448 



GENERAL ZOOLOGY 



covered by an operculum (Figs. 268 and 269). A swim bladder 

 is usually present. 

 Series 1. Ganoidei. Skeleton cartilaginous or bony, with 

 heterocercal or homocercal (outwardly symmetrical) tail fin. 

 Scales may be (1) the ganoid type (rhomboidal bony plates 

 covered with enamel or ganoin), (2) the cycloid type (bony 

 plates with evenly curved borders), (3) the ctenoid type (free 

 edge of scale spiny), or (4) the scute type (bony plate with 

 enameled spines and articulating with one another to form 

 a stiff armor. A swim bladder is present with an open duct 

 connecting with the pharynx or esophagus. There is a spiral 

 valve in the intestine. 



Fig. 268. — Diagram of one side 

 of the mouth and pharynx of a 

 fish, split lengthwise, c, gill- 

 clefts, separated by gill arches; N, 

 nasal pit; o, esophagus; t, tongue. 



Fig. 269. — Trutta irideus, the rainbow 

 trout, with the gill cover bent back to 

 show the gills. (After Jordan and Kel- 

 logg, Animal Life and Evolution.) 



Order 1. Crossopterygii. Jointed pectoral fins; continuous 

 median fin; ganoid scales; and paired swim bladder. Mostly 

 extinct. 



Example: Polypterus bichir, a living species found in the 

 River Nile. 



Order 2. Chondrostei. Cartilaginous skeleton ; persistent noto- 

 chord; heterocercal tail; body naked or covered with bony 

 plates. 



Examples: Acipenser fulvescens, lake sturgeon, length up to 

 6 ft.; Acipenser oxyrhynchus, common sturgeon found in the 

 North Atlantic and tributary streams, length up to 9 ft.; 

 Polyodon spathula, the spoon bill or paddle fish of the Missis- 

 sippi Valley, skin naked, length up to 6 ft. 



Order 3. Holostei. Bony skeleton; ganoid or cycloid scales; 

 terminal mouth provided with teeth ; lunglike air bladder con- 

 nected with the esophagus. 



