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 Miillerian fibres of Ammoccetes, 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 Miillerian fibres of Ammoccetes. I do not know 
how far this system is represented in Limulus or Scorpio. 
It is, to my mind, improbable that the Mauthnerian 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 VIIIth 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 Paleostraca 
on the other. 
The muscular system requires further investigation, not so much 
