VIII 



AKTHEOPODA 



215 



ming appendages. In subsequent moults a varying number, but in 

 no case do all the remaining segments of the thorax develop swimming 

 appendages. Thus in Crangon only one extra segment develops 

 forked appendages (Fig. 162), in Cheraphilus two segments develop 



FIG. 162. " Mysis " larva of Crangon Allmanni, lateral view. (After Sars.) 



Ul, telson ; erp 1 " 4 , the four exopodites borne by the three niaxillipedes and great chela respectively ; 

 pi, pleopods (abdominal appendages) ; ur, uropods (last abdominal appendage) ; th+ s , the appendages 

 which will form the five pairs of walking legs. 



appendages with exopodites, in Pontophilus two, and in Sabinea one 

 only. Those segments that do not develop swimming appendages 

 give rise to simple, unforked appendages, which at the last moult 

 develop directly into the hinder walking 

 legs of the adult, as in the case of the 

 Loricata, and we may add, as in the 

 case of the Mysis larva which develops 

 out of the Zoaea larva of Thalassinidae, 

 another family of the Caridea. 



Finally, in the Brachyura all the 

 thoracic segments behind the first two 

 which bear the swimming appendages 

 of the Zoaea, develop only rudimentary 

 bud-like appendages whilst the larval 

 swimming stage persists, but when this 

 is over and the larva begins life at the 

 bottom, then these appendages develop 

 directly into the walking legs without 

 ever passing through a forked stage. 



Thus, in the life-history of the crabs 

 the Mysis stage has been completely 

 eliminated, but nevertheless the Zoaea 

 does not change into the adult stage 

 but into a form called a Megalopa, in 

 which the carapace is longer than broad, and in which all the seg- 

 ments of the abdomen possess well-developed swimming appendages. 

 This larva obviously represents a Macruran stage in the ancestry of 



FIG. 163. "Megalopa" larva of the 

 Crab Pilummis, dorsal view. 

 (After Cano.) 



Letters as in preceding figures. 



