THE SKELETON 35 



Their posterior ends give origin to the M. sub-hyoideus, while the 

 anterior ends serve for the insertion of M. subarcualis rectus i. 



3. The First Branchial (Third Visceral) Arch. 



This consists of a narrower arcuate bar articulating with the copula 

 on either side and comprising hypo- and cerato-branchial elements 

 fused together homocontinuously (i.e. without interruption). Each 

 bar is a flat oval in cross-section and its posterior end curves dorsal- 

 wards approximately parallel with the cerato-hyal, thus forming the 

 posterior cornu of the hyoid. From the posterior end of this cartilage 

 the M. subarcualis rectus i takes its origin. 



4. The Second Branchial (Fourth Visceral) Arch. 



The second branchial {^fourth visceral) arch of the adult is repre- 

 sented only by the paired hypo-branchial elements (c.h.br. II.). These 

 are slender curved bars articulating mesially with the posterior end 

 of the copula, and fusing laterally with the previous arch at a point 

 about two-thirds along its length. The space between the two arches 

 is covered by a tough membrane, the memhrana intercartilaginea 

 (Driiner) (me. i.e.), except over the mesial third, where the membrane 

 is incomplete to allow for the passage of the M. rectus cervicis 

 profundus. 



5. The Copula. 



The copula (c.cop.) is the median cartilage with which those of the 

 visceral arches articulate. It probably represents the fused basi- 

 branchials, but it never shows signs of segmentation even in the larva. 

 Its anterior end bears two pairs of small horn-like processes — the 

 radials. As already indicated the anterior pair (c.h.hy.) of the latter 

 are the hypo-hyals, but the posterior pair (c.rad.) have been shown by 

 Driiner (1901) to arise secondarily during metamorphosis, and are 

 therefore not strictly a part of the visceral skeleton. They, like the 

 hypo-hyals, serve to attach the copula to the tongue. 



6. The Os Triangulare. 



The OS triangulare (o.t.) is a small, broadly triangular bar of bone 

 lying across the ventral side of the pharynx in an inscriptio tendinea 

 at the junction of the M. genio-hyoideus and M. rectus cervicis 

 superficialis. It is the only portion of the hyo-branchial skeleton to 

 ossify. It represents the expanded end of the uro-branchial, and, in 

 the larva, it is connected with the copula by a longitudinal rod which 

 disappears during metamorphosis, although very occasionally small 



