216 



INVERTEBRATA 



CHAP. 



crabs, just as clearly as the Mysis larva represents a Schizopod 

 stage in the ancestry of Macrura. The Megalopa is transformed 

 into the aduit by one or two moults. 



The life -history of the Anomura closely resembles that of the 

 Brachyura; in their case also the Mysis stage is omitted, but the 

 third appendage of the thorax, the third maxillipede, becomes func- 

 tional before the critical 

 moult which ends the free- 

 swimming life. The post- 

 larval stage of the Paguridae 

 or hermit crabs, which corre- 

 sponds to the Megalopa stage 

 of Brachyura, is distinguished 

 by the possession of a sym- 

 metrically-developed ab- 

 domen an indication that 

 these asymmetrical forms 

 are descended from ancestors 

 which were bilaterally sym- 

 metrical. It has been found 

 that the abdomen of the 

 young Pagurid becomes 

 asymmetrical before the 

 animal seizes on an empty 

 shell in which to shelter its 

 abdomen. This fact is of 

 extraordinary interest on 

 account of its bearing on the 

 nature of heredity. 



The curious groups of 

 Stomatopoda, which agree 

 with Schizopoda in having 

 only three pairs of appendages 

 modified as jaws, and in 



FIG. 164. -Two views of first post-larval stage h J V mg exopodites On 801116 



of Eupagurus bernhurdi corresponding to the * tne thoracic appendages ; 



Megalopa stage of Brachyura. (After Sars.) but which differ from them, 



A, dorsal view. 13, lateral view, br, branchiae attached and indeed from all Other 



to thoracic limbs seen through the carapace ; W ,th* , the TV/To ! i j.r 



two last pairs of thoracic limbs, reduced in size. ' MalaCOStraca, in having the 



first five pairs of thoracic 



appendages modified into grasping claws, and in having gills 

 developed on the abdominal appendages, present a life-history which 

 affords further confirmation of the laws of larval modification, laws 

 which have made themselves clear in the course of our study. There 

 is some reason to believe that the life-history of some Stomatopoda 

 begins with a Nauplius larva. At any rate Lister (1898) has 

 captured in the open sea a Metanauplius in which the mandible is 

 reduced to its blade, and in which the rudiments of the maxillae 



