214 GENERAL SUMMARY. 



tract and a common centre, e.g. , the segmental gustatory nerves. Or the nerves from 

 a special group of neuromeres may break up into several sets of components which 

 then reunite to form new groups. For example, in the vagal and branchial complex 

 of arachnids, which represents seven neuromeres, we may recognize the following 

 groups of more or less independent components: a. the combined, almost exclu- 

 sively sensory nerves of the first two or three vagal appendages (scorpion) ; b. and c. 

 the cardiac and intestinal components; d. the mixed nerves supplying the append- 

 ages, gills, lung books, or operculum; and e. the combined motor components 

 that constitute the hypobranchial nerve which supplies the compound hypo- 

 branchial muscle derived from all the branchial and vagal segments. 



19. The extent to which segmental nerves have been reduced to a single set 

 of highly specialized components, or broken up into several independent sets of 

 components, decreases in a cephalo-caudal direction. For example, each of the 

 three forebrain neuromeres in arachnids contains only the purely sensory nerves 

 of the olfactory organ and of the parietal eye and lateral eyes. In the six follow- 

 ing thoracic segments the nerves are largely sensory, the motor components being 

 reduced to the relatively small nerves supplying the muscles of the legs and those 

 passing from one side of the carapace to the other, or to those holding the endo- 

 cranium in place. All the procephalic and mesocephalic haemal muscles have 

 disappeared. 



20. A still further reduction of motor components took place in the verte- 

 brate brain with the fusion of the anterior thoracic appendages to form the im- 

 movable anterior arch of the mouth (pre-maxillae and maxillae) ; with the reduction 

 of the free thoracic appendages to external gills; with the fixation of the endo- 

 cranium by its fusion with the exoskeleton; and finally with the atrophy of nearly 

 all the branchial musculature in the air-breathing vertebrates. 



21. The most important events in the conversion of the arachnid type of 

 brain into the vertebrate type were the transfer of the lateral eye placodes to the 

 interior of the cerebral vesicle; the closing of the neurostoma by the closure of the 

 medullary plate; the transfer of the optic ganglia to the roof of the midbrain; and 

 the union of the branchial neuromeres with those of the vagus region. 



