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Contributions to Marine Bionomics. 



By 



Walter Garstang, M.A., 



Fellow and Lectuier of Liucoln College, Oxford. 



II. The Function of Antero-lateral Denticulations of the 

 Carapace in Sand-burrowing Crabs. 



The antero-lateral margins of the carapace in many of the crabs of 

 our own and of foreign coasts are beset with a row of teeth or spines, 

 which vary in character and number in different species and genera. 

 In the Oxyrhyncha (Spider-crabs) the whole surface of the carapace 

 is generally studded with spines and stiff hairs of a peculiar character, 

 but there is no general restriction of these processes of the carapace 

 to the antero-lateral margins of the body. These crabs, moreover, do 

 not adopt burrowing habits. Their armature of spines, tubercles, and 

 hairs is employed, as is well known, for protective purposes : in some 

 cases possibly as an actual defence against attack, in others {i.e., 

 Eurynome as2Jera) as a means of protective resemblance to their 

 surroundings; but in the great majority as mere pegs and hooks for 

 the fixation of foreign bodies, such as alga^, hydroids, polyzoa, and 

 ascidians, for purposes of concealment and disguise. 



In the Catometopa (Land-crabs, etc.) the carapace is usually smooth 

 over its whole surface. These animals often burrow in sand, but for 

 the most part their burrows are permanent subterranean tunnels and 

 chambers. 



In the Cyclometopa, however — the group which includes most of our 

 commoner British crabs — the back of the carapace is generally smooth, 

 while the antero-lateral margins are in most forms conspicuously 

 serrated. Most of these animals inhabit sandy or gravelly areas, 

 and show a marked propensity towards burrowing habits. Their 

 burrows are never* permanent channels or tunnels in the sand, but 

 are mere temporary excavations, the sand, mud, or gravel being in 

 actual contact with their bodies when imbedded. 



So far as I am aware no one has hitherto elucidated the remarkable 

 constancy of antero-lateral serrations of the carapace in this group of 

 crabs. I here present evidence which tends to show that the presence 



• i>ci/lla scrrala of the Natal coast appears to bo exceptioual in this resi^ect. (Kkauss, 

 Die i)iUi(i/ricanischc7i Craslacccn, 1843, p. 12.) 



