78 HIGH SCHOOL ZOOLOGY. 



flivision of the caudal fin, but they have no air-bladder. The 

 eggs ai-e of large size and are either laid covei-ed by a peculiar 

 horny shell or else the young are bom alive. 



29. Only another sub-class remains to be discussed, that of the 

 Dipnoi, so called on account of the fact that they breathe by the 

 modified air-bladder as well as by gills. Three genera belong 

 here, widely separated geographically, but all similarly situated 

 in that, during the dry season they may have to depend wholly 

 or partly on their lungs for the oxygenation of the blood. They 

 are named Lepidosiren, Pi-otopterus (Fig. 55) and Ceratodus 



Fig. 55.— The African Lung Fish. Protopterus annectens. A* 

 (After Claus.) 



and they are found respectively in the Amazons region of South 

 America, on the west coast of Africa, and in Queensland- As 

 regards the structure of the air-bladiler they resemble Lepidos- 

 teus and Amia, but in respect to the opening place of the air- 

 bladder into the oesophagus as well as to the skeleton and fins, 

 they more nearly resemble Polypterus. The group is of special 

 interest on account of the amphibious habits, and the changes 

 in the respiratory and vascular system rendered necessary there- 

 by. A point in which they i-esemble the true Amphibia is that 

 the nasal chamber open into the mouth, which is not the case 

 in any of the other fishes. 



.30. Two aberrant gi-oups of Vertebrates are generally associ- 

 ated with the fishes on account of their fish-like appearance, 

 although in structure they are very unlike them. These are 

 the Lampreys and the Lancelets. The former no doubt owe 

 some of the peculiarities of their structure to their parasitic 

 habits. They attach themselves by their round (Cyclostomi) 



