THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS 163 



line of the pharynx. At its distal end it divides into two sacs. The tube is the trachea or wind- 

 pipe, and the two sacs at its end are the lungs. Both trachea and lungs subsequently enlarge 

 and become subdivided, and their structure is further complicated by the addition of mesoder- 

 mal tissues (connective tissue, smooth muscle, cartilage) to the original simple entodermal 

 layer. 



h) The swim bladder: This is an outgrowth from the digestive tract, occurring only in the 

 teleostome fishes. It is at first connected with the digestive tract, generally with the esophagus, 

 on either dorsal or ventral side by a duct, which persists throughout life in ganoids. The swim 

 bladder lies just internal to the dorsal body wall of teleostomes and is supposed to have hydro- 

 static functions. It is not improbable that the swim bladder is the forerunner of lungs. 



i) The liver: The liver is a very large gland which arises by outgrowth from the intestine. 

 The stalk of the outgrowth becomes the bile duct, which enlarges on the surface of the liver to 

 form the gall bladder. The liver is situated between the two layers of the ventral mesentery 

 and is also located in the transverse septum but has grown so large that it projects extensively 

 out of these structures (see Figs. 44.B and 48). 



j) The pancreas: This gland arises by one to four, generally three, outgrowths, from the 

 intestine slightly posterior to the origin of the liver. The stalks of. the outgrowths become the 

 pancreatic ducts. The outgrowths combine to form one gland of various form in different 

 vertebrates. The pancreas may lie between the two layers of the dorsal mesentery, the 

 ventral mesentery, or both. 



k) The yolk sac: In the embryos of most vertebrates a yolk-filled sac projects from the 

 intestine. The wall of this sac is part of the intestinal wall, which may be regarded as having 

 been stretched out into a sac. The yolk in the sac is used for the growth of the embryo, and 

 at the completion of the embryonic stage the yolk sac is greatly reduced in size and is drawn 

 into the intestinal wall. 



/) The urinary bladder: This is a large sac growing out from the ventral wall near the 

 posterior termination of the intestine. In the embryos of reptiles, birds, and mammals it is 

 greatly enlarged, projecting beyond the limits of the body, and is called the allantois. The 

 urinary bladder is inclosed between the two layers of the ventral mesentery. 



The structures discussed in this introduction will be better understood after they have 

 been studied in the dissections. 



References on the coelom, digestive, and respiratory systems are: K, pages 6-20, 220-23, 

 232-85; W, pages 257-71, 283-316; Wd, pages 308-13, 324-92; P and H, pages 84-90. 



C. THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS OF ELASMOBRANCHS 



The following directions apply chiefly to the spiny dogfish but may also be 

 used for the smooth dogfish and skate, the minor differences between these 

 animals being specified where necessary. 



1. The body wall and the pleuroperitoneal cavity.— Make an incision from 

 the left side of the cloaca forward through the pelvic girdle slightly to the left 

 of the midventral line up to the pectoral girdle. The incision will probably 

 cut through the skin first and should then be extended through the muscle layer. 

 To assist in exposing the interior a transverse incision may be made in the 

 middle of the lateral body wall on each side. In the skate cut along the left side 

 of the cloaca and then along both lateral borders of the body cavity but not 

 anteriorly leaving the flap of body wall adhering to the pectoral girdle. The 

 large internal cavity is the pleuroperitoneal cavity and constitutes the greater 



