, NATURAL HISTORY OF AMERICAN LOBSTER. ' 24I 



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 

 Ifevel its fibrillae 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-fibrillse ; 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- 

 fibrillaeare 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 



