PISCES 109 



the most primitive teleostome fishes, the Crossopterygii, it is used as 

 a lung when the water is foul; in A mia it is constantly functional 

 as an air-breathing apparatus; while in the Dipneusti (lung-fishes) it 

 is an elaborately pouched lung, used to tide the fish over a period of 

 drought. 



In certain other fishes that have acquired terrestrial habits, such 

 as the Climbing Perch, Anabas (Fig. 59), and in the air-breathing eel, 

 Clarias (Fig. 60), there is an extensive post-branchial chamber pro- 

 vided with labyrinthine or arborescent elaborations of the epithelium 

 that are highly vascular and play a pulmonary role. 



THE INTEGUMENT OF FISHES 



The integument of fishes differs from that of land vertebrates in 

 being soft and slimy on the surface, though usually well protected 

 beneath the soft epidermis by means of scales or plates composed of 

 hard materials such as bone and ganoin. The slimy condition of the 

 epidermis is not due to a deposit from the water, as is commonly 

 believed, but is a mucous secretion from numerous cutaneous 

 glands. 



The scales of fishes differ materially from those of reptiles, birds, 

 and mammals in that they are composed exclusively of dermal ele- 

 ments. In the land vertebrates the scales are mainly cornifications 

 of the epidermis. The great majority of fishes are more or less com- 

 pletely covered with scales of moderate size, but some fishes such as 

 the eels, cat-fishes, etc., have secondarily lost their scales or have 

 them in a degenerate condition, imbedded deeply in the skin; other 

 fishes have the scales replaced by bony plates, which may form a more 

 or less solid armor, as in the trunk fishes (Fig. 93) and the porcupine 

 fishes in which the plates are provided with spines. The possession 

 of a coating of small, equally distributed scales is considered as the 

 primitive or generalized condition for fishes, and the loss of scales 

 or the development of heavy plates, as specialized or senescent condi- 

 tions. 



Four main types of scales are distinguished: placoid, ganoid, cy- 

 cloid and ctenoid. The placoid scale, characteristic of elasmobranch 

 fishes, is believed to be the original type of scale. It consists of a 

 basal plate of bony substance derived from the dermis and a spine- 

 like external protuberance covered with an enamel-like substance de- 

 rived from the epidermis. In the sharks and their kin it is clear that 



