MARINE VERTEBRATES 



341 



often seen close to our shores and in the harbours and estuaries, 

 swimming in shoals with a graceful undulatory movement. Por- 

 poises move forward entirely by the vertical action of their powerful 

 horizontal tails, and extend their flippers only to change their 

 course or to arrest their progress. At short intervals they rise to 

 the surface, exposing their slate -coloured backs and dorsal fins for 

 a moment, and then immediately dive downwards in such a manner 

 as to appear to turn a series of somersaults. Occasionally they 

 will leap quite out of the water, exhibiting their white under surfaces, 

 which shine with a sudden flash when illuminated by the rays of a 



FIG. 244. THE COMMON POKPOISE 



bright sun. The blow-hole is the first part exposed, and if one is 

 sufficiently near the shoal a fountain of spray may be seen to 

 shoot into the air, and the outrush of the expired air may be 

 heard as each one makes its appearance. 



The true nature of the spouting of a cetacean seems to be very 

 generally misunderstood, the fountain of spray produced at each 

 exhalation giving the idea that the animal is expelling a quantity 

 of water from its nostrils. This, of course, is not the case ; for the 

 cetacean, being an air-breather, has no need to take in a supply of 

 water, as the gill -breathing fishes have. Air only is expelled through 



