META^IORPHOSIS OF CORTSTES CASSIVELAUNUS. 475 



first maxillipede has attained its final lamellar form (fig. 16). 

 The abdomen, however, has changed considerably. It is now 

 kept normally hout up under the body, tlie young- crab having 

 taken definitel}' to a burrowing habit. The first two segments 

 are broad and flattened at the sides, while the remaining 

 segments narrow out posteriorly and bear no lateral spines. 

 All the segments bear setae on their lateral margins. The 

 telson has now an evenly rounded posterior margin. 



The pieopods are no longer swimming organs, having lost 

 all their setee. The first four i^airs remain biramous, and of 

 abont the same size as before, but the fifth pair is reduced to 

 a simple stump. There is still no appendage upon the first 

 abdominal segment, so that apparently in the female this 

 appendage never develops, while in the male it is retarded 

 till at least the second post-larval stage. I have hitherto 

 obtained no later stage than that now under consideration, 

 so that I cannot say at what period the distinctive sexual 

 characters appear. The specimens of the first post-larval 

 stage in my possession show also no difference in the relative 

 size of the chelipeds. 



Conclusion. 



The Corystidas, though placed by Milne Edwards (1834) 

 and by Heller (1863) among the Oxystomata, have by more 

 recent authors, such as Clans andMiers (1886), been assigned 

 to the Cyclometopa. The resemblance between the Corystidae 

 and the true Oxystomata has been shown by Mr. Garstang 

 (1897, etc.) to be largely superficial, and due to adaptive 

 modifications of an essentially different character, though 

 directed to the same ends. He has, in fact, brought forward 

 clear evidence that the Corystidae and the Oxystomata have 

 been independently derived from Cyclometopous ancestors. 



This view is to some extent supported by my observations 

 on the development of Corystes, though the great uniformity 

 in the structure of the zoaea throughout the Brachyura pre- 

 vents any conclusion being drawn from the earlier stages. 



