102 THE ANCESTRY OF VERTEBRATES 



466) and HOFFMANN (1898, p. 265) this little group of 

 muscles arises from the occipital somites. They are sup- 

 plied by tiny branches from the cervical plexus, as we 

 may call part of the cervico-brachial plexus which in 

 other groups of Vertebrates is separate from the brachial 

 plexus. 



The cervical plexus in Selachians is composed of a 

 varying number of spinal nerves, from four to twelve, the 

 least numbers being found in the sharks, the highest in 

 the Rajidae. This can be determined by carefully splitting 

 the cervico-brachial plexus into its components. Evidently 

 there is a certain relation between the total number of 

 roots of the cervico-brachial plexus and the depth to which 

 secondarily the series of gill-slits has extended backwards 

 into the trunk region, as indicated by the distance of the 

 shoulder-girdle from the head. The number of occipital 

 nerves in different Elasmobranchs may vary trom 1, or 

 even 0, to 5, the highest number being met with in the 

 most primitive forms (Notidanidae), the lowest in the 

 more differentiated, such as the rays. They are designated by 

 FURBRINGER with the final letters of the alphabet: v. w, 

 X, y, z, of which then the most anterior ones show a ten- 

 dency to disappear. The spinal nerves following behind 

 them and forming together with them the cervico-brachial 

 plexus are numbered: 1, 2, 3 etc., or, as far as in Teleos- 

 teans and Amniotes they are assumed to have been incor- 

 porated into the skull (FiiRBRlNGER's occipito-spinal nerves, 

 the hypoglossus of Amniotes) : a, b, c, etc. 



The circumstance that the occipital nerves, though passing 

 through the cranium, yet in Selachians join the cervico- 

 brachial plexus, which takes a wide curve round behind 

 the last gill-slit to the hypobranchial musculature, indeed 

 seems to plead strongly for the post-branchial origin of 

 the corresponding occipital somites. For if the occipital 

 nerves did ab o/igine belong to the branchial region, we 

 might expect them to 'pass between the gill-slits and not 

 behind them to the ventral side. The same holds good for the 

 ventral muscle-buds from the occipital myotomes which 

 grow out equally round behind the last gill-slit to participate 

 at the formation of the hypobranchial or, casu quo, 

 the tongue musculature, as has been shown e. g. by VAN 

 Wyhe (1882) for Selachians, by VAN Bemmelen (1889) 

 for Reptiles and by FRORIEP (1885) for Mammals. 



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