Emhryology of certain of the Loiver Fishes. 195 



which are primitively in close proximity. The motor nerve 

 trunks, in fact, are merely the much^ enlarged portions of 

 an originally diffuse nervous network which connect up 

 two once adjoining regions of the original epithelial wall — 

 continuous from ectoderm through protostoma to endoderm 

 — which have become specialised to form central nervous and 

 muscular systems. 



External Gills. 



A set of organs which have proved to be of much interest 

 in the development of those forms with which I have con- 

 cerned myself are the external or dermal gills. 



The importance of these structures has been, in my 

 opinion, by no means properly appreciated by morphologists. 

 This is not surprising when we consider that among the more 

 accessible vertebrates they occur in only one group — that of 

 the Amphibia. The researches of recent years have shown 

 that they are also highly conspicuous features in the develop- 

 ment of both existing Crossopterygians and of two out of 

 the three surviving Dipnoans ; and a study of their develop- 

 ment in the three groups in which they occur leads me to 

 believe that they are by no means to be lightly dismissed as 

 mere recent adaptations to a larval existence, evolved in- 

 dependently in the three groups, but that, on the contrary, we 

 have in them truly homologous structures, and consequently 

 structures of great antiquity in the evolution of the verte- 

 brate phylum. 



Typically, an external gill forms a conspicuous projection — 

 carrying lateral branches, often arranged in a pinnate manner 

 — from the side of the body in the region of the visceral arches. 

 The lateral branches have a rich development of superficial 

 capillaries, whose blood supply is carried by a conspicuous 

 afferent and an efferent vessel running along the interior of 

 the organ. The external gill is provided with voluntary 

 muscles. By means of these muscles the external gills can 

 be twitched actively at the will of the animal, so as to renew 

 the water which bathes their surface. 



Each external gill originally forms a projection from a 

 visceral arch, in the broad sense, i.e., from the mass of tissue 



