MONOGRAPH OF THE EXISTING CRINOIDS. 



131 



derived from the same stock, though along radically different lines. Practically 

 the entire body of the crab is compressed within the enormously enlarged and rigid 

 cephalothorax, which is commonly broader than long. Locomotion, instead of 

 being chiefly or entirely in the direction of the longitudinal axis of the body as in 

 other bilaterally symmetrical animals, is in any direction, but most commonly at 



FIG. VS. DORSAL VIEW OF A SPECIMEN OF COMATITLA ETHERIDGEI FROM THE ARU ISLANDS, SHOWING THE INNER OR INTERNAL 

 (LONGER) ARMS DIFFERENTIATED FROM THE OUTER OR EXTERNAL (SHORTER) ARMS. 



right angles to this axis; roughly it may be said to be best developed in the direc- 

 tion of the longer axis of the cephalothorax in any given type. The number of 

 fully developed metameres within the cephalothorax is always five. Asymmetry 

 of the anterior ambulatory appendages or of the abdomen or of both is the rule 

 among the crabs. In the echinoderms the entire body (except for the appendages 

 in the crinoids) is enclosed within a typically heavily calcified and closely knit test 



