1 PARAMECIUM 69 



contraction (systole) of the contractile vacuole is repeated rhythmically 

 at intervals — commonly about five minutes, but varying with circum- 

 stances and with the individual. 



The complexities of the ectoplasm are not exhausted by the features 

 already mentioned : there remains one of its most characteristic features 

 namely that it projects outwards in the form of innumerable little 

 protoplasmic hairs or cilia. Each cilium is actively movable, being able 

 to bend suddenly in a definite direction and then recover slowly. The 

 cilia are present in enormous numbers, one arising from the centre 

 of each of the little dimples that give the ectoplasm its honeycomb 

 surface. The cilia work in unison, the rapid bending taking place 

 in the same direction towards one end of the body. The result of the 

 movements of the cilia, like thousands of little paddles, is to cause the 

 Paramecium to glide slowly through the water, one definite end — that 

 which is above in Fig. 28 — being normally in front, although the move- 

 ment is capable of reversal. 



The endoplasm of the Paramecium is, so far as visible structure is 

 concerned, comparatively simple. It is very fluid ; it is granular in 

 appearance — the granularity being due to the presence of minute particles 

 of various kinds, such as excretory matter or reserve food-material. In 

 the living Paramecium the endoplasm shows a slow circulation within 

 its bounding layer of ectoplasm. 



Embedded in the endoplasm lies the very characteristic nuclear 

 apparatus. This consists of a large maoronueleus (or meganucleus. 

 Fig. 28, N) — kidney-shaped, and serving to carry out the ordinary 

 nuclear function of controlling the general processes of metabolism — 

 and a small rounded mleronucleus ^ (Fig. 28, n) lying usually in the 

 concavity of the surface of the macronucleus. So far as is known the 

 micronucleus becomes functionally active only at the period of repro- 

 duction, in which process it plays a very important part. 



The fact that the fluid endoplasm is enclosed within a comparatively 

 stiff ectoplasm gives the Paramecium its definite form — rather elongated, 

 slightly pointed towards the ends, the front end rather less pointed than 

 the hinder one, and somewhat flattened. A broad shallow valley — the 

 peristome (Fig. 28, p) — starts from one edge of the Paramecium near its 

 front end and extends somewhat diagonally backwards across about 

 one-half of its width. The somewhat oblique position of the peristome 

 and especially of its hinder boundary causes the water pressure on the 

 surface of the Paramecium as it glides along to give it a movement of 



' In P. caudatum. P. aurelia a very similar species, also common, possesses 

 two micronuclei. 



F2 



