14 MANUAL OF ZOOLOGY. 



physiological. One animal differs from another either morpho- 

 logically, in the fundamental points of its structure ; or physio- 

 logically, in the manner in which the vital functions of the 

 organism are discharged. These constitute the only modes in 

 which any one animal can differ from any other; and they may 

 be considered respectively under the heads of Specialisation of 

 Function and Morphological type. 



a. Specialisation of function. All animals alike, whatever 

 their structure may be, perform the three great physiological 

 functions ; that is to say, they all nourish themselves, repro- 

 duce their like, and have certain relations with the external 

 world. They differ from one another physiologically in the 

 manner in which these functions are performed. Indeed, it is 

 only in the functions of correlation that it is possible that there 

 should be any difference in the amount or perfection of the 

 function performed by the organism, since nutrition and repro- 

 duction, as far as their results are concerned, are essentially the 

 same in all animals. In the manner, however, in which the 

 same results are brought about, great differences are observable 

 in different animals. The nutrition of such a simple organism 

 as the Amoeba is, indeed, performed perfectly, as far as the 

 result to the animal itself is concerned as perfectly as in the 

 case of the highest animal but it is performed with the simplest 

 possible apparatus. It may, in fact, be said to be performed 

 without any special apparatus, since any part of the surface of 

 the body may be extemporised into a mouth, and there is no 

 differentiated alimentary cavity. And not only is the nutritive 

 apparatus of the simplest character, but the function itself is 

 equally simple, and is entirely divested of those complexities 

 and separations into secondary functions which characterise 

 the process in the higher animals. It is the same, too, with 

 the functions of reproduction and correlation ; but this point 

 will be more clearly brought out if we examine the method in 

 which one of the three primary functions is performed in two 

 or three examples. Nutrition, as the simplest of the functions, 

 will best answer the purpose. 



In the simpler Protozoa, such as the Amoeba, the process 

 of nutrition consists essentially in the reception of food, its 

 digestion within the body, the excretion of effete or indigestible 

 matter, and the distribution of the nutritive fluid through the 

 body. The first three portions of this process are effected with- 

 out any special organs for the purpose, and for the last there is 

 simply a rudimentary contractile cavity. Respiration, if it can 

 be said to exist at all as a distinct function,, is simply effected 

 by the general surface of the body. 



