14 



gleich insofern nicht vollkommen durchführen, als die Bothriocepha- 

 liden ja schon ursprünglich nur zwei Bothrien besitzen, durch deren 

 Verschmelzung dann natürlich nur ein unpaares und scheitel- 

 ständiges Saugorgan entstehen kann, wie es für die Gattungen 

 Bothrimonus und Cyathocephalus characteristisch ist, während wir 

 andererseits bei denTetrarhynchen trotz der erwähnten Verschmelzung 

 immer noch paarige und seitliche, sei es marginale sei es laterale, 

 Bothridien finden, als natürliche Folge der ursprünglichen Vierzahl. 



4. On the Formation of the Pelvic Plexus, with especial Reference to 

 the Nervus Collector, in the Genus Mustelus. 



By E. C. Punnett, B.A., Cambridge. 



(Paper read before the Royal Society, London, November 16, 1899.) 



(Abstract.) 



eingeg. 24. November 1899. 



The main object of this investigation was to ascertain whether at 

 any period in the development of the animal selected, the number of 

 branches composing the nervus collector was greater than that found 

 in the adult. As a logical consequence of Gegenbaur's theory we 

 should expect such to be the case, and the ontogenetic history of the 

 nervus collector recorded in this paper, its maximum development in 

 young embryos, and its subsequent gradual decrease through the 

 later stages of embryonic existence leading to its condition in the adult, 

 must, if there is any truth in the recapitulation theory, all point to its 

 primitive character. 



The history of the posterior collector, the very existence of which 

 has not hitherto been described, throws important light upon the 

 theory mentioned above. Here we have a collector formed in the em- 

 bryo, from which in later stages the component nerves separate and 

 run singly into the fin. Such a fact points very strongly to the collec- 

 tor condition being more primitive than that condition in which the 

 nerves reach it without previously effecting any junction with one 

 another. 



It is further shown that the formation of this collector is due to 

 migration of the whole fin rostrally, an not merely to a contraction of 

 the fin area, and in support of this the following evidence is brought 

 forward. The two species, M. laevis and M. vulgaris, differ from one 

 another chiefly in the more rostral position of the pelvic girdle in the 

 former. That it is highly improbable such a condition should be due 



