CONTRIBUTIONS TO MARINE BIONOMICS. 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 Bathynectes; 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-lke 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 in the Leucosiide the floor of the afferent branchial channel 
is also provided by one of the appendages, in this case by the external 
maxillipeds. The relations of the afferent channel in the Leucosiide to 
the external channel which I have now described in 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 Portunidee 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 Bathynectes longipes is represented by 
Bell and Risso. 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 
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