156 



AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



The very young scales are covered by the epidermis, through 

 which the spines later on force their way. 



3. Endoskeleton. In an embryo dogfish a firm cellular rod, 

 the notochord, underlies the central nervous system, much as in 

 Amphioxus, but in this case only extending as far forwards as the 

 middle of the brain. A cartilaginous sheath is soon formed round 

 the notochord ; in the head-region the floor of a firm cartilaginous 

 brain-case is developed in connection with this ; in the trunk and 

 tail the sheath is transversely segmented into a series of joints, 

 flexibility being thus much increased. Further modifications and 

 additions result in a complicated endoskeleton, composed almost 

 exclusively of cartilage. 



'Pfi.br. 

 -Jlp. br. 



Fig. 43. DOGFISH. Skull and part of vertebral column (reduced). Au, 

 Auditory capsule; JVa, nasal capsule; //. , ///., /F., <fcc., nerve-exits; 

 /./, upper jaw; L.J, lower jaw; H.m, hyo-mandibular ; Cer.h, cerato- 

 hyal; B.hy, basi-hyal; Ph.br, pharyngo - branchials ; Ep.br, epi- 

 branchials; Cer.br, cerato-branchials ; B.br, basi-branchials ; c, centra, 

 continued up into neural plates; Sp, neural spines; Int, intercalary 

 pieces ; F. d, F. r, foramina tor dorsal and ventral roots of spinal nerves, 

 indicated for two nerves. 



It is convenient to consider the skeleton under two headings : 

 I. Skeleton of the body axial skeleton ; II. Skeleton of the fins 

 appendicular skeleton. 



I. Axial Skeleton (Fig. 43). This is divisible into (1) Skull, 

 (2) Vertebral Column arid Ribs. 



(1) The skull is characteristic of Yertebrata proper (Vertebrata 

 Craniota). It includes a cranium or brain-case olfactory and 

 auditory capsules ; and the visceral skeleton, consisting of jaws 

 and respiratory skeleton. 



