THE PROSOMATIC SEGMENTS OF AMMOCCGETES 297 
cartilage is coloured purple, the soft cartilage blue, and the muco- 
cartilage red, so that the position of this bar is well shown. This 
bar may be looked upon as bearing the same relation to the muco- 
cartilaginous plate of the lower lip as the opercular bar does to the 
muco-cartilaginous plate over the thyroid; and seeing that these two 
plates form one continuous ventral head-shield of muco-cartilage 
(Fig. 118, B), and also that this bar fuses with the opercular bar, we 
may conclude that the segment represented by the lower lip is 
closely connected with the hyoid or opercular segments. In other 
words, if the lower lip arose from the metastoma, then this pair of 
skeletal bars might be called the metastomal bars, which formed the 
supporting skeleton of the last pair of prosomatic appendages and, as 
is likely enough, arose in connection with the posterior lateral horns 
of the plastron; these posterior lateral horns, like the rest of the 
plastron, would.give rise to hard cartilage, and so form in Ammoccetes 
the two lateral so-called pterygoid projections. 
In the branchial region the muscles which marked out each 
branchial segment were of two kinds—ordinary striated visceral 
muscles and tubular muscles, Of these the former represented the 
dorso-ventral muscles of the branchial appendages, while the latter 
formed a separate group of dorso-ventral muscles with a'separate 
innervation which may have been originally the segmental veno- 
pericardial muscles so characteristic of Limulus and the scorpions. 
In Figs. 116, 117, the grouping of these muscles in each branchial 
segment is well shown, and it is immediately seen that the hyoid 
segment possesses its group of striated visceral muscles (ms) supplied 
by the VIIth nerve in the same manner as the posterior groups, as 
has already been pointed out by Miss Alcock in her previous 
paper. Passing to the segment in front, Fig. 116 shows that the 
group of visceral muscles (mg) corresponds in relative position with 
respect to the metastomal bar to the hyoid muscles with respect 
to the opercular bar or to the branchial visceral muscles with 
respect to each branchial bar. What, then, is this muscular group ? 
The series of sections show that these are the dorso-ventral muscles 
belonging to the lower lip, which, as seen in Fig. 119 (Jf), form a 
well-marked muscular sheet, whose fibres interlace across the mid- 
ventral line of the lower lip. This group of lower lip-muscles is very 
suggestive, for these muscles arise, not from the trabecule, but from 
the front dorsal region of the cranium, just in front of the two lateral 
