STAR-FISH, SEA-URCHINS, ETC. 



655 



The echinoderms now living are divided into five classes — Asteroidea, star- 

 fish ; Ophiuroidea, brittle-stars ; Echinoidea, sea-urchins ; Crinoid^a, sea- 

 lilies, feather-stars ; and Holothuroidea, sea-cucumbers. There are two 

 extinct classes — first, the Cystidea, which are closely allied to the ancestors of 

 all echinoderms ; and second, the Blastoidea, which are perhaps nothing 

 more than an offshoot from the crinoids. In the star-fish and brittle-star 

 the mouth (seeFiw. 6) is in the centre of the under side of the body, directed 

 towards the sea-floor ; while the anus (Fig. 2, As), when it exists, is on the 

 other side and directed upwards. In a regular sea-urchin (Fig. 7) the mouth 

 and anus are in a similar position, but in the irregular sea-urchins (Fig. 8) 

 the body has become somewhat elliptical in shape and the mouth has moved 

 a little forward ; while the anus has moved down from the top of the body 

 to its lower surface, so that both mouth aud anus lie on the under surface at 

 either end of the long axis. In a holothurian (Fig. 9) the body is cucumber- 

 shaped, with the mouth at one end and the anus at the other, and the animal 

 usually crawls along on one side of its body. In the crinoids both mouth 

 and anus are on the upper surface of the body, the mouth usually in the 

 centre (Fig. 15, M) and the anus (Fig. 15, As) a little on one side, and both 

 are directed upwards ; while the opposite or under side of the body is pro- 

 longed into a stem 

 (Figs. 13, 14) by which ....- ^j 



the animal is generally 

 attached to the sea- floor 

 or some other object. 

 Echinoderms cannot 



live on land, since they 



require water to work 



their hydraulic appar- 

 atus, neither can they 



live in fresh water, 



where they would not 



find enough lime-salts 



from which to build 



their skeletons : in the 



sea, however, they have 



a universal distribution. 



Hence their calcareous 



remains have often 



been preserved as 



fossils, and are found 



in the rocks from the 



earliest period in which 



animals are known to 



have existed. Echino- 

 derms move little from 



)>lace to place during 



adult life ; many of 



them, however, have 



rather larger powers of 



locomotion in the lar- . ,, 



val stRge, and the free-swimming larvae are occasionally carried to consider- 

 able distances by ocean- currents. 



Fi'<7. 



>. —Dissection of Common CRoss-Fisn {Aslerias ruhens 

 Two-thirds natural size. 



