CHAPTER XIV 



PHYLUM ECHINODERMATA 



THE class of animals known as the Echinodermata comprises 

 the well-known Star-fish or Five-fingers, the equally well-known 

 Sea-urchins, the less familiar Sea-cucumbers and Brittle-stars, 

 lastly the graceful Feather-stars. The name is derived from two 

 Greek words, extvos, which means hedgehog (and was also used for 

 the sea-urchin), and Sep/xa, the skin. The prickles and spines with 

 which many members of this Phylum are covered constitute a 

 very prominent feature in their appearance. Spines, it is true, are 

 sometimes absent, but in every case, whether this is so or not, the 

 skin contains a skeleton consisting either of plates or of rods, and 

 the spines are merely rods belonging to this skeleton which project 

 outwards and are still covered by the skin which they push before 

 them. 



Class L ASTEROIDEA. 



The most familiar of all the British Echinoderms is probably 

 the common star-fish, Asterias rubens, which may be found at low 

 water on almost any part of the coast where shell-fish, its favourite 

 food, abound. Very similar species, Asterias vulgaris and Asterias 

 polaris, abound on the American coast, the first-named on the New 

 England coast, the second further north in the Gulf of St Lawrence. 

 , The species represented in Fig. 150 belongs to a different genus, 

 Echinaster, but in all essential features of its anatomy it agrees 

 with Asterias. Echinaster sentus is common on the N. American 

 coast. The name " star-fish " denotes that the shape resembles the 

 conventional figure of a star. The body is produced into five arms 

 or lobes which are arranged like the spokes of a wheel round the 

 centre of the body or disc, on the under side of which the mouth is 

 situated. These arms are termed radii, and the re-entrant angles 

 between them interradii. 



