CONTRIBUTIONS TO MARINE BIONOiMICS. 399 



Similar observations and experiments were made upon numerous 

 specimens of Atelecyclus heterodon, a crab belonging to an altogether 

 different family. In this crab the antero-lateral margins are provided 

 with as many as nine teeth, but the function of the teeth was found 

 to be essentially similar. Owing to the different form of the body, and 

 the different shape of the cheliped in the two crabs, the orifice of the 

 channel between cheliped and carapace is of greater relative extent in 

 Atelecyclus than in Bathynectcs ; but the length of the denticulated 

 margin of the carapace was found to correspond precisely with the 

 extent of the inhalant gap in each case. The following conclusions 

 may be drawn, therefore, from these observations : — 



(1) Antero-lateral denticulations of the carapace in crabs may 



subserve a sieve-like function. 



(2) The extent of the denticulated area corresponds with the extent 



of the inhalant gap between the carapace and the cheliped 

 when the latter appendage is approximated to it in the 

 flexed position. 



It is also obvious that a new function must be ascribed to the 

 chelipeds of sand - burrowing crabs provided with antero-lateral 

 denticulations of the carapace. In such cases the chelipeds act as 

 organs temporarily subservient to the respiratory process by providing 

 a broad operculum to the exostegal channel. Attention may be 

 recalled in this connection to the fact elucidated by Milne-Edwards in 

 1839, that iu the LeucosiidEe the floor of the afferent branchial channel 

 is also provided b}' one of the appendages, in this case by the external 

 maxillipeds. The relations of the afferent channel in the Leucosiidie to 

 the external channel which I have now described iu the Cyclometopa 

 are discussed by me in the paper to which reference has already been 

 made (1897). 



The subservience of the chelipeds to the respiratory process enables 

 me, moreover, to explain the function of a remarkable spine which 

 in the Portuuid;e is almost universally present on the inner margin of 

 the distal extremity of the carpal joint (carpopodite or wrist) of the 

 cheliped. This carpal spine, though usually strong and conspicuous, 

 presents various minor modifications of form which are employed by 

 systematists in the discrimination of different species. 



The appearance of the spine in Bathynectcs longipes is represented by 

 Bell and liisso. When the cheliped is fully extended the carpal 

 spine projects freely from its anterior margin; but when the propodite 

 is flexed towards the proximal part of the cheliped, it is arrested at a 

 certain angle with the carpopodite by the carpal spine in question. If 

 now the arm (meropodite) of the cheliped be approximated to the carapace 

 in the position requisite for the completion of the exostegal canal, it 



