638 THE INVERTEBRATA 



number, shaped like bunches of grapes and varying in size with the 

 season. They are attached to the body wall by their ducts, which open 

 one on each side at the base of each arm. 



Asterias (Figs. 433, 436, 440, 441, 443). A typical member of the 

 class. Its principal features have been mentioned above. British. 



Astropecten. Without anus; without suckers on the tube feet; and 

 with conspicuous marginal ossicles. Lives on a bottom of hard sand, 

 into which it burrows, and upon which its tube feet are adapted to 

 walk. British. 



Asterina. With the arms short and wide, so that the body is 

 pentagonal ; and without pedicellariae. Has a shortened development, 

 with a larva which is not a Bipinnaria. British: between tidemarks. 



Brisinga. With numerous, long, slender arms, sharply distinct 

 from the disc, which is small. A deep-sea genus. 



Class OPHIUROIDEA 



Star-shaped Echinodermata ; whose arms are sharply marked off from 

 the disc and do not contain caeca of the alimentary canal; with 

 madreporite on the oral side; ambulacral groove covered; tube feet 

 without suckers; and no pedicellariae. 



The special features of the organization of a brittle star are con- 

 nected with the fact that the animal moves, not by means of its tube 

 feet, but by pushing and pulling upon surrounding objects with its 

 arms. In adaptation to this the arms are sharply distinct from and 

 freely movable upon the disc, on the underside of which they are 

 inserted. They are armoured by skeletal plates (Figs. 444, 445), in an 

 upper, two lateral, and an under series. The epidermis is vestigial; 

 there is a strong cuticle ; spines on the side plates give grip ; and the 

 under plates, covering in the ambulacral groove, which is thus con- 

 verted into an epineural canal, protect the nerve cord during the 

 movements of the arm. The ambulacral ossicles of each pair fuse to 

 form one of a series of vertebrae, which articulate by an arrangement 

 of knobs and sockets and can be moved upon one another in various 

 directions by four muscles. The large vertebrae reduce the peri- 

 visceral cavity in the arm to a canal, in which there is no room for 

 caeca of the alimentary canal. The nerve cord bears ganglia corre- 

 sponding to the muscles between the vertebrae and formed by increase 

 of the coelomic (deep oral) component of the cord. The perihaemal 

 vessel is shallow and not divided by a septum. The tube feet have no 

 suckers and no ampullae and are often provided with warts of sense 

 cells. 



The alimentary canal (Fig. 446) is a mere bag, not protrusible 

 through the mouth, which is armed with an arrangement of spines 



