THE PROSOMATIC SEGMENTS OF AMMOCGETES 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 branchie. 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 Ammoccetes (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, mm, and yet again a muco-cartilaginous bar, sk,, 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 (m;) are the 
muscles of the upper lip, the skeletal elements form a pair of large 
bars of muco-cartilage (sk), 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 
