28 MECHANICAL FUNCTIONS. 



tions never ceasing so long as the animal is alive. This 

 motion, when considered merely with respect to the 

 mechanism by which it is produced, cannot but excite 

 intense curiosity ; for we have no analogy in the organi- 

 zation of any other animal with which to compare it, 

 nor from all we know on the subject should we believe 

 it possible that a circular motion of a part of an animal 

 could be continued for any length of time in the same 

 direction. What animal or other substance will with- 

 stand perpetual twisting in one direction ? and yet if 

 there is no deception with respect to the revolutions of 

 these wheels, one would be led to suppose such a sub- 

 stance. The appearance is undoubtedly that of a con- 

 stant revolution of the wheel itself, but recent observers, 

 however, believe it to be only apparent, and the decep- 

 tion to be caused by a peculiar and exceedingly rapid 

 motion of the cilia on the margin of the wheels. 



Physalia, or Portuguese Man-of-War. This animal 

 greatly excels in size those we have heretofore descri- 

 bed, but scarcely ranks above them in organization ; its 

 powers of motion being merely such as to enable it to 

 rise and sink in the water. 



It consists of a large air bladder, of perhaps a quart in 

 capacity, which floats on the surface of the ocean, and 

 which serves it as a sail. Below this there is a bundle 

 of tentacula, with a mouth and stomach, the whole appear- 

 ing more like an inorganic mass than a living creature, 

 Fig. 16. fig. 16. These animals are very 



abundant in most parts of the At- 

 lantic ocean, where they appear 

 at a little distance like so many 

 large soap bubbles floating along 

 before the wind, only that they pre- 

 I sent the most vivid hues of color. 

 'Nothing, it is said, can exceed 

 the beauty of the spectacle pre- 

 sented by a numerous fleet of these 

 animals quietly sailing on the 

 smooth surface of a tropical sea. 

 Whenever the surface is ruffled by 

 the slightest wind, they suddenly 



