156 THE INVOL UNTAR Y NER VO US S YSTEM 



which has been called branchiomeric and mesomeric, and by me 

 splanchnic and somatic, so as to include the musculature of the 

 non-branchial arches as well as the branchial. 



A three-root system is found in the segmental nerves of 

 Limulus and the scorpion, and one of the roots contains conspi- 

 cuously both sensory and motor fibres and supplies entirely the 

 appendage of each segment. Here also a double segmentation 

 is recognized, due to body segments and appendage segments ; 

 the body segments form a somatic segmentation corresponding to 

 that of the vertebrate, and the appendage segments would cor- 

 respond to the splanchnic segmentation of the vertebrate if the 

 masticatory and branchial muscles were derived from the muscles 

 of the masticatory and branchial appendage of the invertebrate. 

 The study of the lowest vertebrate, Ammoccetes, shows how natural 

 it is to look upon the branchial unit as a buried branchial ap- 

 pendage, and leads to the conclusion that the branchial region 

 of the new-formed alimentary canal was formed as a branchial 

 chamber by a number of fused branchial appendages, the 

 muscles of which have thus become the striated muscles supplied 

 by the facial, vagus and glossopharyngeal nerves. At first the 

 respiratory chamber so formed extended close up to a similar 

 chamber, formed possibly by appendages, at the anal end of 

 the body, and opened into that chamber ; so that originally, as in 

 the arthropods, the double segmentation due to appendages and 

 body segments existed throughout the whole length of the ani- 

 mal. When the animal became a vertebrate with a smooth body 

 surface and changed its locomotion from that of a crawling animal 

 to that of a swimming animal, new segments were formed, by 

 which greater mobility was gained ; in these new-formed segments 

 the appendicular segmentation would not be represented, and the 

 new-formed gut would simply lengthen and its muscles would be 

 supplied from those already formed. 



At the time when the vertebrate first appeared, neither 

 arthropods nor arachnids like those of the present day had been 

 evolved. We must therefore regard the ancestor of the verte- 

 brate as being much nearer the annelid stage, and can presume 

 that his muscles were not all striated as in present day arthro- 

 pods, but some only had reached this stage, while others were 

 unstriped. A living example of a very primitive arthropod is 

 Peripatus, and here, as pointed out by Balfour, the muscles of 



