382 



Bashford Dean lAemorial Volume 



THE AXIAL MUSCLES 



In fishes the axial muscles comprise (a) the great masses of muscle contributing to the 

 formation of the body wall and tail; (b) a group of muscles in the hypobranchial region; 

 and (c) the muscles that move the eyeballs. 



Muscles of the Trunk and Tail. — Metamerism is such a striking feature of the 

 trunk muscles of fishes that it overshadows the longitudinal division into muscle bundles 

 or layers and the incipient diiferentiation into indi'V'idual muscles — a development that, 

 in the higher vertebrates, quite reverses the picture. 



In surface views of the six large embryos 

 of Chlamydoselachus in the American Museum, 

 ranging from 190 mm. to 374 mm. in length, the 

 myomeres are more or less sharply defined. 

 Along the lateral surfaces of the trunk and tail 

 they are clearly outHned, and in some specimens 

 they may be traced ventrally as far as the tropeic 

 folds. Dorsally, they are usually obscure and in 

 this situation better views were obtained by 

 removing patches of skin from one of these em' 

 bryonic specimens. In the adult specimens, only 

 slight indications of the body musculature could 

 be seen until after the skin had been reflected; 

 then the myosepta stood out boldly. It is ap' 

 parent, even from a cursory study of our mate- 

 rial, that the myomeres of the trunk region of 

 Chlaynydoselachus conform to the primitive 

 elasmobranch type and bear a close resemblance 

 to those of Heptarichus as described and figured 

 by Maurer (1912) and Daniel (1934). From 

 Daniel (1934, p. 89) I quote the following para- 

 graph which is illustrated by my Text-figure 53 : 



In a side view, the muscles of the body oi Heptanchus maculatus axe divided at the lateral 

 line (n.) into dorsal bundles (d.h.) which attach to the cranium, and ventrolateral bundles 

 which attach to the pectoral girdle. Both the dorsal and the ventrolateral muscles extend 

 to the tip of the tail. In these bundles the myosepta (ms.) are bent into rigsag shape. Above 

 the lateral Hne one of the columns has the apices of its myosepta directed forward, the other 

 backward. Below the Hne there appears to be a single column with apex pointed posteriorly. 

 Some of the anterior fibers of the ventral bundle are specialized as the pectoral muscles of 

 the pectoral fin. 



Howell (1933, p. 249) attaches considerable significance, from a developmental 

 point of view, to the longitudinal division of the trunk musculature of fishes into dorsal 

 and ventrolateral bundles. His account of the developmental processes leading to this 

 condition follows: 



-'Y 



Text-figure 53. 



Lateral view of the body musculature in the 



pectoral region of Heptanchus maculatus. 



cl., gill-clcft; d.b., dorsal bundle; d.f., dermal fin rays; 



d.r.m., dorsal radial muscles of pectoral fin; I.b., lateral 



bundle; 11., lateral Une; ms., myoseptum; tr., trapezius 



muscle; v.h., ventral median muscle. 



After Daniel, 1934, Fig. 90. 



