32 ELEMENTS OF BIOLOGY. 



is a much more complicated process. There is now a dis- 

 tinct mouth (a) provided with a masticatory apparatus, and 

 opening into a gullet which is furnished with salivary glands 

 {b). The gullet conducts to the stomach, which, in turn, 

 opens into a long and convoluted intestine {dd), w^hich is 

 completely shut off from the general cavity of the body, and 

 which terminates in a permanent aperture {e), by which the 

 indigestible portions of the food are got rid of. A well- 

 developed liver is also present. The nutritive products of 

 digestion are now propelled through all parts of the organ- 

 ism by a permanent contractile organ or heart {h). Lastly, 

 the function of respiration is carried on by distinct and com- 

 plex organs or gills (^), whereby the blood is submitted to 

 the action of the oxygen contained in the surrounding 

 water. 



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 pro- 

 vided, the complexity of the organism thus necessarily 

 increasing J>ari passu with the complexity of the function. 

 This gradual subdivision and elaboration is carried out 

 equally with the other two physiological functions — viz., 

 reproduction and correlation — and it constitutes what is 

 technically called the " specialisation of functions," though 

 it has been more happily termed by Milne-Edwards "the 

 principle of the physiological division of labour." As has, 

 however, been already remarked, in any physiological com- 

 parison of organisms one with another, it is at once seen 

 that the functions of relation stand in quite a different posi- 

 tion to that occupied by the functions of nutrition and re- 

 production. As far as these last are concerned, there can 

 be no difference in the amount or perfection of the function 

 discharged by the organism. The simplest and most 

 degraded of animals — say a sponge — nourishes itself as 

 perfectly, as far as the result to itself is concerned, as does 



