912 



CLASS PISCES. 



Fig. 832. — Cycloid scale, 

 enlarged. 



Fig. 



3.— Ctenoid scale, 

 enlarged. 



Mr Smith - Woodward, is to brigade the Teleostei and Ganoidei 

 together in a subclass under the name of Teleotomi, with ordinal 

 divisions differing somewhat from the subordinal ones employed 

 below, and to raise the Dipnoi, Chimeroidei, Elasmobranchei, and 

 Cyclostomi to the rank of subclasses, with the concomitant eleva- 

 tion of their respective suborders to the rank of orders. It seems, 

 however, scarcely to harmonise with the divisions adopted in the 

 other classes of Vertebrates to regard a group like the Chimeroidei 

 as a subclass, and accordingly the view of Professor Huxley is 

 provisionally followed of regarding the Elasmobranchei and Chi- 

 meroidei as divisions of ordinal value. 



Although the body in all Fishes must be adapted for progression 

 through the water, yet there is an enormous range of variation in its 



contour among the differ- 

 ent groups, as we may ob- 

 serve when we contrast a 

 Lamprey, a Shark, a Flat- 

 fish, a Ribbon-fish, and a 

 Globe-fish. The dermal 

 structures termed scales, 

 which are so characteristic 

 of Fishes, present many 

 types of structure. In the Teleostei they usually form thin plates, 

 frequently marked by concentric lines and not formed of true bone. 



When the posterior margin is simple 

 (or entire) such a scale is termed cycloid 

 (fig. 832), but when denticulated, cten- 

 oid {fig. 833). Other examples of this 

 type are shown in figs. 834, a, b. In 

 many Ganoids and a few Teleosteans 

 the scales are much thicker, and consist 

 of a variety of true bone covered exter- 

 nally with an enamel-like substance 

 termed ganoine. Such scales, of which 

 specimens are shown in fig. 834, e, and 

 fig. 835, are termed ganoid; they are 

 arranged in oblique rows, and connected 

 together by a peg-like projection, their 

 shape being oblong. Scales of ganoid 

 structure may, however, be much thin- 

 ner, and resemble the cycloid type in 

 their contour and their imbrication. 

 Lastly, the bony dermal scutes or plates, frequently armed with a 

 spine, which occur in the skin of the Sharks and Rays (fig. 834, 

 c, d), are strictly comparable in structure to teeth, consisting of 



Fig. 834.— a, Cycloid scale of Pike 

 Esox) ; b, Ctenoid scale of Perch 

 (Perca) ; c, Dermal plate of Thorn- 

 back (Raia); d, Do. of Monkfish 

 (Squatina); e, Ganoid scales of Palcz- 

 oniscus. a and b enlarged. 



