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water. It must be remembered that in such cases the medium 

 in which the organisms are immersed affords them very con- 

 siderable support, not only for their bodies, as a whole, but for 

 all their appendages and outgrowths, if such be present. This, 

 of course, is due to the fact that the majority of such organisms 

 are, in the main, made up of tissues having a specific gravity 

 very little greater than that of water, and therefore the most 

 delicate and jelly-like creatures can live in water without 

 difficulty. And, as a matter of fact, the most delicate and 

 fragile of all known organisms are found in water. In con- 

 sequence of this equable support, and the small amount of 

 friction involved, very little energy has to be expended to pro- 

 duce slow movements, and if the organisms are actually of the 

 same specific gravity as water, and can get their nourishment 

 directly from the substances dissolved therein, as sometimes 

 happens, it is not necessary for them to be provided with 

 swimming organs at all. They can then depend entirely for 

 change of position upon the never-ceasing movements, due to vari- 

 ous physical causes, taking place in the water itself. As water, 

 compared with dry land, offers such a small amount of resistance, 

 it is at once apparent that for movement at any but the slowest 

 speeds the locomotive organs must either possess very large 

 surfaces, so as to get a grip of the water, or, if small, they 

 must act in a very rapid manner. Numerous instances of both 

 of these arrangements occur among microscopic aquatic organisms. 

 The simplest mode of progression is that found in Amoeba, the 

 second is produced by the action of flagella, and, in the third 

 type, by the action of cilia. More specialised modes follow, 

 and include medusoid motion, the method adopted by Nema- 

 todes, another method in the case of Salpa, and the highest 

 method of locomotion — that due to appendages actuated by 

 muscles — in more complete forms. A point for consideration 

 in connection with movement under water may be here referred 

 to. It is the difficulty of keeping a straight course owing to 

 the want of a datum line, or even fixed points, for both 

 horizontal and vertical directions, from which bearings may be 

 taken. In submarine vessels some automatic arrangement is 



