II] PARAMECIUM 39 



Paramecium, it appears after the Flagellata (see p. 31) on which it 

 appears to feed. It is often termed the Slipper animalcule because 

 it is of an elongated oval outline and seen sideways it has a thin 

 scoop-like anterior end and a thick posterior part, so that it is 

 usually described as slipper-shaped. It is covered all over with 

 cilia of the same size arranged in regular lines ; this arrangement of 

 cilia, which is called the holotrichous arrangement (0X05, entire; 

 Optg, hair), is always associated with the absence of a stalk and with 

 a free-swimming life. Vorticella, on the other hand, is said to be 

 peritrichous (TTC/OI = around). Paramecium like Vorticella possesses 

 two nuclei, one large and easily visible and one, the micronucleus, 

 small and difficult to detect. It has a well-developed mouth and a 

 deep pharynx situated on one side and lined with specially long cilia. 

 Paramecium is a beautiful form in which to study the contractile 

 vacuole ; there are two of these present, one in the anterior and 

 another in the posterior portion of the animal. If one of these 

 vacuoles be watched it can be seen to contract and then slowly to 

 re-appear. In the process of re-appearance six radiating channels are 

 seen through which the liquid streams so as to form a perfectly 

 spherical drop. These seem to be permanent channels in the proto- 

 plasm leading to a central space which becomes filled with fluid and 

 and forms the vacuole. 



Paramecium possesses peculiar organs named trichocysts 

 (5, Fig. 13) embedded in the outer layer of the protoplasm. These 

 look like minute rods. When the Paramecium is irritated as 

 for instance if it is deluged with dilute iodine these are suddenly 

 shot out, assuming the form of long threads, and they are thought 

 to exercise a stunning effect on any small animal with which they 

 come in contact, though no good evidence for this has been adduced. 



Prof. Jennings has carefully studied the behaviour of Para- 

 mecium. Although to the casual eye its movements appear to be 

 guided by intelligence yet all its actions may be reduced to variation 

 in the intensity of one reaction. According to Jennings, when 

 Paramecium is not irritated its normal condition is one of actively 

 swimming forwards. When irritated it stops and the antejcior cilia 

 become motionless, then the beat of all the cilia is reversed and the 

 animal is thus forced backwards. After retreating for a short 

 space it turns round an axis running from right to left through 

 the middle of the body, the front end bending downwards through 

 a moderate angle. The reaction is now at an end and the normal 



