438 THE DOGFISH CHAP. 



cartilage of the frog (p. 44), presents at. its posterior end 

 a rounded surface against which fits a corresponding 

 concavity on the lower jaw, so that a free articulation 

 is produced, the lower jaw, or mandibular cartilage 

 (p. 44), working up and down in the vertical plane. 



The upper and lower jaws correspond to the first of 

 a series of seven pairs of visceral arches, and are there- 

 fore often spoken of as the mandibular arch. The 

 remaining six pairs have on either side the form of 

 cartilaginous half-hoops, lying in the walls of the 

 pharynx, and united with one another below so as to 

 form a basket-like apparatus supporting the gills. The 

 second of these arches is distinguished as the hyoid, and 

 is situated immediately behind the jaws. It consists of 

 two parts : a strong, rod-like hyomandibular (Fig. 112, 

 hy. m) y which articulates above with the auditory 

 capsule and is connected below by fibrous tissue with 

 the jaws, thus helping to suspend them to the cranium 

 and serving as a suspensorium formed in the frog by 

 the quadrate (pp. 40 and 44) ; and a hyoid cornu or horn, 

 which curves forwards inside the lower jaw, and is 

 connected with its fellow of the opposite side by a 

 median basi-hyal plate which supports the tongue 

 (compare Fig. 9). 



The remaining five arches (br. a. i-br. a. 5) are called 

 the branchial arches. Each is formed of several separate 

 pieces, united by fibrous tissue so as to render possible 

 the distension of the throat during swallowing : the 

 fifth is connected below with its fellow by a large median 

 basi-branchial plate, which supports the roof of the 

 pericardial cavity. Both they and the hyoid give 

 attachment to delicate cartilaginous branchial rays (br. r, 

 br. r', Fig. 118, r) which support the gill-filaments. 



In the frog-tadpole the gills are similarly supported by 

 cartilaginous arches, which become greatly reduced, and 



