126 AN INTRODUCTION TO ZOOLOGY 



and is slightly sharper in P. caudatum than in P. aurelia. An 

 asymmetrical groove, the peristome groove, commences as a fairly 

 wide, shallow depression near the anterior end, runs backwards with 

 a slight spiral twist, getting deeper and narrower until about the 

 middle of the body it finally leaves the surface and runs in as a 

 funnel-shaped structure. The hole where the groove leaves the 

 surface is the cell mouth or cytostome, and the funnel-shaped con- 

 tinuation, termed the gullet, oesophagus or cytopharynx, ends blindly 

 in the endoplasm of the cell. Thus we are able to distinguish not 

 merely anterior and posterior ends, but, the side on which the mouth 

 is situated being ventral, we have dorsal and ventral surfaces, and so 

 also right and left sides. The animal thus has a definite shape which 

 does not change, although it is elastic enough to allow it to squeeze 

 through an aperture slightly smaller than itself, and also to bend 

 round any obstacles that lie in its path. It is enabled to swim 

 comparatively rapidly and uniformly by means of a well-developed 

 locomotor apparatus in the form of a covering of minute motile 

 hair-like processes, the cilia. They are fairly evenly distributed 

 over the whole surface, over the peristome groove, and even in a 

 modified form down the cytopharynx. Owing to the way in which 

 they strike the water and the twist at the front end Paramcecium 

 rotates on its own axis as it moves forward much in the same way 

 as a bullet from a rifle or a shell from a gun. 



Under the high powers of the microscope further details of its 

 structure can be made out. The protoplasm, like that in Amceba, 

 is clearly differentiated into an outer layer, the ectoplasm or cortex, 

 strongly marked off from the more granular endoplasm or medulla. 



The ectoplasm itself is divided into two distinct layers. The 

 outer layer is a thin, tough elastic membrane known as the cuticle, 

 formed by the modification of the outer ectoplasm, and giving 

 to the animal its definite shape. The surface of the cuticle exhibits 

 a characteristic sculpturing, being divided up by series of fine grooves 

 into a number of minute hexagons, from the centre of each of which 

 a cilium arises. Each cilium is a minute protrusion of the ectoplasm 

 perforating the cuticle, and it can be traced into its deeper layers 

 where it takes its origin in a tiny speck, the basal granule, 



The remaining deeper part of the cortex under the cuticle is far 

 thicker and appears transversely striated owing to the presence in it 

 of a large number of very minute spindle-shaped bodies, the tri- 

 chocysts, set at right angles to the surface. Even under the highest 

 powers of the microscope the trichocysts only appear as small simple 

 rods-, in which no details of their internal structure can be made out. 

 When the animal is treated with an irritant fluid such as very dilute 

 acetic acid, however, the trichocysts each discharge a very fine 



