AND CIRCULATION. 117 



from the ventricle to the gills, which are their chief respir- 

 atory organs ; thence it passes into arteries for distribution 

 to the system 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 nev- 

 ertheless to the heart of a mammal, and not to one half of 

 it, as has often been maintained, for the gills are not lungs. 



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

 cle, without an auricle. 

 In the mollusks, there is 

 likewise but a single ven- 

 tricle, as in Natica, (Fig. 

 88, h.) Some have in 

 addition one or two auri- 

 cles. These auricles are 



,. . . -, Fig. 88. 



sometimes so disjoined 



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

 Among Radiata, the sea-urchins are provided with a tubular 

 heart, 



