SUMMARY 155 



hypothesis the muscular groups of the vertebrate must have 

 arisen from corresponding groups in the annelid ancestor. It is 

 worth while here in this final chapter to give my views on the 

 possible ways in which such conversion took place. 



The fundamental hypothesis is that the central nervous 

 system of the invertebrates has formed the central nervous sys- 

 tem of the vertebrate by growing round and enclosing the ali- 

 mentary canal of the former, so that the alimentary canal of the 

 vertebrate is a new formation derived from structures already ex- 

 isting in the invertebrate ancestor. The most important fixed 

 structure common to both the invertebrate and the vertebrate is 

 the central nervous system with the nerves arising from it ; those 

 nerves are arranged in the same manner in the two groups of 

 animals. First the supra-infundibular nerves, the optic and 

 olfactory, correspond to the supra-cesophageal nerves, then the 

 infra-infundibular nerves correspond to infra-cesophageal and give 

 origin to the nerves of mastication and respiration, and finally 

 the spinal nerves correspond to the nerves from the ventral chain 

 of ganglia in the invertebrates. Further, the infra-infundibular 

 nerves, which like the infra-cesophageal may be divided into a 

 prosomatic and mesosomatic group (using the terms employed to 

 characterize the foremost divisions of such animals as Limulus or 

 the scorpion), are not built up of two roots as in the case of the 

 spinal segmental nerves, one ventral and motor and one dorsal 

 and sensory, but of three roots, one ventral and motor, one lateral, 

 and one dorsal and sensory, the lateral root being both motor 

 and sensory. Now the cranial region is older than the spinal 

 region, so that this three-root system must be looked upon as the 

 more primitive system, and indeed the two-root system can be 

 derived from it by leaving out the lateral root. The lateral root 

 nerves form a definite group of prosomatic and mesosomatic nerves, 

 and supply with motor fibres the striated muscles of mastication 

 (fifth nerve), respiration, deglutition and facial expression. This 

 group of muscles is differentiated from other striated muscles in 

 their origin, since they are all formed from muscles surrounding 

 branchial and visceral arches, and represent the muscles of a dis- 

 tinct segmentation due to the branchial segmentation in the 

 mesosomatic region, and a similar but non-branchial segmentation 

 in the prosomatic region. 



There is thus a double segmentation in the cranial region 



