118 Basic Structure of Vertebrates 



VISCERAL SKELETON 



The visceral skeleton, as seen in embryonic stages of all verte- 

 brates and in adult sharklike fishes, consists of a series of cartilaginous 

 rings or "arches," each more or less completely encircling the anterior 

 region of the digestive tube (Figs. 116, 117, 120). The first or most 

 anterior arch is close behind the oral aperture and constitutes the 

 upper and lower jaws, therefore (at least in the adult) completely 

 surrounding the digestive tube. It is called the "mandibular arch." 

 The second, or hyoid arch, is just behind the mandibular. It is in- 

 complete dorsally, extending from the ventral side of each otic region 

 of the cranium downward to the midventral line. The remaining arches, 

 placed in close succession to one another behind the hyoid arch, attain 

 their maximum development in fishes, serving mainly for the support 

 of the gills and hence called "branchial arches." They are all incom- 

 plete dorsally. The several visceral arches alternate in position with the 

 pharyngeal respiratory apertures of the gill-breathing vertebrates and 

 with the pharyngeal pouches of the embryos of land vertebrates. In 

 sharklike fishes, the spiracle, a modified gill-cleft, passes between the 

 mandibular and hyoid arches; the first branchial cleft is between the 

 hyoid and the first branchial arch; and the gill-clefts and arches alter- 

 nate thus to the posterior end of the series (Fig. 116). 



The maximum number of visceral arches occurs in a genus of 

 shark, Heptanchus, having a pair of spiracles and seven pairs of gill- 

 clefts and, accordingly, nine visceral arches. In most fishes the number 

 of arches is seven or six, but in some cases less. 



(There is frequent confusion in the use of the names of the arches. 

 They are all visceral arches. The mandibular is the first visceral, 

 the hyoid is the second visceral, the first branchial is the third 

 visceral, and so on.) 



Each arch is divided into several parts. The embryonic mandibu- 

 lar arch consists, on each side, of one elongated dorsal cartilage called 

 the "palato-pterygo-quadrate." This cumbersome name is derived 

 from the fact that the palatine, pterygoid, and quadrate bones of the 

 upper jaw are derived from the cartilage or in close relation to it. The 

 embryonic lower jaw consists, on each side, of a slender cartilage 

 known as the mandibular (or Meckel's) cartilage. The hyoid arch 

 consists, on each side, of a dorsal hyomandibular cartilage (hyo- 

 mandibula or pharyngohyal) and below it, in the order mentioned, 

 an epihyal, a ceratohyal, and a hypohyal. The two hypohyals join 

 a midventral basihyal. In the branchial arches the maximum num- 

 ber of parts in an arch is nine — four pairs and a median ventral part 

 (Fig. 118). Named in order from above downward, they are the pha- 



