THE PROSOMATIC SEGMENTS OF AMMOCCETES 303 



segment which is, in all respects, comparable with the segments 

 supplied by the facial, glossopharyngeal, and vagus nerves, except 

 that it does not possess branchiae. This simply means that the 

 appendages which these nerves originally supplied were prosomatic, 

 not mesosomatic, and corresponded, therefore, to the chilarial or 

 metastomal appendages. 



A comparison of the ventral surface of Slimonia, as given in 

 Fig. 8, p. 27, with that of Ammocoetes (Fig. 119), when the thyroid 

 gland and lower lip muscles have been exposed to view, enables the 

 reader to recognize at a glance the correctness of this conclusion. 



The Tentacular Segments and the Upper Lip. 



Anterior to this metastomal segment, Fig. 116 shows a group of 

 visceral muscles, mi, and yet again a muco-cartilaginous bar, ski, but, 

 as already stated, no tubular muscles. These visceral muscles indicate 

 the presence in front of the lower lip-segment of one or more segments 

 of the nature of appendages. The muscles in question (mi) are the 

 muscles of the upper lip, the skeletal elements form a pair of large 

 bars of muco-cartilage (ski), which start from the termination of the 

 trabecule, and pass ventralwards to fuse 'with the muco-cartilaginous 

 plate of the lower lip (Figs. 117 and 118). This large bar forms the 

 tentacular ridge on each side, and gives small projections of muco- 

 cartilage into each tentacle. In addition to this tentacular bar, a 

 special bar of muco-cartilage exists for the fused pair of median 

 tentacles, the so-called tongue, which extends in the middle line 

 along the whole length of the lower lip, being separated from the 

 muco-cartilaginous plate of the lower lip by the muscles of the lower 

 lip. This tongue bar of muco-cartilage joins with the muco-cartilage 

 of the lower lip at its junction with the thyroid plate, and also with 

 the tentacular bar just before the latter joins the muco-cartilaginous 

 plate of the lower lip. This arrangement of the skeletal tissue 

 suggests that the pair of tentacles known as the tongue stand in a 

 category apart from the rest of the tentacles ; a suggestion which is 

 strongly confirmed by the separate character of its nerve-supply, as 

 already mentioned. 



For three reasons, viz. the separateness both of their nerve-supply 

 and of their skeletal tissue, and the importance they assume at trans- 

 formation, this pair of ventral tentacles must, it seems to me, be put 



