THE GRAPSOID CRABS OF AMERICA. 11 



Order DECAPODA. 



Suborder REPTANTIA. 



Tribe BRACHYURA. 



Definition. The Brachyura are the short-tailed crabs, in contra- 

 distinction to the Anomura, which include the hermits, the hippas, 

 and others. The Brachyura are characterized by having the cara- 

 pace fused with the epistome at the sides and nearly always in the 

 middle; the last of the thoracic sternal somites fused with the rest, 

 its legs usually like the others ; the basis and ischium of cheliped and 

 legs immovably united; the abdomen brachyurous (small, straight, 

 symmetrical, bent under the thorax, showing no traces of other func- 

 tion than reproduction, and without biramous limbs on the sixth seg- 

 ment) ; by lacking a movable antennal scale ; and by having the third 

 maxillipeds broad. 



Development. Crabs, as a rule, pass through two or more free- 

 swimming stages of development after leaving the egg and before 

 attaining the form of the adult. The first stage is known as the 

 zoea; its carapace is relatively stout and usually spined, the eyes 

 are conspicuous, the thoracic legs are undeveloped, the abdomen is 

 long and slender. The zoea may molt several times, with slight 

 and very gradual changes, but eventually in shedding its skin it 

 suddenly develops into a very different larval stage, the megalops, 

 which has more in common with the adult than does the zoea. In 

 the megalops the long spines of the carapace have disappeared; 

 the eyes are at the ends of movable stalks; the five thoracic feet 

 are developed and similar to those of the adult ; and the maxillipeds 

 are no longer used for locomotion: the telson or abdomen may be 

 partially bent forward under the ventral surface of the body. 1 



Some crabs are exceptions to the above rule of postembryonic larval 

 metamorphoses. Such are the river-crabs of the family Potamon- 



1 For details of the metamorphosis of a crab, see : 



Faxon, W., On some Young Stages in the Development of Hippa, Porcollana, and 

 IMnnixa. Bull. Mus. Comp. Zool. at Harvard College, vol. 5, No. 11, Cambridge, June, 

 1879, pp. 253-208, pis. 1-5. 



Czerniav^ky, Voldemar, Megalopidea s. Larvae Anomuriformes Crustaceorum Bracbyu- 

 rorum. Proc. En torn: Soc. St. Petersburg, vol. 11, 1880, pp. 51-96, pis. 2 and 3. With 

 bibliography and dichotomous table of genera. (In Latin and Russian.) 



Brooks, W. K., Handbook of Invertebrate Zoology. Boston : S. E. Cassino, 1882, Chap. 

 21, pp. 207-223. 



Faxon, Walter, Selections from Embryological Monographs. Compiled by Alexander 

 Agusslz, Walter Faxon, and E. L. Mark. I. Crustacea. By Walter Faxon. Mem. Mus. 

 Comp. Zool. at Harvard College, vol. 9, No. 1, Cambridge, July, 1882, pis. 1-14 with 

 explanations (not paged). 



Ortmann, A. E., Bronn's Klassen und Ordnungen des Thier-Reichs, vol. 5, Abth. 2, 

 Lief. 47-49, Leipzig, 1898, pp. 1078-1105, pis. 110-112. 



Caiman, W. T., The Life of Crustacea. London, 1911, Chap. 4, pp. 66-87. 



