350 FISHES CHAP. 
a double wedge, the thick part of which is represented by the head 
and one of the thin edges by the free hinder margin of the caudal 
fin. The body is bounded by smooth flowing contour lines, unbroken 
by any sharp separation of the body regions from one another, and 
with no points of resistance to its forward motion through the 
water. The body being thicker in front than behind, and, as seen 
in transverse section, broader above than below, it follows that its 
centre of gravity will be nearer the head than the tail, and nearer 
the dorsal than the ventral surface. The dorsal position of the 
centre of gravity necessarily renders the equilibrium of the body 
unstable, and were it not for the balancing action of the paired 
fins the Fish would float belly upwards, as is always the case 
after death. Most Fishes are provided with a membranous gas- 
containing sac, the air-bladder, the principal function of which is 
to render the Fish, bulk for bulk, of the same weight as the 
water, so that in this position of equilibrium, or plane of least 
effort, the animal can execute its various locomotor movements 
with a minimum expenditure of muscular effort—an advantage 
which no other animal possesses." To give stability to the body, 
and to steady its course when swimming, the Fish has a dorsal 
and a ventral keel, formed by the anal and dorsal fins, which, 
like the sliding keel of a yacht, can be raised or lowered as 
occasion requires. When these fins are removed the course of 
the Fish becomes zigzag, and the animal wobbles. 
The organs more directly concerned with swimming are the 
tail and the caudal fin, and the pectoral and pelvic fins, but the 
relative share which these structures take in the actual pro- 
pulsion of the Fish differs greatly. The principal organ of loco- 
motion in the typical Fish is the powerful muscular tail, which, 
in swimming, is lashed from side to side by the alternating con- 
traction of the great longitudinal muscles on opposite sides of the 
vertebral column.” In such movements the tail is first flexed or 
bent, say to the right side: this stroke has been termed the non- 
effective or back stroke. By a stroke in the reverse direction the 
tail is then extended and straightened, that is to say, the Fish 
makes the forward or effective stroke. By a rapid succession of 
such strokes to the right and left sides alternately the Fish is 
1 See Chapter XI. 
* Pettigrew, Animal Locomotion, Internat. Sci. Series, London, 1874, p. 64 ; 
Gadow, Science for All (Cassell), v. p. 302. 
