THE PROS OM 'A TIC SEGMENTS OF AMMOCCETES 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 (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 



TJ 



