THE PROSOMATIC SEGMENTS OF AMMOCCETES 295 



out these two segmental muco-cartilaginous bars and their attendant 

 muscles, and see to what sort of segments their investigation 

 leads. 



The bar which comes first for consideration (sl 3 ) arises imme- 

 diately behind the auditory capsule from the first branchial cartilage 

 very soon after it leaves the sub-chordal cartilaginous ligament ; the 

 soft cartilage of the sub-chordal ligament ceases abruptly in its 

 extension along the notochord at the place where the hard cartilage 

 of the parachordal joins it, and in a sense it may be said to leave the 

 notochord at this place and pass into the basal part of the first branchial 

 bar. The most anterior continuation of this branchial system is this 

 muco-cartilaginous bar (sk 3 ), which passes forward and ventral wards, 

 being separated from the axial line by the auditory capsule (cf. Fig. 

 118, A, B, C). Its position is well seen in a sagittal section, such 

 as Fig. 117. It follows absolutely the line of the pseudo-branchial 

 groove (ps. br., Fig. Ill), and ventrally joins the plate of muco- 

 cartilage which covers the thyroid gland. It forms a thickened 

 border to this plate anteriorly, just as the branchial cartilaginous 

 bars border it posteriorly. In fact, it behaves with respect to the 

 hyoid segment in a manner similar to the rest of the cartilaginous 

 bars with respect to their respective segments. 



It represents, although composed of muco-cartilage, the cartila- 

 ginous bar of the operculum in Limulus, which also forms the termi- 

 nation of the branchial cartilaginous system, as fully explained in 

 Chapter III. ; it may therefore be called the opercular bar. 



The next bar (s& 2 ) is extremely interesting, as we are now out of 

 the branchial or mesosomatic region, and into the region corresponding 

 to the prosoma. It starts from a cartilaginous projection made of 

 hard cartilage, just in front of the auditory capsule, called by Parker 

 the 'pedicle of the pterygoid' — a projection (ped.) which defines the 

 posterior limit of the trabecular on each side, where they join on to 

 the parachordals, — and winding round and below the auditory capsule, 

 joins the opercular bar (cf. Fig. 118), to pass thence into and form part 

 of the muco-cartilaginous plate of the lower lip. In the section figured 

 (Fig. 116), this projection of hard cartilage is not directly continuous 

 with (sk^), owing to a slight curvature in the bar ; the next few 

 sections show clearly the connection between (ped.) and (sh-z), and 

 consequently the complete separation by means of this bar of the 

 hyoid segment from the segment in front. In the figures, the hard 



