20 MANUAL OF ZOOLOGY. 



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

 bf 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. 



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 performed 

 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 ; 

 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 pan 

 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 function," 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, when com- 

 pared with the development of the nutritive and reproductive 

 functions. 



b. Morphological Type. The first point in which one 

 animal may differ from another is the degree to which the 



