chap, iv.i INTRACELLULAR DIGESTION. 103 



protoplasm of the cell, which is capable of separating 

 out from the food such parts as are nutritious, and 

 of converting them into protoplasmic matter ; what 

 is useless is discharged, or got rid of. 



This direct mode of assimilation by a living cell is 

 spoken of as intracellular digestion; it is the 

 only mode of nutrition which is known to obtain 

 in the Protozoa, but it is very important. to observe 

 that the phenomenon is by no means limited to that 

 division of the animal kingdom ; it obtains also in 

 various lower groups of the Metazoa, and even after a 

 distinctly defined mouth has become developed. It 

 is, therefore, associated with a number of characters 

 which indicate an advance in the complexity of 

 organisation; and, on the other hand, it is found 

 also in forms which have, under the influence of a 

 parasitic habit, become degraded as compared with 

 their ancestors. The simplest mode of seizing food is 

 observed in the Amoeba, where the protoplasmic body 

 seems to engulf its nutriment by flowing and closing 

 around it. And this ingestion of food does not 

 take place at any definite point in the body of the 

 Amoeba, but now at one spot, and now at another. 

 When the form of the body becomes more definite, 

 the protoplasmic processes act as organs by which 

 the food is drawn towards the central body-mass. 



A much more elaborated mode is to be seen in 

 the ciliated Infusorians, where a definite orifice 

 (" cytostome ") acts as the sole entrance for food 

 into the body ; in many cases this so-called <l mouth " 

 has also an anal function, but in a few forms it has 

 been distinctly observed that a second orifice is pre- 

 sent ; by means of this " cytoproct," the undigested 

 portion of the food passes from the body. The 

 presence of a definite oral orifice is no doubt to be 

 associated with the greater elaboration of the organi- 

 sation of an Infusorian, and we find also that some 



