140 CLASS IV. 



effected by means of these muscular fibres, and in no degree by 

 forcing fluid into and out of them. To how great an extent these 

 parts can be elongated may be judged by the length of the spines 

 of the shell in sea-urchins : they must have the power of extending 

 beyond the longest spines. In specimens that have been kept in 

 spirits these tubules are found to be short and mutually entangled. 

 The teeth, and the curious apparatus that encloses them (ARISTOTLE'S 

 Lantern) have a number of proper muscles. As to the general 

 muscular system, that which moves the whole body, it is found 

 especially developed in Holotliurice. Here ten muscles run the whole 

 length of the body, beneath the skin: they are arranged in pairs, 

 which are separated by spaces in which transverse circular fibres are 

 found that cover the entire inner surface of the skin. By means of 

 the longitudinal muscles the body can be shortened and bent : by 

 the transverse fibres it is contracted transversely, and so attenuated 

 and elongated. The longitudinal muscles are inserted into a cal- 

 careous ring composed of five large and five smaller pieces, and 

 surrounding the commencement of the intestinal canal. 



All these animals inhabit the sea. They are found in all parts 

 of the world : yet, most of the species only in limited regions. Since 

 we have only a partial acquaintance with foreign species, many 

 more of which will doubtless be hereafter discovered, we are not in 

 a condition to give a survey of the geographical distribution of the 

 genera. Unlimited confidence is not to be reposed in the accounts 

 given by some writers of the localities in which the species are 

 native. Of the Asterice nearly one-third of the known species are 

 found in the East-Indian seas, one-fifth in the seas of Europe. Of 

 Opliiurce, on the other hand, the European and African species 

 are more numerous than the East-Indian. The western hemisphere 

 has, on the whole, fewer star-fishes than the eastern : in America 

 there are no species of Scytaster, as, for instance, of Culcita, Astro- 

 gonium, Stellaster ; Echinaster, on the other hand, is peculiarly 

 American. The species of Euryale are chiefly found in the Arctic 

 and in the Indian seas. To the species which are very generally 

 distributed belongs that which occurs on the coast of Holland, 

 Asterias (Asteracanthion) rubens. The Red sea so rich in species of 

 the class of Polyps, has only a comparatively small number of star- 

 fishes, and, in this respect is much behind the Mediterranean, which 

 is especially rich in species of Astropecten. The Baltic appears to be 



