490 THE ORIGIN OF VERTEBRATES 



giant-fibres exist, each of which breaks up, at the last abdominal 

 ganglion, into smaller fibres, which pass directly out with the nerves 

 to the tail-fin. Allen has shown that, in addition to these two 

 specially large giant-fibres, there are a number of others, some of 

 which, similarly to the Mvillerian fibres of Ammocoetes, cross the 

 middle line, while some do not. Each of these arises from a large 

 nerve-cell and passes to one or other of the last pair of abdominal 

 ganglia. The latter fibres, he says, send off collaterals, while the 

 two specially large giant-fibres do not. The cells which give origin 

 to all these large, long fibres are situated in or in front of the proso- 

 matic region of the brain, similarly to the giant-cells, which give rise 

 to the corresponding Mullerian fibres of Ammocoetes. I do not know 

 how far this system is represented in Limulus or Scorpio. 



It is, to my mind, improbable that theMauthnerian fibres pass out 

 directly as motor fibres to the muscles of the tail-fin ; it is more 

 likely that they are conducting paths between the equilibration- 

 mechanism in connection with the Vlllth nerve and the spinal 

 centres for the movements of the tail. Similarly, with respect to 

 the arthropod, it is difficult to believe that the motor fibres for the 

 tail-muscles arise in the brain-region. In either case, the striking 

 coincidence remains that the movements of the tail-end of the body 

 are regulated by means of giant-fibres which arise from giant-cells in 

 the head-region of the body in both the Arthropoda and the lowest 

 members of the Vertebrata. 



The meaning of this system of giant-cells and giant-fibres in both 

 classes of animals is well worthy of further investigation. 



Another important piece of comparative work which ought to 

 help in the elucidation of this problem is the comparison of the blood- 

 and lymph-corpuscles of the vertebrate with those of the invertebrate 

 groups. As yet, I have not myself made any observations in this 

 direction, and feel that it is inadvisable to discuss the results of 

 others until I know more about the facts from personal observation. 



The large and important question of the manner of formation of 

 the vertebrate skin has only been considered to a slight extent. 

 A much more thorough investigation requires to be made into the 

 nature of the skin of the oldest fishes in comparison with the skin of 

 Ammoccetes on the one side, and of Limulus and the Pakeostraca 

 on the other. 



The muscular system requires further investigation, not so much 



