THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS 199 



auditory tube and tympanic cavity. The entodermal lining of the visceral pouches persists 

 as certain glandlike bodies, such as the tonsils, the thymus, and the epithelial bodies. The 

 thyroid gland is an evagination from the floor of the pharynx between the second visceral 

 pouches. 



5. The pharynx in fishes leads into a single tube, the esophagus. Above fishes it leads 

 into two tubes, the esophagus and the trachea. At the caudal end of the trachea are the 

 paired lungs. 



6. The upper end of the trachea is expanded into a chamber, the larynx, the walls of which 

 are supported by the modified gill arches. 



7. Both esophagus and trachea become more and more elongated in higher forms, 

 owing to a descent of the viscera posteriorly. This descent leaves a space in the anterior 

 dorsal part of the body cavity into which the lungs grow out from the lower end of the trachea. 



8. Lungs are present in all vertebrates above fishes (they have secondarily disappeared 

 in some urodeles). They are probably homologous with the swim bladder of fishes. Their 

 walls become more and more complicated and divided into air spaces, the higher one ascends 

 in the phylogentic series. In birds the lungs connect with air sacs in the viscera and air spaces 

 in the bones. 



9. The intestine of vertebrates is provided with two glands, the liver and the pancreas, 

 which are attached by ducts to the duodenum (the bile duct is generally and the pancreatic 

 duct sometimes lacking in cyclostomes). 



10. The intestine terminates in all vertebrates except placental mammals (cyclostomes and 

 teleostomes) in a chamber, the cloaca, into which the urinary and genital ducts also open. 

 In mammals (and cyclostomes and teleostomes) the cloaca is lacking and the urinary and genital 

 ducts open separately from the intestine. 



11. In most vertebrates there is a saclike outgrowth, the urinary bladder, from the 

 ventral wall of the cloaca. 



12. In all vertebrates except birds and mammals the coelom is divided into two compart- 

 ments, the pericardial cavity containing only the heart and the pleuroperitoneal cavity contain- 

 ing the other viscera. The two cavities are separated by a partition, the transverse septum. 



13. In cyclostomes, fishes, and urodeles the pericardial cavity is anterior to the pleuro- 

 peritoneal cavity. From Anura on the pericardial cavity is ventral to the anterior part of 

 the pleuroperitoneal cavity. This change is due to a posterior descent of the heart and peri- 

 cardial cavity carrying the transverse septum with them. As a consequence of the descent the 

 wall of the pericardial cavity together with the transverse septum forms a sac, the pericardial 

 sac, around the heart. That portion of the pleuroperitoneal cavity dorsal to the heart is 

 destined to form the pleural cavities. 



14. In birds and mammals the pleuroperitoneal cavity is divided into anterior and pos- 

 terior portions by the formation of a partition which descends from the dorsal body wall and 

 unites with the transverse septum. This partition is called the oblique septum in birds and 

 the diaphragm in mammals. The diaphragm is more complex in origin than the oblique septum 

 and unlike the latter contains a considerable amount of striated muscle. Anterior to the 

 oblique septum or diaphragm are the two pleural cavities, each of which contains a lung; 

 posterior to these partitions is the peritoneal cavity, containing the digestive and reproductive 

 systems. Thus, in birds and mammals the coelom is divided into four compartments the 

 pericardial, the two pleural, and the peritoneal cavities. 



15. A dorsal mesentery supports the digestive tract in all vertebrates and remains practi- 

 cally complete throughout. The ventral mesentery of the digestive tract is absent in the adult 

 except in the regions of the liver and the bladder. There are special mesenteries for the gonads 

 and their ducts. In mammals the mesentery of the stomach develops a special posterior pro- 

 longation called the greater omentum. 



