70 ZOOLOGY FOR MEDICAL STUDENTS chap. 



rotation about its long axis which is very characteristic. At the hinder 

 limit of the peristome is an opening — the mouth (Fig. 28, ni) — at the 

 edges of which the pellicle is turned inwards to form a wide somewhat 

 curved and tapering tube — the oesophagus or gullet — which dips down 

 into the endoplasm and is cut off sharp at its inner open end. During 

 the life of the Paramecium a rapid flickering may be observed within the 

 gullet. This is caused by the movements of the undulating membrane 

 — a thin protoplasmic curtain (formed of a row of large cilia adhering 

 together side by side) down which there pass in rapid succession waves 

 of movement just like those produced on a large scale by moving one 

 end of a curtain backwards and forwards. The undulating membrane 

 performs an important part in the feeding of the Paramecium. Minute 

 food particles such as Bacteria are whirled round by a vortex produced 

 by the cilia of the peristome into the neighbourhood of the mouth where 

 they are caught by the indraught due to the movements of the undulating 

 membrane. They are carried down the gullet, collecting at its lower 

 end and being forced into a drop of water which bulges from the inner 

 end of the gullet into the fluid endoplasm. This drop (Fig. 28, j.v) 

 increases in size as more and more water is forced into it until at length 

 it detaches itself like a soap-bubble from the end of a pipe and passes 

 away into the endoplasm as a typical food-vacuole. As it is carried 

 round in the slow circulation of the endoplasm, much of the water of the 

 vacuole is absorbed by the surrounding endoplasm, so that the vacuole 

 diminishes in size. Acid is secreted into the vacuole to kill the food- 

 organisms and this acid phase is succeeded by an alkaline one in which 

 the food-material is attacked by the digestive ferments, the products of 

 digestion being absorbed while the indigestible detritus is left as faecal 

 material. Finally the vacuole approaches the surface and bursts to the 

 exterior at a point between the mouth and the hinder end at which 

 there is a definite small opening (anus) in the pellicle, the faecal matter 

 being in this way got rid off. 



The life-history of Paramecium, while of much less complexity than 

 that of some of the Protozoa already described, is of great importance 

 especially in relation to its reproductive processes. A healthy Para- 

 mecium, isolated and provided with abundant food, grows rapidly and 

 as in the case of Amoeba this increase in size finds its corrective in a 

 process of fission, the Paramecium becoming gradually constricted across 

 into two individuals which for a time remain connected together end to 

 end but eventually separate. Preparatory to this constriction the two 

 nuclei divide — the macronucleus by a simple process of constriction,, the 

 micronucleus by a mitotic process — so that the young individuals are 



