225 
NATURAL history OF AMERICAN LOBSTER. 
I and 2, plate XLi, the working of this effective mechanism is readily understood. In the 
sectional view of the big claw and walking leg the tendons of the terminal joint lie in 
the plane of the paper, and the axis of articulation is at right angles to it ; a contraction 
of the large flexor muscle (/?. 6) pulls on the large inner tendon and thus closes the 
claw, while an impulse sent into the extensor {e%. 6) draws on the opposite tendon (t. 6), 
which springs from the opposite side of the dactyl, and thus opens the claw. Contrac- 
tion of the flexor of the next segment (/?. 5) would raise the whole claw toward the 
eye, and so on. In this case, where considerable power is required, there is a double 
or divided tendon for this muscle. Owing largely to the variation in the field of move- 
ment of the successive pereiopods, referred to above, the lobster is able to cover a 
wide front in defense, move forward, sideways, or backward, reach every part about 
the mouth, and scratch the underside of its tail. 
Whether the stalked eyes of decapods are metameric appendages or not is a question 
upon which zoologists are not agreed. In the lobster the eye-stalk (fig. i , pi. xxxv) is 
composed of two segments, the basal of which is minute, and imperfectly calcified, as 
in the protopodite of the swimmeret, and that flagella-like outgrowths occasionally 
follow partial excision or injury of the eye is well known. “I think,” says Professor 
Brooks in his monograph on Lucifer, “that the presence of a distinct ocular segment 
in Squilla compels us to recognize an homology between the stalked eyes and an 
ordinary appendage, although it is no doubt true that all the groups in which stalked 
eyes occur can not be traced back to a common ancestor, and also true that the stalked 
eyes themselves can not be traced back to ordinary appendages.” 
The first antenna (fig. 4, pi. xxxv), as we have seen, is first in the order of embryonic 
development, arising on about the ninth day, just behind the thickenings which form the 
optic diks, and before the mouth invagination is formed. The latter appears a few hours 
later than the antennules, and on a line drawn through their posterior margins, so that 
these appendages are essentially prostomial. The mandibles come next in order, followed 
