THE PROTOZOA 127 



thread, very much longer than the cilia, and so clothe the animal in 

 a matted coating of very fine hairs. The exact use of these tri- 

 chocysts is almost as hard to understand as their structure is difficult 

 to determine. They are generally thought to be weapons of offence 

 or defence, but have not been ob- 

 served in use, and are generally only 

 seen when the animal is killed. In the 

 deeper layers of the ectoplasm below 

 the trichocysts are a number of deli- 

 cate threads disposed parallel to the 

 surface, the myoneme fibrillae, which 

 are highly contractile and bring about 

 the bending movements of the animal. 



In one species of Paramcecium, namely, FlG - 4 ; Surface view of cuti- 

 -n 7 . ,, , , . cle of Paramcecium, adapted 



P. bursana, the ectoplasm contains a f rom Butchli. 



number of minute green corpuscles E.G., basal granule; c., ciiium; 

 containing chlorophyll, the colouring ^i5;o c h s t xagonal depressed area ; T - 

 matter of the leaves of plants, and 



similar to those we shall describe more fully in the case of Hydra. 

 They give the individuals of this species a green colour. 



Two further structures are to be considered as ectoplasmic, 

 although they project deeply into the endoplasm, and these are the 

 pulsating vacuoles. They are constant in position, being situated 

 about one-third of the way from each end and, not moving about like 

 the contractile vacuoles in Amoeba, they discharge always at the 

 same spots. When full they appear as spheres containing a clear 

 fluid, and if they are watched they will be seen to contract suddenly 

 and almost disappear. Careful examination shows that in their 

 place is left a small central spot from which radiate a series of from 

 five to ten fine lines. These lines are the collecting canals, and they 

 soon begin to swell up with the accumulation of fluid within them, 

 presenting a very characteristic rosette-like appearance. Finally, 

 they discharge into the central dot, and ultimately they all become 

 merged in the one central vesicle. This process of diastole and 

 systole is continued in a very regular manner as long as the animal 

 lives, and occupies a little less than half a minute. Defaecation in 

 Paramcecium always takes place at one spot on the ventral surface, 

 but there does not appear to be any permanent opening. In some 

 forms allied to Paramcecium, e.g. Nyctotherus, which lives and is almost 

 always to be found in the frog's rectum, a permanent cell anus or 

 cytoproct is present at the hinder end. 



The endoplasm differs from the ectoplasm in being far more 

 granular and more fluid, although there is no sharp line of demarca- 

 tion between the two. The endoplasm is constantly moving round 



