82 



DIVERGENCE AND PARALLELISM 



TABLE OF CONVERGENCE IN THE STRUCTURE OF 

 THE CENTRAL NERVOUS SYSTEM (NEURAL 

 CONVERGENCE) 



Appendiculate. 

 {Segmented invertebrate). 



1. Supra-cesophageal ganglia, 

 giving origin to the nerves of 

 the eyes and antennules, i.e., 

 the optic and olfactory nerves. 



2. Circum-cesophageal com- 

 missures connecting the cerebral 

 ganglia with the infra-cesopha- 

 geal ganglia and the ventral 

 chain. 



3. Infra-cesophageal ganglia 

 and the paired ventral chain of 

 segmental ganglia. Each pair 

 of ganglia gives rise to the 

 nerves of its own segment, 

 motor and sensory ; by the 

 agency of which food is in- 

 gested, respiration and loco- 

 motion effected. 



Vertebrate. 

 {Craniate vertebrate). 



1. The brain proper from 

 which arise only the olfactory 

 and optic nerves. 



2. Crura cerebri, strands of 

 fibres on each side of the in- 

 fundibulum, connecting the 

 higher brain region proper 

 with the lower region of the 

 medulla oblongata and spinal 

 cord. 



3. Region of the mid-brain, 

 medulla oblongata, and spinal 

 cord ; from these arises a series 

 of nerves segmentally arranged, 

 which, as in the invertebrate, 

 give origin to the nerves 

 governing mastication, respira- 

 tion, and locomotion. 



With reference to the first sub-heading in the 

 above table it is to be remarked that Dr Gaskell 

 duly points out that the crustacean antennules 

 are olfactory in function, but he does not add 

 that they are also the carriers of the auditory 

 organs and that the latter are innervated from 

 the cerebral ganglia as they are in Mollusca. At 

 the same time a physiologist may be pardoned 

 for regarding the close correspondence as a mani- 

 festation of homology rather than as a case of 



