162 



THE SEA SHOEE 



large part in the process of respiration. Both arms and disc are 

 hardened by a dense scaffolding of calcareous plates ; and not only 

 are the former attached to the latter by a well-formed joint, but the 

 arms themselves are constructed of a number of segments that 

 are held together by a kind of ' tongue and groove ' joint. Bound 

 the mouth are a number of tentacles that are kept in constant 

 motion with the object of carrying the food towards it, and 



FIG. 107. THE COMMON BRITTLE STAB 



of holding the larger morsels while the act of swallowing is pro- 

 gressing. 



The various species of Brittle Stars live among the rocks and 

 weeds, chiefly in deep water, where they move about by means of 

 the muscular contraction of their arms, the disc being raised on the 

 curved arms as the animal proceeds. Some species are to be found 

 between the tide-marks, and especially abundant on the south-west 

 coast are two small species that live among the tufts of coralline 

 weeds, sometimes so crowded together that dozens may be taken 



