TBLEOSTOMI 



209 



by gills, but when the water gives out or becomes unfit for use 

 the swim-bladder, which may be single or double, is used for 

 lungs. It opens into the ventral side of the gullet and contains 

 air-cells. In this case the air enters through the nose. 



They are interesting as showing how land forms may have 

 originated from aquatic forms. There are only three existing 

 genera: the Lepidosi'ren, of the Amazon; the Cer'atodus (Fig. 

 169), of Australia, and the Protop'terus, of Africa. The Protop- 

 terus (see Fig. 168) "can live out of water, it burrows in the 

 mud at the dry season and builds a cocoon lined with mucus in 

 which it remains quiescent until the wet season."^ 



SUB-CLASS IV. TELEOS'tOMI 



To this extensive sul^-class belong our bony fishes, including 

 most of the living fishes. It contains thousands of species. 



Fig. 170. — Remoras and shark, showing dorsal fins modified into sucking 

 disks, by which the remora attaches itself to the shark in its commensal life, 

 thus securing free transportation. (From Baskett, " The Story of the 

 Fishes," D. Appleton and Co., Publishers.) 



Familiar examples are the perch, sunfish, catfish, trout, carp, 

 pike, cod, and salmon. The mouth is terminal. The nostrils 

 are on the upper surface of the snout. The tail is homocercal 



' Hertwig. 

 14 



