The Anatomy of Chlamydoselachus 



387 



the tropeic groove. The deep muscle that Garman calls the rectus abdominis or keel 

 muscle is called by Braus simply the keel muscle. Braus (1898, p. 337) states that the 

 nerves that innervate the keel muscle lie on its lateral surface, and not on the medial 

 surface as in the case of the musculus rectus abdominis and the obHque muscles of the 

 body wall. He concludes, therefore, that an invagination, leading to inversion, of the 

 ventral body wall has occurred at the mid-line; for it is well known that nerves ending in 

 developing muscles tend to follow these muscles in their migrations. Braus has embodied 

 these conclusions regarding the phylogenetic origin of this muscle in a diagram which 

 I have reproduced as Text-figure 59. 



O.inf. 



Text-figure 60. Text-figure 61. 



Transverse sections of the ventral body wall of Chlamydoselachus showing the inrolling of the 



musculature in the region of the tropeic folds. 

 Text-figure 60. Transverse section of the ventral abdominal wall immediately behind the 



pectoral girdle. 



la., linea alba; P., peritoneum; o.inf., musculus obliquus inferior; R.p., rectus profundus muscle, which is recogniable 



as an inroUed portion of the ordinary musculature of the body wall. 



After Maurer, 1912, Text-fig. 1. 



Text-figure 61. Diagrams showing the condition of the ventral musculature on one side of the 



body in four different regions: A, just behind the pectoral and likewise immediately in front 



of the pelvic girdle; B, in the second quarter, and C, in the third quarter of the trunk. 



R.fi., musculus rectus profundus; a, first; and h, second fold of the rectus profundus. 

 After Maurer, 1912, Text-fig. 3. 



Maurer (1912) has given a somewhat different picture (Text-figures 60 and 61) of 

 the manner of origin of the deeply situated ventral longitudinal muscle, which he calls 

 the rectus profundus. These figures are based on sections taken from four different 

 regions along the ventral body wall of his adult, or nearly adult, specimen. A connection 

 between the rectus profundus and the ventrolateral bundle persists in the region im- 

 mediately behind the pectoral girdle and immediately in front of the pelvic girdle, but 

 is lost throughout the remaining extent of the tropeic folds. A curious feature of all 

 Maurer's drawings of the ventral musculature of his specimens is that in none of them 

 does he show any ventral protrusion of the body wall to form the keel which has been 

 described by Garman (1885.2), Collett (1897), Braus (1898), and by Gudger and Smith 

 (1933). But the most remarkable thing about Maurer's drawings of the musculature of 

 the tropeic folds is that he represents the infolding process not as a simple invagination 

 but as a parting of the musculature of the body wall along the mid-line, after which each 

 edge becomes inroUed independently, like a scroll (Text-figures 55, 60 and 61). This 



