388 



Bashford Dean Ivlemoria] Volume 



does not accord v^-ith the conditions portrayed by other authors in their drawings of 

 transverse sections through the keel muscle. 



As to the function of the deep muscle \^riously called the keel muscle, the rectus 

 abdominis, and the rectus profundus, it clearly aids in a rapid ventral flexion of the body; 

 but why it should be so uniquely set apart from the remaining musculature of the ventral 

 body wall is problematical. 



Text-figure 62. 

 Diagram showing the relation between head somites and body somites, and the origin of the 

 hypobranchial or hypoglossal musculature from trunk myotomes, in a larval Squalus acan- 

 thias. The somites that degenerate in ontogeny are indicated by broken lines. The anlagen 

 of the six eye muscles, which arise from the first three somites, are already differentiated. 



Id, dorsal moiety of the first myotome; Iv, ventral moiety of the first myotome; 2d, 2v, dorsal and ventral moieties 



of the second myotome; 3v, ventral moiety of the third myotome; 7, seventh myotome; a., anterior cavities; 



hyp.m., hypoglossal musculature; M., mouth; ot, otic capsule; sp., spiracle; thr., thyroid. 



After Neal, 1918, F^. 19. 



Goodey (1910.1) studied the relations of the myomeres to neuromeres in the tail 

 and posterior part of the trunk of Chlamydoselachus. In the trunk, he found the limi ts 

 of a myomere corresponding in extent with a monospondylous neuromere. In the main 

 caudal region each myomere is equal in extent wath a diplospondylous neuromere. In 

 the tip of the tail each irregularly divided or heterospondylous neuromere has its myomere. 

 Thus the myomeres of the tail region are not particularly influenced by the secondary 

 segmentation of the vertebral column in this region. 



The Hypobranchial Group. — In fishes, as in other vertebrates, the hypobranchial 

 region has a group of muscles that appear to be a continuation of the longitudinal muscula- 

 ture of the ventral body wall. The muscles of the hypobranchial group are attached 

 posteriorly to the shoulder girdle and anteriorly to ventral portions of the visceral skeleton. 

 This hypobranchial or hypoglossal musculature does in fact arise (Text-figure 62, hyp.m.) 

 as a forward prolongation of some myotomes of the occipital and anterior trunk region 

 which are in strict serial relationship with the myotomes that give rise to the segmental 



