VERTEBRATE ANIMALS 223 



elongated labial cartilages, of which the dorsal in the upper lip is 

 the larger. 



The second or hyoid arch more nearly approaches in structure 

 a typical branchial arch, and it is divided into two portions. The 

 dorsal end, the hyomandibular cartilage, is a stout short rod lying 

 just behind the spiracle, which articulates with the auditory capsule 

 just below the post-orbital groove. This bar is firmly connected 

 with the jaws by the stout symplectic ligament, and so suspends 

 them from the cranium, being for this reason sometimes termed the 

 suspensorium. To its ventral end is also attached the lower portion 

 of the arch, namely, the ceratohyal cartilage. This is a long curved 

 structure more slender than the hyomandibular, and it runs antero- 

 ventrally under the lower jaw to become attached in the middle line 

 to a median basihyal plate, lying in the floor of the pharynx and 

 buccal cavity. A thick ligament joins the ceratohyal to the lower 

 jaw. Two other ligaments just behind the spiracle help to keep the 

 jaws and hyoid arch in position : the first is the superior post- 

 spiracular ligament running from just above the anterior end of the 

 post-orbital groove to the end of the palato-quadrate and hyoman- 

 dibular cartilages ; the second, the inferior post-spiracular ligament, 

 passes from the postero-lateral border of the floor of the skull below 

 the orbit to the hyomandibular and ceratohyal. Both portions of 

 the hyoid arch bear a series of small cartilaginous rods, the branchial 

 rays, on their posterior edges which serve for the support of the front 

 wall of the first gill cleft. 



It will be seen, then, that the spiracle lies in front of the hyoid 

 arch, and between it and the mandibular, although the latter turns 

 forward parallel with the cranial floor, and we find the anterior 

 spiracular wall strengthened by a small prespiracular cartilage, 

 probably the remains of the branchial rays at one time carried by 

 the mandibular arch in an ancestral form. 



The remaining five visceral arches are the branchial arches, 

 numbered i to 5 from before backwards. Each becomes split 

 up into four distinct segments and, connected together by a median 

 ventral unpaired basibranchial plate, form a series of supporting 

 bars for the walls of the branchial pouches. The branchial clefts 

 receive their name from the skeletal element immediately behind 

 them, so that the spiracle is more properly termed the hyoidean 

 cleft, and the rest the true branchial clefts, although, of course, they 

 are strictly homologous. The four segments of the arch on each 

 side are in order dorso-ventrally the pharyngeo-, epi-, cerato- and 

 hypo-branchials. Thepharyngeo-branchials are elongated triangular 

 plates that run from just beneath the vertebral column forwards and 

 outwards in the roof of the pharynx, and the fourth and fifth are 



