The Skeleton of the Fish 



SI 



The Skeleton of Sharks. — In the Elasmobranchs (sharks, 

 rays, chimseras) the tissues surrounding the notochord are seg- 

 mented and in most forms distinct vertebra; are developed. 

 Each of these has a conical cavity before and behind, with a 

 central canal through which the notochord is continued. The 

 form and degree of ossification of these vertebrae differ materially 

 in the different groups. The skull in all these fishes is cartilaginous, 

 forming a continuous undivided box containing the brain and 

 lodging the organs of sense. To the skull in the shark is attached a 

 suspensorium of one or two pieces supporting the mandible and 

 the hyoid structures. In the chimsera the mandible is articu- 

 lated directly with the skull, the hyomandibular and quadrate 



Fig. 4.5. Fig. 46. 



Fig. 45. — Pectoral fin of Heterodontus philippi. (From nature.) 

 Fig. 46. — Pectoral fin of Heptranchias indicus (Gmelin). (After Dean.) 



elements being fused with the cranium. The skull in such case is 

 said to be autostylic, that is, with self-attached mandible. In the 

 shark it is said to be hyostylic, the hyomandibular intervening. 

 The upper jaw in the shark consists not of maxillary and 

 premaxillary but of palatine elements, and the two halves of 

 the lower jaw are representatives of Meckel's cartilage, which 

 is the cartilaginous centre of the dentary bone in the bony 

 fishes. These jaw-bones in the higher fishes are in the nature 

 of membrane bones, and in the sharks and their relatives all 

 such bones are undeveloped. The hyoid structures are in the 

 shark relatively simple, as are also the gill-arches, which vary 

 in number. The vertical fins are supported by interneural and 

 interhsemal cartilages, to which the soft fin-rays are attached 

 without articulation. 



The shoulder-girdle is made of a single cartilage, touching 



