ORIGIN AN D STRLCTURE OF THE HEAD 109 



certain influence on the next following spinal nerves, which 

 causes the ganglia of the latter to atrophy. In such groups 

 where the occipital region of the skull is composed of a 

 restricted number. of segments, as we shall show below to 

 be probable for Amphibians, this sphere of influence may 

 extend beyond the cranio-vertebral limit and cause the an- 

 terior spinal nerves to lose their ganglia and doisal roots, 

 as is the case in Amphibians. In pentanch' Selachians the 

 inlluence of the vagus reaches also a little beyond the 

 cranio-vertebral limit, but in Acanthias. where ihe skull con- 

 tains one segment more than in ScylUum etc., we find in 

 the last segment a well developed spinal ganglion (cf fig. 31) 

 which, hovvever, does not produce a regular dorsal root. 

 In hexanch and heptanch Selachians, however, a spinal 

 ganglion and a dorsal nerve-root is found over the last 

 occipital nerve, fusing with this ventral root in the ^ame 

 way as is the case with the spinal nerves of the trunk. 

 This indicates that these forms probably have one or two 

 segments more incorporated into the skull than Acanthias, 

 the influence cf the vagus no longer reaching as far as 

 the cranio-vertebral limit. 



From these considerations, however, it follows that FuR- 

 BKINGER is wrong in designating the occipital nerves of 

 Hexanchus and Heptanchus as v, w, x, y, z, since, e. g., z 

 is not the same nerve as z in pentanch forms like ScylUum. 

 FiiRBRlNGER himself (I.e. p. 362) designates the last occi- 

 pital myotome in Acanthias as a. Following this nomen- 

 clature I think it probable that the last head segment in 

 Hexanchus and Heptanchus must be called b. or perhaps 

 even c. The nomenclature of the occipital nerves in these 

 forms must be changed accordingly. Already GeGENBAUR 

 (1872, p. 30) had pointed out that the skull in these sharks 

 passes more or less gradually into the vertebral column 

 and that during life an emancipation of vertebral elements 

 from the skull may occur. Thus ihe backward extension 

 of the skull is not constant in different Selachians and the 

 shortest skull seems by no means invariably to be found 

 with the most primitive forms. 



It was only after I had written down the above conclusion 

 resulting from our conception of the hypoglossus-roots that 

 I found to my great satisfaction that VAN Wyhe (1905, 

 p. 320), in a preliminary note on an investigation which 

 until now has not been published in full, had reached 



k 



