288 THE ORIGIN OF VERTEBRATES 
peripheral organ, no marked difference occurs in the arrangement 
of the nerves issuing from the trigeminal motor centre, no new 
nerves are formed to supply the new muscles, but every motor nerve- 
fibre and the motor cell from which it arises increases enormously in 
size, and these giant nerve-fibres thus formed split into innumerable 
filaments corresponding with the proliferation of themuscular elements. 
The clue, then, to the origin of the suctorial apparatus and of the 
nature of the original organs supplied by the trigeminal is afforded 
in this case, as in all other similar inquiries, by the central nervous 
system and its outgoing nerves. Here is always the citadel, the 
fixed seat of government, here is ‘headquarters,’ from which the 
answers to all our inquiries must originate. 
THE TRIGEMINAL NERVE OF AMMOUCETES. 
Striking is the answer. In Fig. 114, Miss Alcock has drawn the 
distribution of the trigeminal nerve as traced by her through a series 
Fig. 114.—DistripuTion oF TRIGEMINAL NERVE IN AMMOCGTES, 
ps. br., pseudo-branchial groove; met., nerve to lower lip, or metastomal nerve; t., 
nerve to tongue; tent., nerve to tentacles. The mandibular and internal maxil- 
lary nerves are coloured red; the purely sensory nerves to the external surface 
are coloured black. 
of sections. It arises, as is well known, from two separate ganglia, of 
which the foremost gives rise to a purely cutaneous nerve, the oph- 
thalmic nerve, and the hindmost to three nerves, the most posterior 
of which is purely cutaneous and passes tailwards over the ventral 
branchial region, as shown in the figure ; the other two nerves, both 
