THE ENDOSKELETON: SKULL AND VISCERAL SKELETON 99 



This streak is the notochord. Its anterior end turns dorsally into the cartilage 

 and terminates at about the level of the small median foramen lying near the 

 anterior end of the basal plate through which the internal carotid artery passes. 

 It will be understood that those portions of the chondrocranium lying to either 

 side of the notochord are the products of parachordals and that the parts anterior 

 to this up to the olfactory capsules are the products of the prechordals. The 

 parachordals are fused laterally with the otic capsules and the prechordals with 

 the olfactory capsules. The lateral and dorsal walls of the brain are produced 

 by extension of these original parts. The posterior end of the ventral surface 

 of the skull is produced at either side of the notochord into a slight process, the 

 occipital condyle, which articulates with the first vertebra. Draw a ventral 

 view of the chondrocranium. 



B. THE VISCERAL SKELETON 



The visceral skeleton or splanchnocranium is that part of the endoskeleton 

 which supports the gills. The gills are located in the ventral and posterior part 

 of the head. To support the gills and enable them to be moved for respiratory 

 purposes a special skeleton is present which is called the visceral skeleton, 

 because gills are part of the walls of the digestive tract, as will be demonstrated 

 later. The visceral skeleton consists of a longitudinal series of crescent-shaped 

 cartilages (or bones) situated between the gill slits in the pharyngeal wall. Each 

 such element is designated a gill arch. There are typically seven gill arches in 

 vertebrates, although some elasmobranchs have nine (Fig. 33). The gill arches 

 in the lower vertebrates are closely associated with the chondrocranium, and in 

 the course of evolution some of them take part in the production of the skull. 



i. The visceral skeleton of the dogfish. Obtain a specimen in which the 

 visceral skeleton has been left attached to the chondrocranium, and study it 

 carefully. The seven gill arches form a series of curved cartilages ventral to 

 the posterior part of the chondrocranium and extending posteriorly to the pectoral 

 girdle. The first gill arch, the mandibular arch, is the largest and most modified 

 of the series. It is seen when viewed from below to consist of dorsal and ventral 

 halves. Each side of the dorsal half is called the palatoquadrate or pterygoquadrate 

 cartilage; in profile view it will be seen that this cartilage is closely applied to the 

 ventral surface of the chondrocranium, to which in life it is attached by a liga- 

 ment. It sends up a well-developed palatal process into the orbit. The pterygo- 

 quadrate cartilages bear teeth and, in fact, constitute the upper jaw of the animal. 

 The ventral half of the mandibular arch consists of two halves, each of which is 

 known as Meckel's cartilage. These bear teeth and together constitute the lower 

 jaw of the dogfish. The wide gap between the two jaws is the mouth opening. 

 At their posterior ends the pterygoquadrate and Meckel's cartilages join at an 

 acute angle called the angle of the jaw, forming a hinge joint, permitting opening 

 and closing of the mouth. 



