508 VERTEBRATE LIFE AND ORGANIZATION 



vertebral column is composed ol vertebrae, each of which has a bicon- 

 cave centrum, which develops around and hirgely rephices the notochord. 

 Dorsal to each centrum is a neural arch encasing the spinal cord. Short 

 ribs attach to the vertebrae. A sternum is absent. The individual verte- 

 brae are rather loosely held together. A strong vertebral support is not 

 necessary in the aquatic environment. 



Most ol the skull oi the dogfish is an odd-shaped box of cartilage 

 encasing the brain antl major sense organs. This belongs to the axial 

 skeleton and is known as the chondrocranium. It forms the core of the 

 skull of all vertebrates. Other basic components of a vertebrate skull 

 include the anterior arches of the visceral skeleton and dermal bones 

 that encase the chondrocranium and anterior visceral arches. These 

 dermal bones have been lost during the evolution of cartilaginous fishes, 

 but they were present in the fishes ancestral to tetrapods. 



The visceral skeleton consists of seven pairs of > -shaped visceral 

 arches. The arches are hinged at the apex of the >; they are intercon- 

 nected ventrally, but are free dorsally. Each arch lies in the wall of the 

 pharynx and supports gills in primitive vertebrates. In jawed vertebrates 

 the first or mandibular arch becomes enlargeci and, together with asso- 

 ciated dermal bones, forms the upper and lower jaws. It forms all of 

 the jaws in the dogfish, for there are no surrounding dermal bones. The 

 second or hyoid arch has moved forward in the dogfish and helps to 

 support the jaws. Its dorsal portion extends as a prop from the otic 

 capsule (the part of the chondrocranium housing the inner ear) to the 

 angle of the jaw. The gill slit that in primitive fish lay between the man- 

 dibular and hyoid arches is reduced to a spiracle. The third to seventh 

 visceral arches are known as branchial arches; they support the gills and 

 complete gill slits lie between them. 



The appendicular skeleton is very simple in the dogfish. A 

 U-shaped bar of cartilage, the pectoral girdle, lies in the body wall 

 posterior to the gill region and supports the pectoral fins. The pelvic 

 girdle is a transverse bar of cartilage in the ventral body wall anterior 

 to the cloaca. It supports the pelvic fins but is not connected with the 

 vertebral column. 



The Mammalian Skeleton. Many changes in the skeleton have 

 taken place during evolution of the skeleton from primitive fishes to 

 mammals (Fig. 25.5). The vertebral column must support the weight 

 of the body in all tetrapods and it has become much stronger. It is 

 thoroughly ossified, and the individual vertebrae are strongly united 

 by overlapping articular processes (zygapophyses) borne on the neural 

 arches. Correlated with changes in the methods of locomotion and 

 the independent movement of various parts of the body, we find 

 that there is more regional differentiation of the vertebral column. 

 Man has seven cervical vertebrae, twelve thoracic vertebrae, five lumbar 

 vertebrae, five sacral vertebrae fused together to form a sacrum that 

 articulates with the pelvic girdle, and three to five reduced caudal 

 vertebrae generally fused into a single piece, the coccyx. Only the 

 thoracic vertebrae bear distinct ribs, most of which connect, via the 

 costal cartilages, with the ventral breast bone, or sternum. Rudimen- 



