CLASS PISCES : ORDEK DIPNOI. 



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. Sift. 



SKELETON OF A FISH. a. Pectoral Fins ; b. Ventral ; c. Dorsal ; d. Anal ; Tail, caudal. 



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. 



General Characteristics. The Double-breathing fishes 

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



Fiy. 815. 



Lepldosiren annectens, Mud-fish. T V 



cles. They thus ease the transition from the Amphibians to 

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

 of tropical rivers. 



