96 



THE ELASMOBRANCH FISHES 



MUSCULATURE OF ELASMOBRANCHS IN GENERAL 



In a consideration of the musculature of Elasmobranelis in general we may 

 first notice the primitive segments or somites which in the eml)ryo are ar- 

 ranged in series from the region back of the ear to the tip of the tail. A trans- 

 verse section through the trunk (tig. 97) shows the somite to be made up of 



an outer and an inner layer between which is a 

 central cavity or myocoele (mc). The outer layer 

 produces the dermatome or cuticle plate (dt.) and 

 the inner layer is divided into an upper myotome 

 i^uy.) and a lower sclerotome (scl.). These two 

 layers extend ventrally as the lateral plate and 

 inclose between them the body cavity or coelom 

 (c). For a time in the Elasmobranchs these two 

 cavities are continuous, but later they are sepa- 

 rated by the fusion of the two layers giving a 

 somite dorsally independent of the lateral plate 

 (seep. 298, fig. 257). 



The sclerotome of the somite, as we have seen in 

 Chapter III, produces the elements of the verte- 

 bral column, while the dermatome gives rise to 

 those connective tissue fibers characteristic of the 

 corium and sometimes may also give rise to a 

 part of the muscle tissue. The myotome, however, 

 produces the mass of skeletal muscle. The lateral 

 plates which enclose the coelom are divided into 

 an inner splanchnic layer (spl.) which produces 

 the muscular layer of the digestive tract; and a 

 somatic layer (so.) which thins out ventrally, 

 forms the peritoneal lining of the body cavity, and gives rise to connective 

 tissue cells. 



The cells of the myotome, the myoblasts, elongate and attach themselves 

 both anteriorly and posteriorly to the connective tissue septa (myosepta) sep- 

 arating somites (fig. 98). Such a muscle cell or fiber furthermore becomes 

 differentiated into longitudinal fibrils and is crossed by a series of transverse 

 stripes or bands. In the body of the adult Elasmobranch the fibers generally 

 retain the simi)le plan of attachment to myosepta. But the myosepta in the 

 adult have secondarily changed their course, always, or at least generally, so 

 as to take a zigzag direction. 



The myotome next extends its boundaries dorsally to the middorsal line, and 

 ventrally it grows toward the midventral line. The fibers formed dorsally 

 between the dorsal septum and the lateral line septum compose the dorsal 

 longitudinal bundles (d.b., figs. 100 and 112) which extend from the occipital 



Fig. 97. Transverse section 

 through developing somite, 

 Pristiurus. (FromEabl.) 



c, coelom; chd., notochord; 

 dt., dermatome ; hch., hypo- 

 chorda ; mc, myocoele ; my., 

 myotome or muscle plate; 

 7i.t., neural tube; scl., sclero- 

 tome ; so., somatic layer mes- 

 oderm ; spl., splanchnic layer 

 mesoderm. 



