NATURAL HISTORY OF AMERICAN LOBSTER. 

 THEIR Chief Functions and Modifications in Larva and Adult — Continued. 



225 



Relation, of appendage to type form. 



Relation of adult to embryonic and larval ap- 

 pendage. 



Apertures of body. 



Uniramous, presimiably through loss of exopo- 

 dite. 



In type form, with endopodital spur in male 



In type form 



The same 



The same 



The same, with protopodite undivided, and 

 2-iointed endopodite underlying exopodite. 



Appear as buds in fifth to eighth stage, and sexu- 

 ally differentiated in eighth to tenth stage. 



Appear as bifid buds beneath cuticle of first larva; 

 released as rudimentary limbs in second larva; 

 fully functional at fourth stage. 



The same 



The same 



The same 



Appear as buds at base of telson in second stage; 

 released in third and completely fimctional in 

 fourth. 



Bifid in an embryo of two weeks; later elongated 

 and forked; released in larva as a triangular 

 swimming plate, with terminal fringe of large 

 spines and small setae, which are more distinctly 

 plumose and greatly elongated at the fourth and 

 later stages. 



Anus on lower side at 

 base. 



I and 2, plate xl,i, 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 (/Z. 6) pulls on the large inner tendon and thus closes the 

 claw, while an impulse sent into the extensor (ex, 6) draws on the opposite tendon (i. 6), 

 which springs from the opposite side of the dactyl, and thus opens the claw. Contrac- 

 tion of the flexor of the next segment (fl. 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-Kke outgrowths occasionally 

 follow partial excision or injury of the eye is well known. "I think," says Professor 

 Brooks in his monograph on I^ucifer, "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, pL 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 



