ELASMOBRANCHII 427 



modified, is called the spiracle. It contains a rudimentary gill struc- 

 ture. The mouth aperture is somewhat the shape of an inverted U, 

 located on the ventral side of the head, and supplied with sharp 

 teeth on the jaws. These teeth are developed by modification of 

 the placoid scales which cover the skin over the body generally. 

 The placoid scales are primitive exoskeletal structures with a hasal 

 plate embedded beneath the skin and a spine projecting on the sur- 

 face. This spine has a pulp cavity, surrounded by dentine, which 

 is covered on its surface by enamel. This structure is considered 

 to be homologous to the vertebrate tooth. The paired nostrils are 

 openings on the ventral side of the snout, anterior to the mouth. 

 The eyes, the lids of which are immovable, are situated on the sides 

 of the head. The cloacal aperture is located between the bases of 

 the pelvic fins. 



Muscular System 



The segmental arrangement of myotomes, separated by myo- 

 eommas, is fairly complete along both sides of the body. The 

 principal specializations of independent muscles are found in the 

 form of myotome modification in the region of the mouth gills and 

 paired appendages. The trapezius found above the branchial area; 

 the superficial constrictors extending from the head to beyond the 

 gill slits and assisting in their operation; and the adductor man- 

 dihularis, connected with the lower jaw, are all examples of special 

 developments. 



Skeletal System 



The endoskeleton of the sharks is composed of cartilage. It con- 

 sists of axial skeleton (skull and vertebral column) ; visceral skeleton 

 (jaw and gill arches) ; and, appendicular skeleton (pectoral girdle 

 and fins, pelvic girdle and fins). The vertebral column and skull are 

 much more developed than in the cyclostomes. The notochord has 

 become segmented and partially replaced by cartilage. The centrum, 

 which has replaced a considerable portion of the notochord in each 

 vertebra, is deeply concave at each end, and is said to be amphicoelous. 

 Some of the remains of the notochord fills these interstices between 

 vertebrae. 



The skull is laid on a foundation of the ventral hasal plate. The 

 dorsal side is fairly well enclosed with cartilage. The anterior ex- 

 tension of the skull is the rostrum and the depression in its dorsal 



