CHAPTER XV 



Pisces 



Classification 



Subclass Elasmobranchii. 

 Subclass Teleostomi. 



Order 1. Crossopterygii. 



Order 2. Chondrostei. 



Order 3. Holostei. 



Order 4. Teleostei, 

 Subclass Dipnoi. 



Characteristics 



1. Aquatic vertebrates. 



2. Breathe by gills (vascular outgrowths of the pharynx). In the 



Dipnoi a single or double swim bladder functions as a lung, 

 and the air is received at the surface of the water. 



3. The swim bladder of the Teleostomi is used as a hydrostatic 



organ. 



4. There are two pairs of fins, pectoral and pelvic, and also unpaired 



median fins. 



5. The skin usually has numerous scales, formed from the dermis, 



but covered with epidermis which may produce enamel. 

 Scales are suppressed in electric fishes, and rudimentary in 

 some other forms. Glandular cells are also found in the skin. 

 There are sensory mucus canals, for touch and chemical 

 sense. Some fishes have well-developed poison glands near 

 the fins. 



6. The lateral line organs along the trunk are for vibrations of 



low frequency. 



7. The muscle segments persist throughout life. There is no muscle 



in the dermis however. 



8. The gut ends in a cloaca in many; in others a distinct anus is 



situated in front of the genital and urinary apertures. 



224 



