CLASS PISCES 



353 



375. Body Form. — The form of a primitive or typical fish is that 

 of a spindle, broadest in front of the middle. This is also the shape 

 of a submarine torpedo and is that shape which enables a body to cleave 



Fig. 249. — African lungfish, Protopterus annectens Owen. X M- {From Packard, 

 "Zoology," after Boas, by the courtesy of Henry Holt & Company.) 



the water with the least amount of retardation from resistance in front, 

 friction laterally, or suction behind. The fins, being thin in the plane 

 of movement, offer little interference. 



C D 



Fig. 250. — Scales of fishes. A, cycloid scale of northern pike, Esox Indus Linnaeus. 

 X 8. B, ctenoid scale of common perch, Perca flavescens (Mitchill). X 9. C, placoid 

 scales of dogfish shark, Squalus acanthias Linnaeus: a, surface view of a number; b, view of 

 dorsal surface of one; c, side view of one, isolated, the surface level being indicated by a line; 

 d, side view of one in position, a, X 30; b, c, and d, X 60. D, ganoid scales of Lepisoateus 

 osseus (Linnaeus). X 2. 



Those fish which are built for speed or live in swift currents approxi- 

 mate the typical spindle shape; those which live in quiet waters tend 

 to become flattened from side to side; and those which are adapted 

 for life close to the bottom become flattened dorsoventrally. A slender, 



