16 OUTLINES OF EVOLUTIONARY BIOLOG 



important function of nutrition. As it crawls slowly about the 

 pseudopodia come into contact with all sorts of solid particles in 

 the surrounding water. Some of these will be inorganic, grains of 

 sand and so forth, others will be the dead or living bodies of other 

 organisms, sometimes much smaller than the Amoeba itself. 

 The Amoeba has the remarkable power of distinguishing amongst 

 these different kinds of particles those which are good for food 

 from those which are not. How it does so we do not know ; we 

 can only say that the presence of a particle which is good for 

 food stimulates the living protoplasm in a way quite different 

 from that in which it is stimulated by the presence of a mere 

 grain of sand. In the latter case the Amoeba will simply pass to 

 one side and avoid the object ; in the former it will put forth 

 pseudopodia which will .close around and envelop it. The food 

 particle is thus passed through the ectoplasm into the interior 

 of the body. There is no definite mouth, but food is taken in 

 wherever it happens to come into contact with the surface of the 

 body, and the aperture closes up after it. Thus a temporary 

 mouth, is formed as occasion demands. Similarly there is no 

 permanent digestive cavity or stomach, but merely a temporary 

 food vacuole into which a digestive fluid is doubtless secreted 

 by the surrounding protoplasm. ^ 



Digestion, as in higher animals, is essentially a process of 

 solii f 'oii, whereby those parts of the food which are digestible are 

 dissolved and *r-endered capable of diffusing' from the digestive 

 cavity into the surrounding body. The higher animals make 

 use chiefly of three classes of food material, proteids, carbo- 

 hydrates (e.g., starches -and sugars) and fats. It is said that 

 Amoeba can only digest proteids, which of course it must obtain 

 from the protoplasmic bodies of other organisms.- When diges- 

 tion is complete a certain amount of insoluble residues from the 

 food will remain over ; these constitute the faeces and have to be 

 got rid of. There is, however, no permanent vent or anus, and 

 the faeces are cast out through the ectoplasm at the hinder end 

 of the body as the animal crawls along. 



Owing to the minute size of the whole organism there is no 

 need for a complex circulatory system, such as is found in the 

 higher animals, for the distribution of the digested food; it merely 

 soaks into the surrounding protoplasmic body from the food 

 vacuoles, and, by anabolic changes which are not fully under- 

 stood, is converted into new, living protoplasm. 



