1013 



where the skull is one segment shorter (Hoffmann, 1894, p. 638, 

 Sewertzoff, 1899, p. 302) than in Acanthias, only two. Thus in the 

 latter forms the skull contains, besides the prostomium, seven segments 

 (equalling the number of visceral archs ^), of which one is post- 

 branchial, in Aoantkias eight, of which two are post-branchial, in 

 Hexanchus and Heptanchus probably still a few more (v. infra). 

 Mesomerism and branchiomerism again correspond (Van Wijhe, 1882, 

 ZiEGLER, 1908, 1915). As in Amphibians the two anterior post- 

 otic somites no longer develop myotomes, but the second somite 

 still forms a rudimentary one. From the remaining occipital somites, 

 however, myotomes are still developed. 



From these the epibranchial musculature is formed (Dohrn, 1885, 

 p. 446, Hoffmann, 1898, p. 265), which in all other Gnathostomes, 

 and also in rays already, is absent. It still reaches its strongest 

 development in the primitive Hexanchus and Heptanchus (Fürbringer, 

 1897, p. 416). The hypobranchial musculature (Musculi coraco- 

 arcuales) originates from the ventral buds of the last epibranchial 

 and the first four post-branchial myotomes, being the 4"^'' — 8''' post- 

 otic somite, according to Neal (1897, p. 450), and only of the latter four, 

 all post-branchial, according to Hoffmann (1898, p. 263). It is supplied 

 by the ventral roots of these latter four myotomes, which partly lie 

 within the range of the skull, partly behind it, forming together 

 the plexus cervicalis. The gill-slits here too dispersing afterwards in 

 a backward direction, this plexus cervicalis again comes to run in a 

 curve round behind the last gill-slit; by the strong elongation of 



pl.cerv. 



pl.brach 



Fig. 2. Plexus cervico-brachialis oi Heptanchus, after Fürbringek. (1897). 



1. primarily epibranchial ventral roots. 



2. plexus cervicalis (hypoglossus). 3. plexus brachialis. 



1) Accordingly two less than the well-known number given by van Wijhe (1882), 

 who considered the praemandibular cavity as the first somite and counted the 

 hyoid segment as two. 



