The nerve to the median ventral tentacle (_t._) 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 {290}to my theory, to the epimeral nerves of Limulus; the other coloured red, which supplies muscles belonging to the visceral or splanchnic muscle-group, and contains also the nerves to the tentacles. This latter group, which is formed by two distinct well-defined nerves, viz. the mandibular and the internal branch of the maxillary, corresponds, according to my theory, to the amalgamated nerves of the prosomatic appendages, and is clearly divisible into three distinct nerves-- 1. The lower lip-nerve or the metastomal nerve (_met._). 2. The tongue-nerve (_t._). 3. The nerve (_tent._) to the upper lip and tentacles. Of these three pairs of nerves it is suggested that the first pair were derived from the nerves to the metastomal appendage. The second pair of nerves ought, on this theory, originally to have supplied the pair of appendages immediately in front of the metastoma--that is, the pair of ectognaths, and therefore the ventral pair of tentacles, known as the tongue, would represent the last remnant of these ectognaths. Similarly, the other tentacles would represent the endognaths, and therefore the third pair of nerves would represent the fused nerves to these concentrated endognaths, which, in the Eurypterids, stand aloof from the ectognaths. Let us consider these three propositions separately. In the first place, have we any right to attribute segmental value to the mandibular nerve? What evidence is there of segments in this region in Ammocoetes? THE SEGMENT OF THE LOWER LIP, OR METASTOMAL SEGMENT.