Page:Proceedings of the Royal Society of London Vol 69.djvu/10

2 " On the Composition and Variations of the Pelvic Plexus in Acanthias mdgaris" By E. C. PUNNETT, B.A., Gonville and Cains College, Cambridge. Communicated by H. GrADOW, F.E.S. Eeceived February 16, Eead March 7, 1901.

Whilst recently engaged upon the development of the pelvic plexus in Mustelus kerns I was much struck with the amount of variation in the number and position of the nerves forming it. This led me at the time to examine all the material which I was able to procure, and the results obtained have been given in a previous paper (12, 1900). Believing, however, that, in Bateson's words, " the facts of variation must be the test of phylogenetic possibility " (4, p. 30), I determined to examine a still larger number of specimens of some other species of shark. The only two species in this country which from their abund- ance lend themselves to such an investigation are Scyllium ainimla and Acanthias mdgaris. The former possesses certain advantages in that it would l)e possible to institute comparisons with the nearly allied 8. mtitlus which co-exists with it in the same locality. Such ad- vantages, however, are to some extent discounted by the greater difficulties which Sri/Hiu/it presents in working. Owing to the greater abundance and toughness of the connective tissue it is not nearly such a favourable object for investigation of this kind as Ai-nnthws. This is also the case with the embryos those of Acanthias giving far clearer preparations with the gold chloride method mentioned below than the Scyllium embryos. The presence of a large " nervns col- lector " in Acanthias also gives it an advantage over ScyUiuin where the " collector " is usually represented only by a single branch.

The research was carried on at Plymouth in the summer of 1899 and of 1900, whilst occupying at the Laboratory tables both from the British Association and from Cambridge University. To the autho- rities of the Biological Station I would express my thanks for the ample supply of material forthcoming. I also take this opportunity of expressing my gratitude to Dr. Gadow for the kind sympathy which he has shown during the course of the work.

The anatomy of the pelvic plexus of Acanthias vulgari* has been studied by Davidoff (1) and by Brans (10). Davidoff's account may be briefly summarised as follows :

The pelvic fin is innervated by seventeen nerves whose serial numbers are 3147. Of these nerves 4047 each divide on reaching the fin, into a dorsal and a ventral portion which innervate respectively the fin musculature lying dorsally and ventrally to the fin skeleton. Before supplying the muscles, however, the ventral branches all run into a longitudinal nerve, lying at right angles to their direction