48 EVOLUTION AND DISEASE. 
of vertebrata is certainly very remarkable, but a much 
more extraordinary instance of change of function is 
manifested in the nervous system now to be considered. 
The Central Nervous System.—Vertebrate animals are 
distinguished from invertebrate not only in the posses- 
sion of a vertebral column, but also by the fact that they 
are furnished with a central nervous system known as 
the spinal cord and brain. Of late years it has been the 
opinion of many biologists that the separation of the 
invertebrata from vertebrata is unjustifiable, and many 
attempts have been made to bridge the gulf supposed 
to exist between these great divisions of the animal 
kingdom. 
Those invertebrates which approach nearest the verte- 
brates are the cephalopods (cuttles, octopods, &c.), and 
in these forms the central nervous system is represented 
by ganglionic masses collected around the cesophagus or 
gullet and united by commisural fibres : this arrangement 
is known as the cesophageal collar, so that in order to 
bring this into harmony with the anatomical disposition 
of the vertebrate gullet, some eminent biologists have 
maintained that the vertebrate mouth is secondary, and 
that the primitive gullet traversed the central nervous 
system by way of the third cerebral ventricle and 
infundibulum, a diverticulum from the original vesicles 
out of which the brain is ultimately developed. This 
hypothesis has not found much favour, but recently 
some observations and speculations have been announced 
which throw much new and important light on the 
matter. 
The central nervous system is traversed by a canal 
