NOTES ON MEDIAN AND PAIllED FINS OF FISH. 341 



that it has migrated backwards from the head region. Now, 

 there is no evidence of a more anterior position of the pelvics 

 in primitive fishes generall}^, either living or extinct. Indeed, 

 the only known fish in which the pelvics are far forward 

 (some Teleostei) are acknowledged to be specialised in this 

 respect. The presence in ontogeny of rudimentary muscle- 

 buds in front of the pelvic fins, is supposed to indicate 

 backward migration. 'J'his is negatived by the fact that 

 similar rudimentary buds are found behind the pelvic fin 

 (Braus 4, PL 22, and in this paper, figs. 1, 4, 25). The fins 

 could not have migrated both ways at once, and there is 

 no reason to believe that they first migrated backwards to a 

 point behind the cloaca, and then forwards towards the head. 



Davidoff (8), Gegenbaur (15), and others have held that 

 the presence in front of the pelvic fin of a collector nerve, 

 composed of branches of a number of spinal nerves, and the 

 greater extent of this plexus in the young than in the adult 

 (Punnett 29, 30), indicates backward migration. But, again, 

 both a similar plexus and extension are found on the pos- 

 terior side of the fin. 



The question of the nerve supply of the fins will be dis- 

 cussed in greater detail later (p. 363) ; but in describing the 

 general nerve-plexus at the base of the fins one must be 

 careful to distinguish between the collector nerve formed 

 by the convergence and combination of branches of a series 

 of spinal uerves and the plexus proper, due to intertwining 

 secondary branches, made up chiefly, if not entirely, of 

 sensory nerve-fibres. The formation of a collector nerve 

 is simply and easily explained as the result of concentra- 

 tion. The mere presence of a connecting plexus (mainly 

 longitudinal) is due neither to concentration nor to migra- 

 tion (p. 367). 



Moreover, both these arguments in support of the theory 

 of migration are sufficiently answered by the fact that 

 rudimentary buds are found both in front of and behind 

 the median fins (Mayer 22 and p. 353 below), and that a longi- 

 tudinal nerve-plexus may extend along their base even when 



