2 ECHINODERMA GENERAL DESCRIPTION 



kingdom Radiata. The presence of a gut distinct from the body- 

 cavity (coelom) is alone enough to mark the superiority of Echino- 

 derm organisation, as was first insisted on by Leuckart. The 

 resemblance of certain Holothurians to the Gephyrea is but super- 

 ficial and secondary ; the above-mentioned characters form sufficient 

 distinction. 



Examples of the Classes. Within the Echinoderma is great 

 diversity of organisation. Between the worm-like, semi-gelatinous 

 Holothurian, Synapta, living in the mud of the shore, and the 

 stalked Pentacrinus of the depths of the sea, or the brittle-star of 

 the rock-pools, there might well seem an impassable barrier. 

 Taking typical examples of the various classes, let us note the 

 more obvious differences. In an ordinary Holothurian (e.g. Holo- 

 thuria, Ciicumaria, Fig. IV. 4, p. 231) the body is cucumber-shaped, 

 with a mouth at one end and an anus at the other ; round the 

 mouth is a ring-canal of the water-vascular system, and from it 

 are given off' five radial canals, running below the surface of the 

 flexible integument and sending podia to the exterior ; two of the 

 avenues (ambulacra) of podia run along that surface of the body 

 which is away from the ground and may be called " dorsal " ; the 

 "ventral" surface, containing the other three ambulacra, is often 

 flattened to form a kind of walki^ sole. A Holothurian has no 

 arms or projecting rays, but its mouth is surrounded by a circlet 

 of tentacles, often branched, retractile at will, and serving to collect 

 food. A Regular Sea-urchin (e.g. Echinus, Cidaris, Figs. VII., XVII. 

 pp. 290, 303) resembles a Holothurian in being without projecting 

 rays ; but it is more spherical in shape, with a rigid test, and 

 moves with its mouth towards the sea-floor, and with its anus at 

 the opposite pole of the body. In a Heart-urchin (e.g. Spatangus, 

 Fig. XLV. p. 324), which moves through and swallows mud and 

 sand, the body has become obliquely elongate, i.e. with the long axis 

 at an angle of 36 to the position it occupies in a Holothurian ; the 

 mouth has moved a little forward, and the anus has moved down 

 from the top of the body to its lower surface, so that both mouth 

 and anus lie on the under surface, at either end of the long axis. 

 In Echinoids, the radial water-vessels are beneath the test ("hypo- 

 thecal ") and stretch from the oral to the anal pole. In a Starfish 

 the mouth is in the centre of the under surface, while the anus is 

 almost in the centre of the upper surface, but is absent in a few 

 forms ; the body, encased in a yielding theca, is either markedly 

 pentagonal in outline or star-shaped ; in the latter case a central 

 " disc " may be distinguished from the " arms." The number of 

 arms varies from five (e.g. Asterias rubens, Figs. I., IV. pp. 240,242) to 

 over forty (e.g. Heliaster). The radial water-vessels, one to each arm, 

 lie in a groove on the oral surface (" epithecal ") and are fringed 

 by podia, which do not pass on to the aboral surface at all. An 



