TORSION OF THE CRUSTACEAN LIMB. 



FRANCIS H. HERRICK. 



The large double claws or chelae, which are so conspicuous in 

 many of the higher Crustacea, have not failed to attract the at- 

 tention of naturalists, both on account of their remarkable re- 

 generative phenomena as well as for the aid which they afford to 

 systematic zoology. It is therefore the more surprising that 

 attention has never been called to a striking instance of torsion 

 in the great chelipeds of two of the best known forms the 

 crayfish and the lobster. 



If we examine the appendages of either animal from above 

 (Figs, i and 2) it will be seen that while in the great claws the 

 dactyles face and open inwards and therefore in a nearly hori- 

 zontal plane, all the smaller chelae open upward and outward in 

 a plane which is nearly vertical. In the lobster, however, at the 

 time of birth (Fig. 4) the three pairs of chelate legs, great and 

 small, all have the same form and position, that is, the claws, - 

 which are laterally compressed, all open vertically with an incli- 

 nation outward. It therefore follows that the position of the 

 great ' forceps ' has been reversed by rotation through 90, in 

 consequence of which their inner or anterior faces have become 

 their under sides (Fig 3). The metamorphosis has been reduced 

 to such a degree in the crayfishes that the young are hatched 

 with the essential characteristics of the adult form. If therefore 

 a torsion really occurs in the limbs of these animals, it must be 

 sought in the egg. 



In the true crabs, or such representative forms as Callinectes 

 and Carcinas, the claws open outward, as in the larval lobster, 

 and the outer or posterior face, corresponding to the upper side 

 of the chela of an adult crayfish, tends to become the under 

 side, and is deficient in pigment. 



In the crayfish again, about one quarter of the weight of the 

 animal is represented by the great chelipeds, while their pro- 

 portional weight in the lobster is one half. The acquisition of 



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