CLASS PISCES : ORDER DIPKOl. 



185 



discharged at the gill-openings. Meanwhile, the blood cir- 

 culating in the gills absorbs the oxygen of the air which the 

 water contains, and is purified. As the heart has but one 



Fig. SlU. 



Skeleton of a Fish.— a. Pectoral Mm ; b. Ventral ; c. Dorsal ; d. Anal ; Tail, coMdal. 



auricle and one ventricle, the blood, after being aerated in 

 the gills, is not returned to the heart, but is driven directly 

 through the body. The mouth is the only prehensile organ. 

 All the species are essentially oviparous, a few being ovovi- 

 viparous. 



ORDER DIPNOI. 



G-eneral Characteristics. — The Double-breathing fishes 

 have both giJls and lungs, two pairs of limbs, and two auri- 



Flg. SIS. 



Mud-fish. tV- 



cles. They thus ease the transition from the Amphibians to 

 the Fishes. There is only one family, the singular Mud-fish 

 of tropical rivers. 



