xm PHYLUM CHORDATA 75 



one on each side of the mesethnioid ; the optic foramina (nv. 2) 

 for the nerves of sight, in the interorbital region ; the trigeminal 

 foramina (nv. ~>) for the fifth nerves, just in front of the auditory 

 capsule ; the auditory foramina (nv. <V) for the nerves of hearing, 

 in the inner wall of the auditory capsules ; and the vagus foramina 

 (nv. 10} for the tenth nerves, immediately posterior to the auditory 

 capsules. 



In addition to the elements of the brain-case parachordals, 

 trabeculse, and auditory capsules there enter into the composition 

 of the skull another set of elements called visceral bars. These are 

 cartilaginous rods formed in the walls of the pharynx between the 

 gill-slits, and thus encircling the pharynx like a series of paired 

 half -hoops (Fig. 769, B, vs. b.). The corresponding right and left 

 bars become united with one another below by an unpaired cartilage 

 (Fig. 773, A, b. br.), forming a visceral arch, and the unpaired ventral 

 pieces may unite successive arches with one another in the middle 

 ventral line, thus giving rise to a more or less basket-like visceral 

 skeleton. It will be noticed that the visceral skeleton has a seg- 

 mental arrangement, being formed of parts arranged in an antero- 

 posterior series, whereas in the cranium there is no clear indication 

 of segmentation. There is, however, no exact correspondence 

 between the segments of the visceral skeleton and the metameres. 

 The visceral arches vary in number from four to nine : the fore- 

 most of them is distinguished as the mandibular arch, and lies 

 just behind the mouth ; the second is called the hyoid arch, and 

 the rest branchial arches, from the fact that they support the gills 

 in water-breathing forms. 



In all Craniata except the Cyclostomes the mandibular arch 

 becomes modified into structures called jaws for the support of the 

 mouth. Each mandibular bar divides into a dorsal and a ventral 

 portion called respectively the palato-quadrate cartilage (Fig. 773, 

 A, pal. qu.) and Meckel's cartilage (nick, c.) : the palato-quadrates 

 grow forwards along the upper or anterior margin of the mouth, 

 and unite with one another in the middle line, forming an upper 

 jaw : Meckel's cartilages similarly extend along the lower or 

 posterior margin of the mouth and unite in the middle line, 

 forming the lower jaw. The quadrate (qu.), or posterior end of 

 the palato-quadrate, furnishes an articulation for the lower jaw, 

 and often acquires a connection with the cranium, thus serving 

 to suspend the jaws from the latter. Thus each jaw arises from 

 the union of paired bars, the final result being two unpaired 

 transverse structures, one lying in the anterior, the other in the 

 posterior margin of the transversely elongated mouth, and moving in 

 a vertical plane. The fundamental difference between the jaws 

 of a Vertebrate and the structures called by the same name in an 

 Arthropod or a Polychsetous Worm will be obvious at once. 



The hyoid bar usually becomes divided into two parts, a dorsal, 



