AXNULOIDA ECHINODERMATA. 85 



Water, or in some moist place, in long chains. They 



have a mouth and alimentary canal, but no anus. The 

 sexes are distinct. 



CLASS 2. Echinodermata. 



121. The class Echinodermata (Gr. echinos, hedgehog ; 

 derma, skin) includes the sea-urchins, star-fishes, sand- 

 stars, brittle-stars, sea-cucumbers, and the almost extinct 

 encrinites or sea-lilies, the remains of which are abun- 

 dantly met with in the older rocks. 



In the mature state, the members of this class are 

 characterized by a radiate arrangement of the various 

 parts, although in the embryonic condition they have 

 bilateral symmetry. The covering is composed of numer- 

 ous calcareous plates, joined together at the edges ; or of a 

 leathery integument, in which there are calcareous grains 

 or spines. There is a water-vascular system, connected 

 with a series of tube feet, termed the ambulacral (Lat. 

 ambulacrum, a walking place) system. In all cases the 

 alimentary canal is completely shut off from the cavity of 

 the body, and is usually furnished with an anal opening. 

 A ganglionated ring of nervous matter surrounds the 

 gullet, from which branches radiate to the different parts 

 of the body. 



122. Skeleton. The form of these animals varies much 

 in the different groups. The sea-urchins are sometimes 

 almost globular, the star-fishes are usually stars of five 

 rays, the sea-cucumbers (holothuria) are worm-like. The 

 covering of the sea-urchins consists of a shell or test 

 which is composed of a great number of five or six-sided 

 calcareous plates, joined together at the edges. Of these 

 plates there are ten double rows or zones. In five of these 

 zones the plates are small, and are perforated, so as to 

 allow the emission of the tube-feet. These are called the 



