1176 



ZOOLOGICAL SOCIETY BULLKTIN 



// 



Life History of the Hlue 

 Cral), Report of tlic Bureau 

 of Fisheries. 190K p. II.) 

 When tlie process of east- 

 ing oft' the skin aetually be- 

 gins, a crack ap])ears in the 

 shell around the sides ami 

 back, so that the up})er part 

 of the sliell or earajiaee can 

 be lifted u|) like a lid. Then 

 the crab begins to back out 

 of the old shell, freeing first 

 the hinder legs and then 

 the others forward succes- 

 sively until only the large 

 pincers remain. On .-iceount 

 of their size the withdrawal 

 of these members is a much 

 more serious process and 

 could scarcely take place at 



all but for the fact that cer- ii,^. |.||.„p p;,,,.,.,.., 



tain portions of the hard 

 sliell of the n.irrow con- 

 stricted bases of the legs break away or become 

 softened for the purpose. The whole process 

 lasts ordinarily only a iew minutes. 



The crab grows very rapidly by absorption 

 of water during this ])rocess and for a short 

 time afterward, so that after shedding it will 

 measure nearly one fourth l.irger. .\s the crab 

 is defenceless and comjiaratively hel]iless in the 

 soft state, it attempts to hide away during the 

 ])rocess of shedding and remains secluded until 

 tlie new shell has h.irdened. which takes only 

 two or three days. 



As soft shelled crabs bring several times as 

 much on the market as the hard shells, the fish- 

 ermen are keen to observe the signs of "shed- 

 ders" as those which are about to moult are 

 called. Sueli crabs are |)ut aside and held in 

 Jiounds until they shed and can be marketed as 

 soft shells. 



This crab illustrates in a remarkable manner 

 by its fifth or hinder pair of limbs how an 

 organ may be modified in adaptation to different 

 nses. If we go back far enough in the racial 

 history of the Crustacea we find forms in which 

 all eiglit pairs of the thoracic legs are adapted 

 for swimming, as. for example, in the Mi/sis 

 shrimp. Later in the evolution of the crabs, 

 the three anterior pairs of thoracic legs became 

 changed into mouth parts while the hinder five 

 j)airs were all fitted for walking. The major- 

 ity of the crabs have remained in this condition 

 and except in the larval stages are unable to 

 swim. Certain members of the group, however, 

 and the blue crab among them, have undergone 



RKAin KOIi A KKMir 



a still further change, in which the fifth or 

 hinder ])air of walking legs became formed like 

 a i)air of oars. With this new arrangement 

 the blue crab is able to swim in a somewhat 

 cumbersome fashion so that it is no longer com- 

 pelled to si)end its life at the bottom, even 

 though it chooses to do so most of the time. 

 Some crabs with this endowment have even 

 gone back to the surface of the sea to live, as 

 for example, the gulf-weed crabs which live 

 among the Sarf/assiim floating ,it the surface 

 over the wide oceans. 



In walking or running the blue crab uses 

 only the three middle ))airs of thoracic legs, for 

 the oar-like hinder pair is held u)) out of the 

 way and the big ))incers, which constitute the 

 first i)air, are also raised. But let no one sup- 

 pose that this reduction in the number of limb^ 

 used in walking interfers with sjieed, for the 

 blue crab can make better time on land than 

 most crabs which use four or all five ])airs of 

 thoracic legs for this purpose. To be sure he 

 is not as agile as the ghost crab {Oci/pnda) of 

 our southern shores which can give the average 

 man a very good chase for a short distance, 

 but, according to my own observations, he can 

 make about a yard a second, which is one-tenth 

 as fast as the fleetest sprinter. 



Of coure this crab can w.ilk forward or back- 

 ward, but when he is in a hurry he always goes 

 sidewise, and he has the big advantage that he 

 can go to the right or left with equal raitidity. 

 His legs have a longer stride in a sidewise di- 

 rection and probably there is less danger of 



