86 K. ZIMMERMANN. 



in size of the abdomen, accompanied by a broadeninjf of the carapace 

 and strengthening of the ambulatory thoracic appendages. The re- 

 duction of the abdomen is well marked in Porcellana plafycheles, which, 

 as a general rule, does not swim, but moves from place to place in a crab- 

 like sidling fashion. The abdomen, which is habitually carried tucked-up 

 beneath the thorax, is relatively very weak, and narrowly triangular in 

 outline, and the tail-fan, though still present, is reduced in size. The 

 abdomen is rarely unfolded except under special circumstances, as when 

 the animal is upset on to its back. In this case, the abdomen is flapped 

 vigorously, so as to raise the animal in the water and alter the inclina- 

 tion of the body, allowing it to fall back into the natural position, while 

 the chelse are extended to catch at any means of support to which it 

 may be possible to cling. A similar action of the abdomen has been 

 observed in Cancer and Carcinus individuals, when subjected to similar 

 conditions, but in these two Brachyuran genera the absence of the tail- 

 fan renders the flapping less effective ; this deficiency, however, is not 

 of much consequence, as their superiority in weight and strength gives 

 Cancer and Carcinus a greater stability than is possessed by Porcellana. 



Decapoda which frequent the shore have the carapace much broader 

 and flatter than in Macrura, and this has led to the sharp lateral folding 

 of the carapace, giving protection to the branchial organs ; this broaden- 

 ing is also of importance in connection with the above-mentioned habit 

 of sideway progression, as we generally find the long axis of any body 

 is turned in the direction of habitual movement. This method of advance 

 in another than the forward direction is peculiarly suited to animals 

 endowed with the Decapod Crustacean's type of sense-organs and fre- 

 quenting obstructed places, and also to those whose walking limbs are 

 spread in the almost radial fashion characteristic of the Porcellanids. 

 The efficient clinging mechanism so afforded is based on the principle 

 which determines the radial spreading of tent-pegs with their ropes, and 

 of the several hooks of a grapnel, and has doubtless been developed in 

 connection with the habit (already marked in Galathea, and most pro- 

 nounced in Porcellana j^lcf'tycheles) of clinging to the under-surfaces of 

 stones, and thus resisting the force of wave-wash as well as that of enemies. 

 The radial arrangement of the limbs also endows the animal with facility 

 of movement in an oblique, as well as in the directly transverse, direc- 

 tion — a valuable asset to a dweller in the intertidal zone of shifting 

 pebbles. 



In connection with the strengthening of the thoracic limbs of Porcel- 

 lana flatyclieles, we note in this species the greatest concentration of 



