xii PHYLUM CHORDATA 339 



plexity. The visceral arches in the dogfish are composed 

 of a system of rods of cartilage. The first visceral arch 

 forms the upper and lower jaws, between which the open- 

 ing of the mouth is situated. The jaws are connected on 

 each side with the skull behind by means of a cartilage 

 known as the hyomandibular, which is a part of the second 

 or hyoid arch ; the rest of the hyoid arch and the branchial 

 arches, which are five in number, lie in the lateral and ven- 

 tral walls of the pharynx and support the gills. 



In both the lizard and the rabbit the branchial arches 

 are not present as such, the only well-developed visceral 

 arches being the first and second. The upper jaw is 

 formed of certain membrane bones, and in the lower jaw 

 also the cartilage completely disappears, its place being 

 taken by bones which are early completely united together, 

 so as to form the bony lower jaw or mandible. In the 

 lizard the mandible articulates on each side with the pos- 

 terior region of the skull through the intermediation of a 

 bone known as the quadrate, which is an element of the 

 first visceral arch. In the rabbit the articulation between 

 the mandible and the skull is direct, no quadrate inter- 

 vening. 



The skeleton of the limbs in the dogfish differs widely 

 from that of the lizard and rabbit. In all three we dis- 

 tinguish the limb-arch from the skeleton of the free part 

 of the limb itself. The limb-arch (pectoral or pelvic) is a 

 cartilage or a system of bones with which the base of the 

 free part of the limb articulates, and has the function of 

 connecting the limb with the trunk and serving for the 

 origin of many of the muscles moving the limb. In the 

 dogfish the entire skeleton of the limbs is composed of 

 cartilages which are so arranged as to support the thin 

 broad expanse of the fin. In both the lizard and the 



