THE PROSOMATIC SEGMENTS OF AMMOCGTES 289 
of which contain motor fibres, are called by Hatschek the mandibular 
and maxillary nerves. Of these the mandibular or velar nerve (met.) 
is a large, conspicuous nerve, which arises so separately from the 
rest of the trigeminal as almost to deserve the title of a separate 
nerve. When it leaves the large posterior ganglion, it passes into 
the anterior part of the velum, runs along with the tubular muscles, 
which it supplies, to the ventral surface as far as the junction of the 
lower lip with the thyroid plate, and has not been followed further by 
Hatschek. Miss Alcock, however, by means of serial sections, has 
traced it further, and shown that at this point it turns abruptly 
headwards to terminate in the muscles of the lower lip. If, then, 
as suggested, the lower lip represents the metastoma—the last pair 
of prosomatic appendages—then this mandibular or velar nerve 
represents that segmental nerve. 
The other nerve—the maxillary nerve of Hatschek—which con- 
stitutes the larger part of the trigeminal, passes forwards from the 
ganglion, and at a point somewhere about the anterior region of the 
eyeball, divides into two, an external (black in Fig. 114) and an 
internal (red in Fig. 114) nerve. The external branch is apparently 
entirely sensory, and supplies the external surfaces of the upper and 
lower lips. The internal branch is mainly motor, and supplies the 
muscles of the upper lip; it contains also the nerves of the tentacles. 
The nerve to the median ventral tentacle (¢.) or tongue leaves the 
internal division of the maxillary immediately after its separation 
from the external; it runs ventralwards, and at the same time passes 
internally until it reaches a position between the muco-cartilage and. 
the epithelium lining the cavity of the throat. It then turns, and 
passing posteriorly (towards the tail) to the point where the median 
ventral tentacle is attached to the lower lip, it supplies some very 
rudimentary-looking muscles which run from the tentacle to the 
adjoining surface, and no doubt serve to move the tentacle from side 
to side. A portion of the nerve still continues to run along the side 
of the median ventral ridge, as far back as the point where the 
muscles of the hyoid segment pass round to the ventral side between 
the velum and the thyroid; in fact, this small nerve passes along 
the whole length of the median ventral ridge. 
This description shows that the trigeminal nerve divides itself 
into two groups: the one represented black in the figure, which is 
purely cutaneous and sensory, corresponding, in the main, according 
U 
