MONOGRAPH OF THE EXISTING CRINOIDS. 



131 



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

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

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

 beinc^ 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 



„!,. rnM.TIILA ETHERIDGEI FKOM TUE .\RU ISL.VNDS, SHOWING THE INNER OR INTERNAL 



right angles to this axis; roughly it may be said to be best f ^^'^P^iJ ^"^1;^^;^;^^- 

 rion of the longer axis of the cephalothorax in any given type The number of 

 fX developed metameres vvithin the cephalothorax is always five. Asj-mmet > 

 ofthe antrio ambulatorv appendages or of the abdomen or of both is the rule 

 !r.Trh; In the echinodorms the entire body (except for the appendages 

 ZZ^^l eid^ird ibin a typically heavily calcified and closely knit test 



