562 THE INVERTEBRATA 



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 



Fig. 419. A, Ophiura. Oral surface of disc and part of the arms. B, Ophio- 

 glypha. Aboral surface. From Woods, b. buccal plates ; d. upper (" dorsal ") 

 plates of arms ; g. genital slits ; /. lateral plates of arms ; r. radial plates ; 

 V. under ("ventral") plates of arms. 



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

 freely movable upon the disc, on the under side of which they are 

 inserted. They are armoured by skeletal plates (Figs. 419, 420), 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 rhe 

 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 7ierve cord bears ganglia corre- 



