ECHINODERMATA 87 



The valves are opened and shut by means of muscles, 

 and in most cases they are hinged, having teeth and 

 sockets near the beak. The mouth faces the middle of 

 the margin opposite the 

 beak ; and on either side of 

 it is a long fringed " arm," 

 generally coiled up, and 

 supported by a calcareous 

 framework. The animal, 



FIG. 43. A Brachiopod 

 (Terebratulina septen- 

 trionalis} . Atlantic coast. 



FIG. 44. Dorsal Valve of a Brachiopod 

 (Terebratula) , showing, in descending 

 order, cardinal process, dental sockets, 

 hinge plate, septum, and loop supporting 

 the ciliated arms. 



having no gills, respires by the arms and the mantle. 

 Brachiopods were once very abundant, over two thou- 

 sand extinct species having been described; but only 

 about a hundred species are now living. 13 These are all 

 marine, and fixed. The animals in this group are 

 related to the mollusca. 



Branch VIII. ECHINODERMATA 



The echinoderms, as starfishes and sea urchins, are 

 characterized by the possession of a distinct nervous 

 system (a ring around the mouth with radiating branches); 

 an alimentary canal, completely shut off from the body 

 cavity, having both oral and anal apertures ; a water- 

 vascular system of circular and radiating canals, con- 

 nected with the outside water by means of the madre- 

 poric tubercle, and a symmetrical arrangement of all the 



