In most, the stone canal (or canals, 

 for there are often several present), is not connected with the 

 body-wall, but opens by means of a perforated madreporite into 

 the body cavity. 



The alimentary canal is a cylindrical tube, which is 

 usually longer than the principal axis,- and forms a large loop. 

 The mouth and anus are situated at opposite poles. 



In most Holothurians two "respiratory trees" open, either 

 separately, or by a short, common stem, into the rectum ; they are 

 large, hollow, arboriform organs by which water is taken into the 

 Tectum and expelled again; their function is respiratory. In some 

 forms "Cuvierian organs" are also attached to the rectum; 

 they are saccular or racemose glandular structures of unknown 

 function. The genitalia are only developed in one inter- 

 radius: the sexual aperture is situated dorsally, usually close to 

 the tentacles. Most Holothurians are of separate sexes, some few 

 are hermaphrodite. 



In the body-cavity of some fornis (Synapta and its allies), especially on 

 the mesenteries, there are small stalked, slipper-shaped bodies, the cavities of 

 which are lined with long cilia: the significance of these ciliate organs is 

 nnknown. 



Many forms feed, like the Spatangidge, by ingesting sand and mud, 

 with the contained organic particles ; others remain with outstretched 

 tentacles and from time to time draw them, one after another, into 

 the mouth in order to obtain the small organisms which have become 

 entangled in their branches. They crawl slowly about by means of 

 their tube-feet ; many bury themselves in the sand. They occur in 

 all seas. 



Many react to a powerful stimulus (a rough touch, or the like) by energetically 

 contracting the body-wall, so that a large portion of the alimentary canal and 



