CONNECTOR NEURONS OF THE NERVOUS SYSTEM 131 



Drepanaspis, etc., and opened into the anus by way of a posterior 

 or cloacal chamber, formed in the same manner as the anterior 

 or branchial chamber. 



Such a conception implies that in the original vertebrate the 

 wall of the alimentary canal was segmented along its whole 

 length. The conversion of a crawling animal into a swimming 

 animal necessitated a greater mobility obtained by elongation 

 of the animal. This was brought about by an increase in the 

 number of segments between the head end and the tail end ; such 

 segments belonged to the somatic segmentation only because, the 

 animal being now a vertebrate, no new appendages of the inverte- 

 brate type could be formed, and consequently no new appendic- 

 ular or splanchnic segments. Elongation of the gut must how- 

 ever take place with the elongation of the body and be supplied 

 with muscles. Such muscles must arise from those of segments 

 already existing, and consequently the splanchnic segmentation 

 of the gut would still be confined to the original branchial and 

 cloacal segments, its musculature being supplied by the bulbar 

 and sacral nerves. 



Now the evidence which I have given in previous chapters of 

 this book shows that the involuntary muscular system (leaving 

 out for the moment the vascular muscles) also falls into two dis- 

 tinct systems ; the one epidermal is supplied by motor fibres 

 from motor cells of the sympathetic system, and the other en- 

 dodermal by motor fibres from motor cells of the bulbo-sacral 

 or enteral system. All the muscles supplied by motor fibres 

 from the sympathetic belong to the former group if, as I believe 

 likely to be the case, the sphincter muscles of the gut turn out to 

 be epidermal, and are differentiated by the action of adrenalin; 

 all the muscles supplied by motor fibres from the enteral system 

 belong to the endodermal group and are differentiated by the 

 action of acetyl-choline. The unstriped muscles under the skin, 

 which form the dermal group, must belong to the somatic 

 segmentation, and the endodermal muscles to the splanchnic 

 musculatures. There is therefore a strong suggestion of a mor- 

 phological differentiation in the involuntary musculature of the 

 same kind as that in the voluntary musculature ; in other words, 

 both striated and unstriated muscle existed in both the somatic 

 and appendicular segments of the invertebrate, from which the 



vertebrate arose ; and therefore the characteristics of the two 



9* 



