CLASS ELASMOBRANCHII 425 



and two posterior auditory capsules. The visceral skeleton com- 

 prises the jaws, the hyoid arch, and five branchial arches. The 

 appendicular skeleton consists of the skeletons of the fins (B, R) 

 and those of the pectoral and pelvic girdles which support them. 



The Digestive System. — The alimentary canal is longer than 

 the body. Following the mouth (Fig. 359, M) is a large pharynx 

 into which open the spiracles and gill-clefts. The pharynx leads 

 into the short, wide oesophagus which opens into the U-shaped 

 stomach (S). The hinder end of the stomach is provided with 

 a sphincter, or circular muscle marking it off from the intestine. 

 The latter is provided interiorly with a spiral fold of mucous 

 membrane, called the spiral valve (I), which furnishes a large 

 surface for absorption and prevents the too rapid passage of 

 food. The liver (L) is large, and consists of two long lobes; its 

 secretion, the bile, is stored up in a gall-bladder and emptied 

 through the bile-duct into the intestine. A pancreas and spleen 

 are also present. 



The Circulatory System (Fig. 361). — As in the cyclostomes 

 and most of the true fishes, the heart (Fig. 361, s.v, au, v, cart) 

 contains venous blood only. This is pumped through the 

 ventral aorta (v.ao) and thence into the afferent branchial arteries 

 (a.br.a), becoming oxygenated in the capillaries of the gills. 

 It then passes into the efferent branchial arteries (e.br.a), which 

 carry it to the dorsal aorta (d.ao). The dorsal aorta supplies 

 the various parts of the body as shown in Figure 361. Veins 

 carry the blood back to the heart, opening into the sinus venosus 

 (s.v). Other veins, called the hepatic portal system (h.p.v), 

 transport the blood from the alimentary canal, pancreas, and 

 spleen to the liver. A third system, the renal portal system 

 (r.p.v), conveys the blood from the hinder portion of the body 

 to the kidneys. 



The Respiratory System. — Respiration is carried on by means 

 of gills. These are folds of mucous membrane well supplied 

 with blood-vessels and borne by the hyoid arch and first four 

 branchial arches. They are supported both by these arches and 



