438 THE DOGFISH CHAP 



spends to the palatoquadrate 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 Meckefs^ 

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



The upper and lower jaws of either side correspond to 

 the first of a series of seven pairs of visceral arches, and are 

 therefore often spoken of as the mandibular arch. The 

 remaining six pairs have 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 support- 

 ing 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. ;//), 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 quad- 

 rate (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. \ 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 gills. 



In the frog-tadpole the gills are similarly supported by carti- 

 laginous arches, which become greatly reduced and modified at meta- 



