OSMOSIS. 49 



the intestine into the body cavity, or, as in Sacculina, art con- 

 veyed to it directly by the suckers. 



We hereby see that the ways and means by which animals 

 obtain the food they need are very various ; even mere external 

 appendages of the body may, like the roots of Sacculina, be 

 transformed into organs of nutrition. But there are other ways 

 in which we see an essential difference in the mode of obtaining 

 nourishment that characterises different animals. All animals 

 that have well-developed internal organs of nutrition are com- 

 pelled, under the influence and guidance of their will and their 

 subjective sensations of hunger and thirst, to make more or less 

 vigorous voluntary efforts to obtain the amount and kind of 

 nourishment they require. Even animals of such simple struc- 

 ture as the Infusoria obey this law, though their intestine, 

 stomach, and mouth do not constitute separate organs, but only 

 are portions of their protoplasmic body, and though, like all 

 other one-celled animals, they absorb their solid food direct 

 into the digesting protoplasm ; still they manifest liking and 

 aversion for different kinds of food, just as much as the higher 

 animals ; they never swallow that which they dislike, even 

 when by some accident it comes into contact with their mouth 

 at the same time as the food they prefer. 



The case is quite different with all those true Parasites which ■ 

 dispense altogether with a true intestinal canal ; they can take 

 up nourishment only in a fluid state, as their skin alone is 

 capable of absorbing it by osmosis, and this is quite independent 

 of the will of the creature. Thus the Cestodea, the curious 

 parasitical snail Entoconcha, several parasitical Crustacea in their 

 fully developed state, the pai-asitical Trematoda (as the liver 

 Distoma), and even some insects — all of which absorb their 

 nourishment through the skin without having any intestinal 

 canal — depend for their life and wellbeing on the osmotic rela- 

 tions between their skin and the fluids surrounding them. 

 Now, as osmosis through the skin — i.e. the absorption of the 

 fiud nutriment in the surrounding medium — can never, so 

 far as we know, be interrupted, all these animals are compelled 

 to be incessantly taking up and assimilating food. Thus the 

 parasite is quite incapacitated from making any choice in the 



