CONTKIBUTIONS TO MARINE BIONOMICS. 397 



of conspicuous serrations on these margins of the carapace is func- 

 tionally related to the exigencies of respiration when these animals are 

 buried in sand. 



The marginal teeth are perhaps best developed and most conspicuous 

 in crabs of the family Portuuidte (Swimming-crabs). As M. Alphonse 

 Milne-Edwards has remarked : " Je ne connais aucun Portunien oii 

 le bord latero-anterieur de la carapace soit entier ou arme d'epines 

 arrondies ou de tubercules obtus." (1860, p. 202.) 



In Bathyncctcs longipcs there are five sharp -pointed teeth on each 

 of the antero-lateral borders. These teeth increase in size regularly 

 from before backwards, and the posterior tooth is a particularly stout 

 and sharp structure. This crab is almost invariably an inhabitant of 

 sandy areas {e.g., Mounts Bay in Cornwall) ; and the individual whose 

 habits I am about to describe was also dredged upon a bottom of fine 

 sand in the neighbourhood of the Eddystoue. 



In an aquarium containing sand the crab burrows into the sand just 

 beneath the surface, leaving its eyes and the transverse slit-like aperture 

 of the buccal frame exposed. The crab is actually imbedded up to the 

 anterior edge of the external maxillipeds ; but it pushes away the sand 

 in front of it by means of these appendages, and when at rest maintains 

 these appendages in a sloping posture, so that they act as a quadrangular 

 sieve-like fence in front of the buccal area. This happens both in very 

 fine siliceous sand and in fine shell sand. The crab was not seen at any 

 time to go completely beneath the surface, though I do not mean to 

 imply by this that the crab never buries itself entirely. This may 

 or may not be the case. Atclecydus heterodon is another sand-burrowing 

 crab, whose habits I have studied for a much longer period ; and this 

 crab has very diversified habits. It may remain partially imbedded at 

 the surface of the sand, with its eyes and a broad funnel formed by the 

 second antennae alone protruding, or it may disappear completely beneath 

 the sand to a depth of several inches. 



When the crab {Bathynectes longiiies) is partially imbedded in the sand 

 as above described, it may be noticed that the chelipeds are flexed and 

 approximated to the under side of the antero-lateral regions of the 

 carapace in an attitude precisely similar to that assumed by Atelecyclus 

 heterodon, or the Oxystome crab Matuta, under the same conditions 

 (1897). The position of the cheliped is such that the marginal teeth 

 of the antero-lateral region of the carapace exactly overhang the slit-like 

 orifice between the distal half of the cheliped (carpopodite and propodite) 

 and the ptcrygostomial fold of the carapace. There is thus produced on 

 each side of the crab, between cheliped and carapace, a channel similar 

 to that which would be produced by the approximation in parallel 

 planes of two fiat plates. This channel cummuuicatu.s below with the 



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