_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 maxillary nerves are coloured red; the purely sensory nerves to the external surface are coloured black.] THE TRIGEMINAL NERVE OF AMMOCOETES. Striking is the answer. In Fig. 114, Miss Alcock has drawn the distribution of the trigeminal nerve as traced by her through a series of sections. It arises, as is well known, from two separate ganglia, of which the foremost gives rise to a purely cutaneous nerve, the ophthalmic 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 {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 constitutes 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.