THE SKELETAL SYSTEM 



229 



Cartilaginous supports for the respiratory system are lacking in the 

 lowest chordates. In amphioxus, the velum in which the mouth lies, 

 and the gill arches are supported by slender rods of a material which from 

 its resemblance to cartilage is called pro-cartilage. A truly cartilaginous 

 visceral skeleton first appears in cyclostomes in the form of a gill-basket in 

 which the number of cartilage rods corresponds with the number of gill- 

 arches. Bdellostoma may have as many as fifteen of these, other cyclo- 

 stomes eight or nine. 



In elasmobranchs the number of visceral cartilages is reduced to corre- 

 spond with the reduced number of gill apertures. The maximum number 



h Idrn ^° ^9 9c bb J^ 



Fig. 186. — Ventral and lateral views of the skull of lamprey {Petromyzon marinus). 

 ad, anterior dorsal cartilage, bb, branchial basket; gc, gill cleft; Ic, labial cartilage; 

 Idni, lateral distal mandibular; Lg, lingual cartilage; nc, nasal capsule; oc, otic capsule; on, 

 optic nerve; pc, pericardial cartilage; pd, posterior dorsal cartilage. (After Parker from 

 Kingsley's "Comparative Anatomy of Vertebrates.") 



is nine, in heptanchus, of which the first, the mandibular, is modified to 

 become the cartilages of the upper and lower jaws, while the second arch, 

 the hyoid, functions slightly as a gill arch, and its dorsal division, the 

 hyomandibula cartilage, acts as a suspensory apparatus for the lower jaw. 

 The dorsal division of the first visceral arch, which forms the cartilage 

 of the upper jaw, is called the palato-pterygo-quadrate cartilage, since 

 palatal, pterygoid, and quadrate bones develop from it in amphibia and 

 reptiles. The lower half of the first visceral arch forms cartilage of the 

 lower jaw, Meckel's cartilage. An articulation is formed between the two, 

 so that in elasmobranchs a biting mouth replaces the sucking mouth of 

 cyclostomes. Nevertheless, the upper jaw does not fuse with the cranium 



