of the Pelvic Plexus in Acanthias vulgaris. 3 



(ventraler Langsstamm) from which the nerves to the muscles are given 

 off. The dorsal branches behave similarly, but the arrangement is 

 slightly more complex owing to the fact that two such longitudinal 

 nerve stems are found, the " medialer " and the " lateraler Langs- 

 stamm."* The ventral branch of the 40th nerve passes through the 

 more medially situated of the two pelvic foramina before joining the 

 longitudinal stem. The dorsal and ventral branches of nerves 

 31 39 are indistinguishable along the greater part of their course, as 

 they all run together in a long nerve stem the nervus collector 

 until just before the point where the girdle is reached. Here the 

 dorsal branches contained in the collector separate from the ventral 

 branches. The former are continued as a branch which runs into the 

 dorsal " Liingsstamm," whilst the latter pierces the more laterally 

 situated of the two pelvic girdle foramina and enters the ventral 

 " Langsstamm." The specimen described by Braus differs considerably, 

 since (1) the nerves entering the fin are 25 45, (2) the girdle is pierced 

 only by the 36th nerve which apparently corresponds to DavidofFs 

 40th nerve, and (3) the ventral branches of the collector do not pass 

 through a foramen. These two cases alone are sufficient to show that 

 a considerable amount of variation occurs. 



The object of this investigation was to ascertain the range of such 

 variations, and to endeavour to find out whether they afford any 

 support or otherwise to either of the two theories which attempt to 

 explain the varying position of the limbs in different species and in 

 different members of the same species, i.e., the theory involving verte- 

 bral inter- or excalation and the theory of limb-migration. For this 

 purpose a number of individuals of both sexes was examined, and the 

 following points in each specimen accurately determined : 



(a) The serial number of the girdle-piercing nerves. 



(b) The number of nerves forming the collector. 



(c) The number of post-girdle nerves. 



(d) The point of junction of the whole- and half-vertebra (" Ganz- 

 und Halb-Wirbel "). 



For the determination of the serial number of the nerves piercing 

 the girdle the arrangement of the occipital region was in each case 

 examined. This was found to be practically constant, thus affording 

 a fixed point from whence to reckon. The nomenclature adopted for 

 the nerves of this region is shown in fig. l.f 



The determination of the point of junction of half- and whole 



* Cf. loc. cit., Taf. 29, fig. 15, and Taf. 31, fig. 27. 



t This nomenclature has been adopted for convenience, and it does not neces- 

 sarily follow that the nerve here denoted as z is homologous with the nerve 

 similarly designated in other forms by Fiirbringer and others. As Fiirbringer's 

 Memoir (8) is not accessible, I am unable to say at present whether for this 

 species his z and my z are identical. 



R 9. 



