254 



PISCES 



the gar-pike. The outer surface " ganoin " is extremely hard and 

 smooth and readily polished. (Figure 138.) 



Teleostean fishes have two types of scales, cycloid and ctenoid. 

 Cycloid scales are circular in shape and overlap. Ctenoid scales are 

 rounded but have a serrated comb-like edge that makes them firmer 

 in position. Flounders have ctenoid scales on the upper side and 

 cycloid scales on the under side. The pipe-fish and the sea horse 



have scales fused into a 

 bony armor. Embryonic 

 eels have scales, but in 

 adults it is impossible to 

 see even traces of them 

 with the naked eye. 



In some elasmo- 

 branchs and t e 1 e o s t s 

 from the depths of the 

 ocean we find epidermal 

 organs modified from 

 glands into luminous 

 organs or "photophores." 

 Some fishes like shad, 

 herring, and menhaden 

 have in the tela subcu- 

 tanea, crystals of guanin, 

 which form the base of 

 Ctenoid; 3, ganoid; 4, placoid. (From Hertwig- pearl essence. (See page 

 Kingsley. Courtesy of Henry Holt & Co.) 1 57-) 



Fish have chromato- 

 phores (melanophores) and change color to match their background, 

 but when blinded the color changing stops. 



Poison Glands. — When the poisonous secretion produced by 

 glands at the bases oi dogfish spines is injected into the lateral line of 

 a fish, respiration is accelerated and the animal soon becomes 

 lethargic. Eagle rays and sting rays, have barbed serrated spines 

 at the base of the tail and the glandular secretion accompanying a 

 blow from the tail, undoubtedly adds to the effects produced by the 

 irritation. Several teleostean fish including the catfish, have poison 

 glands associated with spines on the dorsal fins or in the axillary 

 region. 



Blood. — In fishes the blood consists of a nutritive ^\nd, plasjna^ 



Fig. 138^. Scales of fishes. /, cycloid; 2, 



