THE GEAPSOID CRABS OP AMERICA. 



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Subfamily Sesarminae Dana. 



Scsarnwcca Milne Edwards, Ann. Scl, Nat., 

 ser. 3, Zool., vol. 20, 1853, p. ISl [147]. 



Cyclofjrapsacca Milne Edwards, Ann. 8cl. 

 Kat., ser. 3, Zool., vol. 20, 1S53, p. 191 



[157], piirt. 

 Sesarminae Dana, U. S. Expl. Exped., vol. 13, 

 Crust., pt. 1, 1802, p. 333. 



Front strongly deflexed. The lower bor- 

 der of the orbit commonly runs downward 

 toward the angle of the buccal cavern. 

 Side walls of carapace finely reticulated 

 with granules and hairs or hairs only. 

 External maxillipeds separated by a wide 

 rhomboidal gape; an oblique hairy crest 

 traverses them from a point near the an- 

 tero-external angle of the ischium to a 

 point near the antero-internal angle of the 

 merus; the palp articulates either at the 

 summit, or near the antero-external angle, 

 of the merus, and the exognath is slender 

 and either partly or almost entirely con- 

 cealed. The male abdomen either fills or 

 does not quite fill all the space between the 

 last pair of legs. 



Fritz Miiller^ explains the function of 

 the reticulation of the side walls and the 

 hairy ridges on the maxillipeds in these 

 crabs, which spend much of the time out of 

 the water: 



The small wart-like elevations and ge- 

 niculated hairs form a fine net or hair-sieve 

 extended over the lower surface of the 

 carapace. Thus when a wave of water 

 escapes from the branchial cavity 

 (through the orifices at the anterior angles 

 of the buccal frame) it immediately be- 

 comes diffused in this network of hairs 

 and is then conveyed back to the branchial 

 cavity by vigorous movements of the ap- 

 pendage of the outer maxilliped, which 

 works in the fissure entering the cavity 

 from above the last pair of legs. While 

 the water glides in this way over the cara- 

 pace in the form of a thin film, it 



> Facts and Arsiiinents for Darwin. Translation by 

 W. S. Dallas, Loudon, ISCO, pp. 32-33. 



