THE PROSOMATIC SEGMENTS OF AMMOCCETES 297 



cartilage is coloured purple, the soft cartilage blue, and the rnuco- 

 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 niuco- 

 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 w r ere 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 (ra 3 ) supplied 

 by the Vllth 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 (m 2 ) 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 (M.), 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 trabecula3, but from 

 the front dorsal region of the cranium, just in front of the two lateral 



