216 WALTER QAESTANG. 



that ill Calappa granulata of the Mediterranean, the antero- 

 lateral margins of the carapace are free from denticulations, 

 while the propodial crests of the chelipeds are conspicuously 

 serrated. 



On the other hand, it will be remembered that the presence 

 of teeth along the antero-lateral margins of the carapace is a 

 conspicuous feature of a great number of the less specialised 

 types of crab (Cyclometopa), as, for example, in Carcinus 

 and the Portunidse in general. 



Why are these marginal teeth so commonly found among 

 other crabs, while they are absent in Calappa granulata? 



I have recently determined (1897) by experiments upon forms 

 such as Atelecyclus and Bathynectes, which possess well- 

 developed marginal teeth, and which adopt sand-burrowing 

 habits, that when these crabs are partially or wholly buried 

 in sand, the chelipeds are approximated to the branchial 

 regions of the carapace, as in Calappa and Ma tut a, but in 

 such a manner that the marginal teeth of the antero-lateral 

 regions of the carapace exactly overhang the elongated slit- 

 like orifice between chelipeds and carapace. Moreover, during 

 life a current of water can be demonstrated incessantly 

 pouring into this orifice between the marginal teeth of the 

 carapace, whence it traverses the accessory channel between 

 chelipeds and carapace, in order to reach the afferent branchial 

 aperture at the base of the chelipeds. 



These types, therefore, possess a pair of functional exostegal 

 canals, which differ from those which I have described in 

 Calappa merely in their greater extent and in their less 

 specialised form. I have, moreover, shown that the marginal 

 teeth which overhang the orifices of these canals act as a 

 '^ coarse sieve or grating " which prevents the accidental in- 

 trusion of foreign bodies, such as grains of sand, into the 

 respiratory canal. 



It seems to me to be accordingly probable that the absence 

 of spines and teeth from the antero-lateral margins of the 

 carapace in Calappa granulata is functionally correlated 

 with the restriction of the exostegal orifice in this form to the 



