Hchinoderma — Sea-urchins^ Starfishes, etc. 89 



objects, and so hold itself firm or pull itself along. Other podia GALLEET 

 end merely in a point, and serve only to aerate the contained fluid. _. ^^^^.• 



The subkingdom Echinoderma is divided into seven classes, ^ 

 five of which have representatives in modern seas. A very 15-18, 

 slight examination of a sea-cucumber (Holothurian), a sea-urchin ^*^^®- 

 (Echinoid), a starfish (Asteroid), a brittle-star (Ophiuroid), and 73-78. 

 a feather-star (Crinoid), will show important differences between 

 them. In an ordinary Holothurian (e.g. Solothuria, Cucumaria) 

 the body is cucumber-shaped, with a mouth at one end and an 

 anus at the other; between the mouth and anus run the five 

 ambulacra, of which one or two are often more developed than 

 the others, so that the animal crawls along on that side of its 

 body, with its moulh foremost. A holothurian has no arms or 

 projecting rays, but its mouth is surrounded by a circlet of 

 tentacles, often branched, which can be retracted at will. A 

 regular sea-urchin (e.g. Echinus, Cidaris) resembles a holothurian 

 in being without projecting rays ; but it is more spherical in shape, 

 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), which moves through and swallows mud and sand, the 

 body has become transversely elongate — that is, the long axis is at 

 right angles 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 a 

 starfish, as in a regular sea-urchin, the mouth is in the centre 

 of the under surface, while the anus is almost ia the centre of 

 the upper surface, but is absent in a few forms ; the body is 

 either markedly pentagonal in outline, or it is more or less star- 

 shaped, in which case it is said to consist of a central **disc" 

 extended into " arms." The number of these arms varies from 

 five (e.g. Asterias ruhens) to over forty (e.g. Heliaster). A 

 brittle-star resembles a starfish in which there is a sharp dis- 

 tinction between arms and disc ; the mouth is on the under surface, 

 but there is no anus. Moreover, whereas the " arms " of a starfish 

 are merely extensions of the body, containing the generative glands 

 and processes from the stomach; the arms of a brittle-star are 

 appendages to the body, with a stout internal skeleton of separate 

 ossicles, worked on one another by well-developed muscles, and 

 they contain only blood-vessels, water- vessels, and nerves. The 



