SUBPHYLUM VERTEBRATA 



323 



In all the vertebrates below the mammals the coelom is divided into 

 a pericardial cavity containing the heart and a general body cavity. In 

 the mammals it includes a pericardial cavity, a thoracic cavity containing 



Lung 

 Brain \ Neurali-ube 



Pancreas 



Ovary 



Mesonephros 

 Nofochorcf 



Pharynx 



Coelom 



Cloaca 



Gill 

 slits 



Heart Stomach 



Urinary 

 bladder 



Oviduct 



Fig. 223. — Diagrammatic longitudinal section of the female of a lower vertebrate. 



drawn, with modifications, from Hegner, "College Zoology," after Wiedersheim.) 



(.Re- 



the lungs, and an abdominal cavity filled with the organs of the digestive, 

 excretory, and reproductive systems. 



The general arrangement of organs within the body may best be 

 understood by the aid of diagrams (Figs. 223 and 224) showing ideal 



^Dorsal fin 



Fig. 224. — Diagrammatic cross section of a lower vertebrate. {From Parker and Haswell, 



" Text-book of Zoology.") 



longitudinal and cross sections, neither of which corresponds precisely to 

 the structure of any one vertebrate but both of which bring out general 

 relationships illustrated more or less completely in all. 



