THE ANIMAL KINGDOM 449 



Examples: Lepisosteus osseus, common garpike, in fresh 

 water, length up to 4 ft.; Amia calva, fresh-water dogfish or 

 bowfin, length up to 2 ft. 



Order 4. Teleostei. Bony skeleton; homocercal tail; cycloid 

 or ctenoid scales; gills covered by an operculum; intestine 

 without spiral valve. There is usually a swim bladder. The 

 majority of fishes belong to this order. 



Examples: Coregonus clupeaformis, whitefish of northern 

 lakes; Oncorhyncus tschawytscha, Chinook or king salmon of 

 Pacific Coast; Trutta irideus, rainbow trout; Carpiodes carpio, 

 carp sucker of Ohio Valley southward; Ictalurus furcatus, chan- 

 nel catfish, has no scales; Ameiurus nebulosus, common bull- 

 head, no scales. 



SUBCLASS 2. DIPNOI. Ltjngfishes. A paired or unpaired 

 swim bladder opening into the esophagus functions as a lung, 

 though gills are also present. The nasal passages are respira- 

 tory and connect with the pharynx. The notochord persists 

 as an unsegmented rod. The skeleton is partly ossified 

 cartilage. The pectoral fins are lobate and jointed; the scales 

 are cycloid. To a certain extent lungfishes represent a transi- 

 tion from fishes to amphibians. There are only a few living 

 species. 



Examples: Neoceratodus forsteri, the Australian lungfish, 

 lives in brackish water and frequently comes to the surface to 

 breathe air; Protopterus annectans, one of three African species, 

 aestivates in the dry season by burrowing in the mud, where it 

 forms a slimy cocoon about itself and remains in a state of 

 suspended animation until the return of the rainy season; 

 Lepidosiren paradoxa, a South American species that also 

 aestivates. 



CLASS IV. AMPHIBIA. Salamanders, frogs, and toads. 

 These are both terrestrial and aquatic animals, provided usually 

 with two pairs of pentadactyl limbs, lacking claws or nails on 

 the digits. Limbs and limb girdles are absent in the Apoda. 

 The integument is usually without scales. The eggs develop in 

 water or damp earth into swimming larvae (tadpoles), provided 

 with a tail and breathing by integumentary gills (Fig. 270). 

 The heart has a single ventricle and two atria except in the 

 case of lungless salamanders, in which there is a single atrium. 

 None is marine. 



