STAR- FISH, SEA-URCHINS, ETC. 



655 



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

 fish ; Ophiuroidea, brittle-stars ; Echinoidea, sea-urchins ; Crinoidoa, 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 (see Fig. 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 

 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 net 

 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 

 place to place during 

 adult life ; many of 

 them, however, have 

 rather larger powers of 

 locomotion in the lar- 

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

 able distances by ocean- currents. 



Fig. 2. DISSECTION or COMMON CRCS-FISH (Asterias rubens 

 Two-thirds natural size. 



