H 



MANUAL OF ZOOLOGY. 



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

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

 same results are brought r.Jout, 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 di- 

 gestion 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. 



In a Coelenterate animal, such as a sea-anemone, the func- 

 tion of nutrition has not advanced much in complexity, but 

 the means for its performance are somewhat more specialised. 

 Permanent organs of prehension (tentacles) are present, there 

 is a distinct mouth, and there is a persistent internal cavity for 

 the reception of the food; but this is not shut off from the 

 general cavity of the body, and there are no distinct circulatory 

 or respiratory organs. 



In a Mollusc, such as the oyster, nutrition is a much more 

 complicated process. There is a distinct mouth, and an ali- 

 mentary canal which is shut off from the general cavity of the 

 body, and is provided with a separate aperture for the excre- 

 tion of effete and indigestible matters. Digestion is peiformed 

 by a distinct stomach with accessory glands ; a special contrac- 

 tile cavity, or heart, is provided for the propulsion of the nutri- 

 tive products of digestion through all parts of the organism, 



