1898.] POST-LARVAL STAGES OF THE COMMON CRAB. 209 



are not represented. In Corystes also the tail extends forward 

 onlv to the sternum of the 3rd pereiopods, while in Cancer and 

 Atelecydus it extends to the sternum belonging to the chelipeds. 



I think it will be agreed that the evidence I have detailed is 

 abundantly sufficient to prove that Atelecydus has no claim to a 

 position in the family Corystidae and that ir,s proper position is iu 

 the Cancridse next to Cancer. The resemblances of the adults are 

 enough to establish this proposition, while at the same time it is 

 confirmed by the greater resemblances between the young Cancer 

 and the adult Atelecydus. The peculiarities of the 1st crab-form 

 of Cancer thus indicate that the latter in its evolution has diverged 

 from an ancestral form closely siiuilar to Atelecydus, and that iu 

 the adult condition of Cancer several features which Atelecydus 

 retains throughout life have been considerably modified. 



Mr. Walter Garstang (Journ. Mar. Biol. Assoc, vol. iv. no. 3) 

 has recently described the respiratory adaptations in Corystes 

 cassivelaunus, in which the antennse form a tvibe conveying an 

 anterior afferent current of water to the branchial cavities, and 

 remarks that a similar reversal of the respiratory current occurs 

 in the allied form Atelecydus heterodon. Now, in accordance with 

 this remark, 1 find that there is a certain degree of similaritj'' 

 between the arrangement of the antennae and parts surroundiug 

 the anterior apertures of the respiratory cavities in the two forons. 

 The antennae in Atelecydus are not more than one-third the length of 

 those of Corystes, but they are provided each with a dorsal and 

 ventral fringe of hairs which by their appositiou would form a tube 

 as in Corystes. The anterior edges of the external maxillipeds (2nd 

 segment) are also fringed with long hairs which form a ventral 

 floor to the water-channel as in Corystes. But the second joint 

 of the peduncle of the antennae is not flexed on the first in 

 Atelecydus as it is in Corystes. The first joint of the antenna is 

 fixed in Atelecydus while it is movable in Corystes, and in the 

 former there is a thick fringe of long hairs, extending across the 

 base of the second joint or segment of the external maxillipeds and 

 along the ventral surface of the carapace, which is entirely wanting 

 in Corystes. It is evident therefore that the differences, even in 

 the parts here considered, between the two forms are greater than 

 the resemblances, and all that can be said is that there is a slight 

 adaptive similarity in the two cases. In other words, we find iu 

 Atelecydus a slight development of a structural adaptation for 

 respiratory purposes, which is much more complete in Corystes. 

 The condition of the parts in question in Atelecydus is not such 

 as to indicate any close affinity between the two genera. 



DESCRIPTION OF PLATE XXI. 



Fig. 1. Early crab-form of Cancer pagurus, the Edible Crab. Actual size of 

 specimen 25 mm. across carapace. Drawn with Zeiss oc. 3, obj. a^, 

 without camera lucida. 

 2. Transition stage of Cancer pagurus ; from a specimen 4 mm. across 

 carapace. Drawn under the same conditions as fig. 1. 



Piioc. ZooL. See— 1898, No. XIV. 14 



