BRACHYURA. 



295 



transition from the swimming to the creeping manner of life, and in 

 the most important points of its segmentation already agrees with 

 the adult. In the condition of the abdomen, the Megalopa is some- 

 what on a level with an adult Anomuran Decapod. The anterior 

 part of the body with the limbs already shows the typical Brachyuran 

 character. The youngest Megalopa stages, however, still in most 

 cases carry on the dorsal shield traces of the former Zoaean spines 

 (Fig. 139 A). The limbs have now attained their final shape; 

 the maxillipedes have lost the locomotory function and are com- 



A 



FIG. 139. Three ontogenetic stages of Carcinus maenas. A, younger, and B, older Megalopa 

 stage. G'Jyouug crab. (A, after SPENCE BATE ; B and C, after BROOK), d, dorsal spine ; 

 ?, rostrum. 



paratively smaller. The ambulatory limbs, on the contrary, have 

 developed ~greatly. The abdomen is still stretched out backwards, 

 and shows the][pleopoda as swimmerets provided with long setae ; 

 these consist ofa basal joint and an oval terminal plate provided 

 with setae (exopodite), while the quite short endopodites furnished 

 with small hooks/[as retinacula, serve to connect the right and left 

 swimmeret, and consequently bring about a simultaneous movement 

 of the two limbs. The forked telson of the Zoaea has changed 

 into a rounded caudal plate. 



