35O 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. 1 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. 2 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. 



