386 THE MECHANICAL FUNCTIONS. 



Diodons and Tetrodons are remarkable for being 

 provided with the means of suddenly assuming a 

 globular form by swallowing air, which, passing 

 into the crop, or first stomach, blows up the whole 

 animal like a balloon. The abdominal region being 

 thus rendered the lightest, the body turns over, the 

 stomach becoming the uppermost part ; and the 

 fish floats on its back, without having the power of 

 directing itself during this state of forced disten- 

 sion. But it is while lying thus bloated and passive 

 at the mercy of the waves, that this animal is really 

 most secure ; for the numerous spines, with which 

 the surface of the body is universally beset, are 

 raised and erected by the stretching out of the skin, 

 thus presenting an armed front to the enemy, on 

 whatever side he may venture to begin the attack. 



There is a numerous family of fishes, found in 

 the seas of India, so constructed as to be able to 

 crawl on land to some distance from the shore. 

 One of these, the Perca scandens, is even capable 

 of climbing on the trees which grow on the coast.* 



If we consider the density of the medium which 

 fishes have to traverse, the velocity with which they 

 move will appear surprising. They dart through 

 the water with apparently as much ease and ra- 

 pidity as a bird flies through the air.f Although 



* See the account given by Lieutenant Daldoiff in the Trans- 

 actions of the Linnean Society, III. 62. I shall have occasion to 

 notice, in the sequel, the remarkable conformation of the respiratory 

 orsfans of these and other fishes, which enables them to live for a 

 time out of their natural element. 



t Lacepede computes the velocity of the Salmon at eight metres 

 per second, which is about three miles in a minute, or 180 miles an 

 hour ! Some error must surely have crept into a calculation pro- 

 ductive of so extraordinary a result. 



