194 COMPARATIVE ANATOMY 



and metabolism. The most obvious foreign substances in the 

 endoplasm are the food-vacuoles and little balls of foreign 

 substance taken in as food. The food-vacuoles, which, of 

 course, must not be confounded with the contractile vacuoles, 

 are clear spherical spaces looking like bubbles in the protoplasm, 

 and filled sometimes with fluid only, sometimes with solid food 

 particles surrounded by fluid. The structure and behaviour 

 of the food-vacuoles of Paramecium are the same as those of 

 Amoeba (p. 131). Besides the food-pellets there are numerous 

 minute crystalline particles which are probably excretory 

 products. It has also been shown that the endoplasm of 

 Paramecium contains diffused animal starch or glycogen, 

 giving the characteristic port-wine red colour on treatment 

 with iodine. Since the contents of the food-vacuoles and the 

 food-pellets are in various stages of disintegration, and since 

 glycogen is shown, in higher animals, to be a reserve material 

 formed by the activity of protoplasm from the substances taken 

 in as food, we can have no doubt that a portion at any rate of 

 the food of Paramecium is converted into glycogen in the 

 endoplasm, is further built up as required into the substance 

 of the body, and that the waste materials resulting from the 

 breaking down of the protoplasm are excreted partly by the 

 contractile vacuoles, partly in the form of the crystalline 

 particles found in the endoplasm. The endoplasm is clearly 

 the seat of the digestive and to a great extent of the anabolic 

 activities of the animal. Surrounded as it by the firm 

 ectoplasm and the external cuticle, solid food can only be 

 introduced into it by means of the mouth. Paramecium feeds 

 chiefly on minute infusoria and flagellata which are swept into 

 the mouth by the cilia lining the peristomial groove and the 

 entrance to the mouth itself, and it has already been shown 

 that the cilia in these regions are so disposed as to direct the 

 currents towards the mouth. A drawing of the mouth as seen 

 under a magnification of 1000 diameters is given in fig. 41, B. 

 It commences as a wide funnel-like cavity at the posterior end 

 of the peristomial groove. The cuticle is reflected so as to line 

 the cavity, and the cilia are also continued into it. The mouth 

 narrows somewhat rapidly to form a narrow ciliated tube which 

 is directed at first upward, then turns downward with a slight 

 spiral twist and ends in the endoplasm of the posterior third 

 of the body. The entrance to the narrow tube is guarded by 



