266 



BUIvLETlN OP THE BUREAU OP FISHERIES. 



DEVELOPMENT OF THE GREAT FORCEPS. 



How has the differentiation of the great claws been brought about? It is easy to 

 follow the history of their development molt by molt from the first larval stage onward. 

 This history clearly shows that the toothed claw represents an original or an older 

 type, and that the crusher claw was later developed by a modification of this primitive 

 pattern. 



In the first larval stage of the lobster the future big claw (fig. 14) is distinctly of the 

 embryonic type, relatively short and thick, and armed with few tactile bristles, its tips 



being drawn out, as it were, into 

 long sharp-pointed spines. The 

 dactyl, which bears the longer 

 and straighter spine, is larger than 

 the undeveloped index. This in- 

 equality is much more marked in 

 the smaller chelipeds, where the 

 index appears as a bud-like out- 

 growth, setate and bearing one or 

 more stiff, barbed, or serrated 

 bristles (fig. 2). 



In the second and third larvae 

 (fig. 41 and 42) the claws become 

 broader and more voluminous, 

 while their spinous tips are re- 

 duced and both index and dactyl 

 are curved. 



In the fourth stage (fig. 9 and 

 pi. xxxi) the great chelipeds sud- 

 denly become very conspicuous, 

 bearing long slender forceps which 

 now for the first time serve as 

 organs of prehension with marked 

 success. The jaws of the forceps 

 are slender, dentate, and tufted 

 with tactile hairs. The condition 

 of symmetry, with this general structure, on right and left sides, continues through the 

 fifth and in some cases up to the seventh or eighth stage, when the first traces of asym- 

 metry begin to appear, though not necessarily apparent to the naked eye. (Fig. 15 and *" 

 16.) By the ninth stage, when a total length of about one and one-quarter inches has 

 been reached, the differentiation of the crusher claw is easily recognizable, but the 

 changes registered at each molt are slight. In the account which follows we shall con- 

 sider in more detail the beginnings of asymmetry and the development of the teeth 

 and tubercles which characterize the two types of big claw in the adult animal. 



Fig. 14. — Left great claw-foot of first larva of the lobster, from above, show- 

 ing outwardly opening dactyl, before any torsion of the limb has occtirred, 

 the short ischium (3), with free joint at future breaking plane (i), and 

 base of swimming branch (Ex). Compare with text figures 6 and 9. 



