178 CHARACTERS OF ANIMALS AND PLANTS LESS. 



In this case also our selected organisms agree with animals 

 and plants generally. Animals, with the exception of some 

 internal parasites, ingest solid food, and they must all have 

 their nitrogen supplied in the form of proteids, being unable 

 to build up their protoplasm from simpler compounds. 

 Plants take their food in the form of a watery solution ; 

 those which possess chlorophyll take their carbon in the 

 form of carbon dioxide and their nitrogen in that of a nitrate 

 or ammonia salt : those devoid of chlorophyll cannot, ex- 

 cept in the case of some bacteria, make use of carbon 

 dioxide as a food, and are able to obtain nitrogen either 

 from simple salts or from proteids. Chlorophyll-less plants 

 are therefore nourished partly like green plants, partly like 

 animals. 



This difference in the character of the food is connected 

 with a morphological difference. Animals have, as a 

 rule, an ingestive aperture or mouth, and some kind of 

 digestive cavity, either permanent (stomach) or temporary 

 (food-vacuole). In plants neither of these structures 

 exists. 



Another difference which was referred to at length in an 

 early lesson (p. 32), is not strictly one between plants and 

 animals, but between organisms with and organisms without 

 chlorophyll. It is that in green plants the nutritive processes 

 result in deoxidation, more oxygen being given out than is 

 taken in : while in animals and not-green plants the precise 

 contrary is the case. 



There is also a difference in the method of excretion. In 

 Paramoecium there is a special structure, the contractile 

 vacuole, which collects the superfluous water taken in with 

 the food and expels it, doubtless along with nitrogenous and 

 other waste matters. In Vaucheria and Mucor there is no 

 contractile vacuole, and excretion is simply performed by 



