NATURAL HISTORY OF AMERICAN LOBSTER. 
2 4 I 
THE MUSCLES. 
The muscles of the lobster’s body are of two kinds, the striped or striated and the non- 
striated, distinguished in higher animals as the voluntary and involuntary muscles. The 
involuntary muscular tissue is inconsiderable in quantity, excepting the “fine meat” at 
the tips of the claws, being mainly confined to the walls of the alimentary canal, the 
blood vessels, and sexual organs. The heart and powerful skeletal muscles are composed 
of distinctly striated fibers. 
The skeletal muscles, of which the large adductor of the mandibles is a good example, 
are attached to the hard shell on the one hand, and to tendinous ingrowths of the softer 
cuticle on the other. Just how the union with the shell is effected is a somewhat vexed 
question. In the first larval stage of the lobster the prominent muscle just referred 
to is distinctly striated up to the basement membrane. (Fig. 2 , pi. xlvi, bm.) At this 
level its fibrillse are directly continuous with attaching fibers within the cells of the epi- 
dermis; the basement membrane is accordingly penetrated at this point. Examination 
of earlier embryonic stages shows essentially the same conditions. The epidermis of 
the shell in the area of attachment ( fb . ep.) is modified in a characteristic manner; its 
cells are columnar and elongated, and their cytoplasm develops fibers which appear to 
fuse with those of the muscle-fibrillae ; moreover, their nuclei are eventually reduced 
and spindle-shaped, though this was not the case in the specimen figured. The base- 
ment membrane in this region is a distinct cuticular sheet, to which blood cells and 
other elements (ms.) presumably of mesoblastic origin also attach themselves, with long 
axes parallel with the surface, thus making a distinct lamella. The horizontally placed 
lamellar cells can be detected beneath the modified epiblast, where the cuticular portion 
of the membrane appears to be reduced or absent. In some cases the epiblastic fibrils 
brush out perceptibly at their periphery against a concavo-convex layer of chitin, upon 
which the outermost stratum of the shell is molded. Since the clearer inner chitinous 
layer frequently peels off in preparations, it may represent a renewal of the shell at this 
point previous to molting. 
In his study of regenerating limbs in the lobster, Emmel (97) has found that the 
striated muscles are regenerated from ectoderm, and that the outer ends of the myo- 
fibrillseare differentiated as tensile elements, which pass between the proper epidermic cells, 
are frequently spread out in branches, and are fused directly to the chitin of the shell. 
The muscles of the tail, which form a great part of the edible flesh of the lobster (pi. 
xxxiii) consist of two paired masses, the dorsal extensors, by the contraction of which 
the abdomen is straightened, and a much larger pair of ventral muscles, mainly flexor in 
function, which form the principal source of power for locomotion. As we have seen, the 
segments of the shell in this region are united by flexible membrane, and move over artic- 
ular surfaces as well as upon double hinges of the typical ball-and-socket form, and that 
the parallel and horizontal arrangement of their articular axes limits the flexion of the 
tail to the vertical plane. The ventral muscles are very complex, being composed of 
external bundles attached to the side walls of successive segments, and of interlooping or 
enveloping strands, which are fixed to the lower or sternal parts of the skeleton. A 
48299° — Bull. 29 — 11 16 
