480 



DECAPODA. 



swimming feet. In the course of these changes the larva becomes a true 

 Decapod in the arrangement and number of its appendages ; and indeed it 

 was united with this group before its larval character was made out. In 

 addition to the appearance of new appendages certain changes take place in 

 those already present. The two posterior maxillipeds, in the Palinurus 

 Phyllosoma at any rate, acquire again an exopodite, and together with the 

 biramous ambulatory feet develop epipodites in the form of gill pouches. 



The mode of passage of the Phyllosoma to the adult is not known, but 

 it can easily be seen from the oldest Phyllosoma forms that the dorsal 

 cephalic plate grows over the thorax, and gives rise to the cephalo-thoracic 

 shield of the adult. 



There are slight structural differences, especially in the antennas, between 

 the Phyllosoma of Scyllarus and that of Palinurus, but the chief difference 

 in development is that the first pair of maxillipeds of the Palinurus embryo, 

 though reduced in the embryonic state, does not completely vanish, at any 

 rate till after the free larval state has commenced ; and it is doubtful if 

 it does so even then. The freshly hatched Palinurus Phyllosoma is very 

 considerably more developed than that of Scyllarus. 



Brachyura. All the Brachyura, with the exception of one or 

 more species of land crabs 1 , leave the egg in the Zo?ea condition, 

 and though there are slight variations of structure, yet on the 



FIG. 223. THE APPENDAGES OF A CRAB ZO/EA. 



At. I. first antenna ; At. II. second antenna ; met. mandible (without a palp) ; mx. 

 i. first maxilla ; mx. 2. second maxilla ; mx. 3. third maxilla ; mxp. i. first maxilliped ; 

 mxp. i. second maxilliped. 



ex. exopodite ; en. endopodite. 



whole the Crab Zoaea is a very well marked form. Immediately 

 after leaving the egg (fig. 210) it has a somewhat oval shape 



1 It has been clearly demonstrated that the majority of land-crabs leave the egg in 

 the Zoiva form. 



