12 MANUAL OF ZOOLOGY. 



examine the method in which one of the three primary func- 

 tions 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 

 without any special organs for the purpose, and for the last 

 there is simply a rudimentary contractile cavity. Respira- 

 tion, 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 Ccelenterate animal, such as the 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 circula- 

 tory 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 performed 

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

 tractile cavity, or heart, is provided for the propulsion of the 

 nutritive products of digestion through all parts of the 

 organism, and the function of respiration is performed by 

 complex organs specially adapted for the purpose. 



It is not necessary here to follow out this comparison 

 further. In still higher animals the function of nutrition 

 becomes still further broken up into secondary functions, for 

 the due performance of which special organs are provided, the 

 complexity of the organism thus necessarily increasing pari 

 passu with the complexity of the function. This gradual sub- 

 division and elaboration is carried out equally with the other 

 two physiological functions, viz. reproduction and correlation, 

 and it constitutes what is technically called the ' specialisa- 

 tion of functions,' though it has been more happily termed by 

 Milne-Edwards ' the principle of the physiological division of 

 labour.' It is needless, however, to remark that in the higher 

 animals it is the functions of correlation which become most 

 highly specialised, disproportionately so, indeed, wl^en com- 

 pared with the development of the nutritive and reproductive 

 functions. 



