2 68 TEXT-BOOK OF ZOOLOGY 



The rays of the caudal fin are firmly united with the posterior vertebrae 

 (compare with the tail of birds). 



(b) A fish is capable of floating at rest in any particular spot of its 

 own selection. It follows, hence, that its body is invariably of the same 

 weight as the quantity of water which it displaces. (What would happen 

 if its body were heavier or lighter than this bulk of water ?) The fish 

 being thus supported by the water, its limbs do not require to be strong, 

 especially as they take no considerable share in the animal's locomotion 

 (see Section 2, d). (Contrast their function in this respect in most other 

 vertebrates.) 



The most important organ of locomotion is, in fact, the tail and its fin. 

 If we watch a fish suddenly darting forward from a position of rest, we 

 notice that it is the tail and caudal fin which are called into action, and 

 not the limbs, while in the case of a fish rapidly swimming along, the 

 forward movement results from alternate right and left strokes of the 

 same organs. We may get some idea as to the manner in ichich this 

 motion is effected by watching a kind of rowing often employed in 

 harbours and by watermen, which is called sculling. Here the oars- 

 man standing at the end of the boat handles a single oar, which rests 

 in a notch in the edge of the boat, moving it with a twist from left to 

 right, and vice versa. At each stroke the oar exerts a pressure directed 

 obliquely backwards against the water, which has the effect of pushing 

 the boat a certain distance forwards. In a similar manner the fish 

 propels itself forwards by twisting, sideward strokes of its tail. 



The importance of the tail as an organ of propulsion also explains 

 its size and strength. (Compare with the tail of other vertebrates, and 

 consider also the action of this organ in lizards, snakes, and tailed 

 amphibians.) Hence the tail of fishes is formed apart from the 

 vertebral column, which forms its central support exclusively of 

 muscles, which extend forward in four large bands as far as the head. 

 (Separate these muscular bands in a cooked fish or red-herring.) The 

 two stouter of these bands lie along the back, the thinner ones at the 

 sides of the body and tail; and all of them are composed of a large 

 number of transverse muscular segments (myotomes), which fall away 

 from each other when the fish is boiled. (Where are the largest muscles 

 situated in other vertebrates, and why ?) The force exerted by these 

 muscles is very considerable. Thus, a salmon is able to accomplish a 

 distance of from 24 to 27 feet per second, and can leap over weirs and 

 waterfalls up to 13 feet in height. (How does a fish behave when it 

 happens to get thrown upon dry land ?) 



If it desires to alter its course, the fish bends its body into a 

 curve, so that head and tail are pointed towards the side it wishes to go. 



