3 o VERTEBRATE ZOOLOGY 



or hyoid arch,— and a posterior group, consisting of the posterior 

 five arches, which support gills and are called the branchial or gill 

 arches. A further distinction between these two groups is that 

 the arches constituting the first alone appear as a portion of the 

 head proper, those constituting the second group having migrated 

 backward into the trunk. This condition is a secondary one and 

 has been brought about by the shortening of the dorsal, or cranial, 

 portion of the hinder part of the skull without a corresponding 

 shortening of the ventral, or visceral, portion. The primitive posi- 

 tion of all the visceral arches is on the head. 



The mandibular arch forms the jaws of the animal. The dorsal 

 half, or upper jaw, is formed of a pair of cartilages called the 

 palatoquadrate cartilages ; the ventral half, or lower jaw, is formed 

 of a pair called Meckel's cartilages. Each palatoquadrate carti- 

 lage is joined with the base of the cranium by a strong ligament 

 near its middle, and at its hinder end articulates with the lower 

 jaw. The two halves of the lower jaw are joined together by a 

 median ligament. The jaws are also joined with the second or 

 hyoid arch by means of a ligament, and also with the base of the 

 cranium. Slender, accessory labial cartilages are present in the 

 lips at the side of both the upper and the lower jaw, two pairs being 

 in the former and one in the latter. 



The hyoid arch forms the principal support, or suspensorium, of 

 the jaws; it supports also the tongue and bears the foremost 

 pair of gills. Its dorsal half on each side is called the hyomandib- 

 ular cartilage ; it projects outward from the cranium, with which 

 it articulates, and at its outer end is joined by short ligaments 

 with the jaws and also, by a long ligament, with the base of the 

 cranium. Projecting from its hinder border are several deli- 

 cate cartilaginous rods called the gill rays, which support the 

 gill filaments. 



The ventral half of the hyoid arch on each side is called the 

 ceratohyal cartilage ; it bears gill rays on its hinder border. Join- 

 ing the two ceratohyals ventrally and projecting forward is the 

 basihyal cartilage, which supports the tongue. 



The five branchial arches are essentially alike in structure. Each 

 arch is divided into four segments on each side, two dorsal and 



