370 



COLLEGE ZOOLOGY 



see the headpiece at the beginning of this 

 chapter. Over the jaws the placoid scales 

 are modified as teeth with their points di- 

 rected backward and are used for holding 

 and tearing prey. A placoid scale consists of 

 a bony basal plate, with a spine in the center 

 composed of dentine, and is covered with a 

 hard enamel-like dentine. The method of 

 embryonic development of the scale and its 



dentinal nature indicate that placoid scales 

 are homologous with vertebrate teeth. Al- 

 though the best evidence denies the pres- 

 ence of enamel on the surface of the placoid 

 scale, the enamel-forming organ is present 

 as in developing teeth. The homology of the 

 teeth and scales is due to the fact that the 

 mouth lining is inturned skin and hence 

 possesses skin structures. 



Enamel-like 

 surface 



Basal plate 

 Dermis 



Figure 245. Detail of a placoid scale (dermal denticle) as seen in section. Because of the 

 pointed scales, the skin is rasplike in texture. Thus, if a shark brushes against a man in the 

 water, it could inflict a severe wound in his skin. (After Kerr.) 



Skeletal system 



The skeleton is composed entirely of 

 cartilage (gristle). The cartilaginous skele- 

 ton in the elasmobranchs is in all probability 

 a degenerate and not a primitive characteris- 

 tic as was formerly believed. There are two 

 main subdivisions of the skeleton: (1) the 

 axial and (2) the appendicular. The axial 

 skeleton consists of the vertebral column and 

 the skull. The vertebrae are hour-glass- 

 shaped (amphicoelous), and the notochord 

 persists in the lens-shaped spaces between 

 them. The skull is much more highly de- 

 veloped than that of the cyclostomes. It is 

 composed of ( 1 ) the cranium or brain case; 

 (2) two large anterior nasal capsules and 

 two posterior auditory capsules; and (3) the 

 visceral skeleton, made up of the jaws, the 

 hyoid arch, and 5 branchial arches support- 

 ing the gill region. The appendicular skele- 

 ton consists of the cartilages of the fins and 

 those of the pectoral and pelvic girdles 

 which support them. 



Digestive system 



The digestive tract is longer than the 

 body. Following the mouth (Fig. 244) is a 

 large pharynx into which open the spiracles 

 and gill slits. The phar\'nx leads into the short 

 wide esophagus which opens into the 

 U-shaped stomach. The posterior end of the 

 stomach ends at a circular sphincter muscle, 

 the pyloric valve. The intestine follows and 

 terminates in the cloaca and cloacal open- 

 ing. A slender, fingerlike rectal gland, which 

 apparently secretes mucus, attaches dorsally 

 at the junction near the point where the 

 small and large intestines join. Within the 

 intestine is a spiral fold of mucous mem- 

 brane called the spiral valve (Fig. 246), 

 which prevents a too rapid passage of food 

 and thus allows increased absorption. The 

 liver is large and consists of two long lobes; 

 its secretion, the bile, is stored up in a gall 

 bladder and empties through the bile duct 

 into the intestine. A pancreas and spleen 

 are also present. 



