AND CIRCULATION. 91 



from one side of the heart to the other ; except during the 

 embryonic period, when there is an opening between the 

 two auricles, which closes as soon as respiration commences. 



242. In reptiles (Fig. 86), the venous blood from the body 

 is received into one auricle, and the oxygenated blood from 

 the lungs into the other. These throw their contents into the 

 single ventricle below, which propels the mixture in part to 

 the body, and in part to the lungs ; but as only the smaller 

 portion of the whole quantity is sent to the lungs in a 

 single circuit, the circulation is said to be incomplete. In 

 the Crocodiles, the ventricle has a partition which keeps 

 separate the two kinds of blood received from the auricles ; 

 but the mixture soon takes place by means of a special 

 artery which passes from the pulmonary artery to the aorta. 



243. In fishes (Fig. 87) the blood is carried directly from 

 the ventricle to the gills, which are the respiratory organs ; 

 whence it passes into the arteries for distribution to the sys- 

 tem in general, and returns by the veins to the auricle. 

 Here the blood, in its circuit, passes but once through the 

 heart ; but the heart of a fish corresponds nevertheless to 

 the heart of a mammal, and not to one half of it, as has 

 often been maintained. 



244. Crabs and other Crustacea have but a single ventri- 

 cle without an auricle. 



In the mollusks there is 

 likewise but a single 

 ventricle, as in Natica 

 (Fig. 88, h). Some have 

 in addition one or two 

 auricles. These auricles 

 are sometimes so dis- Fi - 88 ' 



joined as to form so many isolated hearts, as in the cuttle- 

 fish. Among Radiata, the sea-urchins are provided with 

 a tubular heart. 



