212 



ZOOLOGY 



SECT. 



eye rather crude and glaring, they appear to be distinctly pro- 

 tective, harmonising with the brilliant hues of the Coral Polypes 

 and other members of the reef fauna. Pelagic fishes, such as the 

 Mackerel and Herring, are usually steely-blue above, white 

 beneath. 



Many deep-sea Teleostei are phosphorescent : in some of these 

 definite luminous organs (Fig. 834) are arranged in longitudinal 



FIG. 834. Stomias boa. The white dots are the luminous organs. (From Hickson., after 



Filhol.) 



rows along the body, each provided with a lens, like that of the 

 eye, the whole organ having thus the characters of a minute 

 bull's-eye lantern. Some species of the same order, such as the 

 Weaver (Trachinus), possess poison- glands, opening either on one 

 of the dorsal spines, or on a spinous process of the operculum, or, 

 as in the Cat-fishes (Siluridse), on the spine of the pectoral fin. 



Exoskeleton. In many Teleostomi, such as Polyodon and 

 the Eels, the skin is devoid of hard parts, but in most cases a 

 dermal exoskeleton is present. In Amia and in the majority of 

 Teleostei this takes the form, as in the Trout, of scales, rounded 

 plates of bone imbedded in pouches of the derm and overlapping 

 one another from behind for- 

 wards. When the free border A ^gsnz B 

 of the scales presents an even 

 curve, as in Amia and most 

 Physostomi and Anacanthini, 

 they are called cycloid scales 

 (Fig. 808) ; when, as in most 

 Acanthopteri, the free edge 

 is produced into small spines 

 (Fig. 835, A) they are dis- 

 tinguished as ctenoid scales. 

 In exceptional cases the 

 scales may be so large and 



strong as to form a rigid armour. In the Sturgeon (Fig. 824) 

 there is a strong armour, formed of stout bony plates, or scutes, 



i'i<;. 635. A, ctenoid scale; B, ganoid 

 (After Giinther.) 



