v. 6 BRANCHIAL ARCHES 145 



the auditory capsules, the cranium is completed by the addition of a 

 number of segmented elements, evidently modified vertebrae. 



The problem is, therefore, to determine the nature of the pro-otic 

 part of the skull. Before we can settle this we must consider the 

 visceral or branchial arches. These are pairs of rods of cartilage 

 developed in the walls of the mouth and pharynx, between the gill- 

 slits. In the dogfish each typical branchial arch (Fig. 96) consists of a 

 series of four pieces, the pharyngo-, epi-, cerato-, and hypo-branchials. 

 Ventrally some of the arches join a median basibranchial plate. These 

 rods lie in the pharynx wall and on their outer sides carry a series of 

 projecting rods, the branchial rays and extrabranchial cartilages, 

 whose function is to support the lamellae of the gills. 



There are five such branchial arches, differing only slightly from 

 each other. In front of these lie two arches, the hyoid and mandibular, 

 which, though modified, are obviously of the same series. The hyoid 

 the more nearly resembles a typical branchial arch. Its most dorsal 

 element, the hyomandibular cartilage, is a thick rod attached dorsally 

 to the skull by ligaments and at its lower end forming the support 

 for the hind end of the jaw. It apparently corresponds to the epi- 

 branchials. The more ventral elements, cerato- and basihyal, resemble 

 the corresponding members of more posterior arches. The jaws them- 

 selves (mandibular arches) depart more widely from the form of a 

 typical branchial arch, but the two thick rods of which each is com- 

 posed, the upper palato-pterygo-quadrate bar and the lower Meckel's 

 cartilage, are recognizably members of the branchial series. Looking 

 at the whole apparatus with a thought to the embryological processes 

 that have produced it, with as it were a manufacturer's eye, we can 

 see at once that the jaws and hyoid arch have been produced by a 

 modification of the processes that make the branchial arches. 



6. The jaws 



Study of the serial relationship of the jaws and branchial arches 

 gives us an understanding of the course of evolution of the mouth. 

 We may suppose that the ancestors of the gnathostomes possessed a 

 nearly terminal mouth, either on the front end of the body or on the 

 ventral surface. The pharynx was pierced by a series of gill pouches, 

 beginning shortly behind the mouth and separated by arches, each 

 containing a set of cartilaginous bars (Fig. 96). There is some evidence 

 that this condition persisted in the cephalaspids (p. 125), where there 

 is found to be a series of ten pairs of gill-slits, beginning far forward 

 on either side of the mouth. The muscles moving the more anterior 



