The Anatomy of Chlamydoselachus 383 



A frequent misconception regarding the development of the musculature is to the 

 effect that the muscles ventral to the lateral Hne are formed by actual growth in that direction 

 of the original, dorsally situated myotomes. Conditions vary in different parts of the body, 

 but in the anterior trunk at least there appears to be a lateroventral muscle mass entirely 

 distinct from the dorsal myotome. Between the two there is a connective tissue septum, 

 and tending further to separate them at early stages of phylogeny are the pronephros and its 

 duct, and the lateral line structures. The lateroventral musculature differentiates by con- 

 densations of mesoderm progressively in a ventral direction, forming a lateral somatopleure, 

 giving rise to the somatic musculature, and a medial splanchnopleure, from which is derived the 

 smooth musculature of the intestinal tract. Whether or not all the striated branchial muscles 

 are also derived from this element is not entirely certain. Between the two plates is a coelomic 

 cavity. In other parts of the body, or in vertebrates that have long since discarded all vestige 

 of a lateral line system, the distinctiveness in origin of the dorsal from the lateroventral 

 musculature tends to become obscured in the embryonic picture. 



Text-figure 54. 



Model of myomere of a selachian 



(Squalus), showing divisions into 



longitudinal muscle bundles. 



DORS.MUSC.DIV., dorsal bundle; LAT. 

 LIHE, lateral line; LAT.M.DIV., lateral 

 bundle; VEHT.M.DIV., ventral bundle. 

 After Howell, 1933, Fig. 3, modified from 

 Langelaan and Daniel. 



VENT. M. DIV.— > 



A model of a single myomere of the trunk region of a selachian is illustrated by 

 Text -figure 54. Regarding the basic segmental features of vertebrate trunk musculature, 

 Howell (1933, pp. 255-256) writes: 



The original plan of vertebrate trunk musculature, well illustrated by cyclostomes, 

 involves a series of segmental muscles each of which is separated from the muscles of adjoining 

 segments by myocommata or myosepta. The axially directed muscle fibers of each segment 

 are basically divided into a dorsal division, above the lateral line on either side of the mid-line, 

 and a continuous lateroventral division below; this constitutes the primary muscular plan. 

 It is a primitive scheme, suited to a low vertebrate that can bend with equal facility in any 

 direction — the essentially vermiform type of control. 



In this plan the myosepta are virtually transverse and usually gently curved. Unlike 

 the situation in mammals, most of whose muscles have one end solidly anchored on bone, in 

 the primitive state the fibers at both ends are attached to yielding connective tissue. Accord- 

 ingly there was originally a tendency for some of the groups of fibers to pull certain parts of 

 the myosepta in a forward and others in a backward direction, as a result of specialized action 

 of the groups concerned. This would have a contortional effect upon the myosepta, and in 

 consequence some parts would have an anterior and others a posterior inclination, as suggested 

 in the given diagram of a myomere of a shark (Fig. 3) [Text-figure 54 herein]. Presumably 

 the swifter the fish (i.e., the stronger the muscle action) the more tortuous the pattern of 

 the myosepta. 



