Phylum Chordata 577 



copulation. Dorsally there are two fins, each preceded by a heavy 

 spine. The caudal fin or tail is heterocercal (Fig. 197), that is, the 

 vertebrae extend into the dorsal lobe. Anterior to each pectoral fin is a 

 series of six gill slits with the first gill slit being modified into the spiracle. 

 The cloacal opening is between the pelvic fins. 



The cartilaginous skeleton consists of three main portions : the 

 axial, the visceral, and the appendicular. The axial skeleton is formed 

 of the vertebrae and skull. The centra of each vertebra is hourglass 

 shaped, with the notochord persisting between them. There are well- 

 developed neural and heinal arches. The skull consists of the cranium, 

 two nasal capsules, and two auditory capsules. Associated with it are 

 portions of the visceral skeleton including the jaws and hyoid arch. 

 In addition there are five branchial arches supporting the gills. The 

 appendicular skeleton is the pectoral girdle, the pelvic girdle, and the 

 cartilage of the fins. 



The muscles of the body wall are segmentally arranged and make 

 possible the lateral undulations of the body b}- which the fish moves. 

 The muscles of the paired fins, head, and branchial region are more 

 highly modified. 



The long digestive tract begins in the mouth where the well-de- 

 veloped teeth grasp and hold the prey. On the floor of the mouth is 

 the flattened tongue. The mouth opens into the wide pharynx which 

 has connections to the gill slits and spiracles. Food passes from the 

 pharynx through the esophagus and then into the large J-shaped stomach. 

 The stomach is separated from the intestine by the pyloric valve. The 

 intestine has its surface enormously increased by a spiral membranous 

 fold, the spiral valve. The intestine terminates in the cloaca. Associ- 

 ated with the digestive tract are a series of digestive glands. The large 

 liver is formed of two lobes and stores its bile in a gall bladder from 

 which the bile passes through a bile duct into the intestine. There is a 

 pancreas, and at the junction of the intestine and cloaca is a small rectal 

 gland which secretes mucus. A spleen is also present in this portion of 

 the coelom. 



A transverse septum divides the portion of the coelom containing 

 the main part of the digestive tract from the heart region. 



Four chambers may be recognized in the heart : the thin-walled 

 sinus venosus which receives the venous blood, an auricle, a thick-walled 

 ventricle, and the conus arteriosus. From the conus arteriosus, the 

 ventral aorta carries the blood forward into the afferent branchial ar- 

 teries. After the blood is oxygenated in the gills, ejferent branchial 



