SECTION III 



FISHES 



CHAPTER I 



INTRODUCTORY 



Definition. General Characters. Position in the Animal Kingdom, and 

 Classification. 



OF the myriads of animals, belonging to widely different 

 types of structure, which live in the sea and the fresh 

 waters, fishes are the most perfectly adapted to 

 motion in the liquid medium. Many marine animals are fixed 

 to the sea-bottom, like the corals and sponges, or crawl about 

 on it, like the star-fishes, sea-urchins, and their allies ; others 

 again merely float passively in the water, drifting with the 

 tides and currents, like the jelly fishes ; others, although they 

 have organs of locomotion, have them only in the form of 

 lateral appendages, like some marine worms and the Crustacea, 

 and cannot swim with any great speed. Some of the squids 

 and cuttle-fishes, belonging to the class Cephalopoda of 

 scientific classification, approach nearer to fishes in their 

 powers of aquatic locomotion, the free-swimming or pelagic 

 forms of this type having elongated bodies provided with 

 lateral fins, by which they are enabled to move easily and to 

 change the direction of their movements at will ; but they are 

 far inferior to the fishes in their mastery of their medium. 



Fishes possess the fusiform or spindle-like shape which is 

 mechanically best suited to motion through water and their 

 jointed backbone and powerful muscles afford them the most 

 efficient means of propulsion. They realise in fact the mecha- 

 nical conditions which human ingenuity has evolved in the 

 screw-propelled steam-ship ; the hull of the vessel in the sub- 

 merged part, and more completely in the submarine, is similar 

 in shape to a fish, but as no revolving motion of one part on 



231 



