262 PARAMCECIUM CHAP, m 



in the intestines of the higher amnials, while others are 

 among the commonest inhabitants of both fresh and 

 salt water. 



A very common ciliate infusor is the beautiful "slipper- 

 animalcule," Paramcecium, which from its comparatively 

 large size and from the ease with which all essential 

 points of its organisation can be made out is a very 

 convenient and interesting object of study. 



Compared with the majority of the organisms which 

 have come under our notice it may fairly be considered 

 as gigantic, being no less than |-J mm. (200-260^) in 

 length : in fact it is just visible to the naked eye as a 

 minute whitish speck. 



Its form (Fig. 71, A) can be fairly well imitated by 

 making out of clay or stiff dough an elongated cylinder 

 rounded at one end and bluntly pointed at the other ; 

 then giving the broader end a slight twist ; and finally 

 making on the side rendered somewhat concave by the 

 twist a wide, shallow groove beginning at the broad end 

 and gradually narrowing to about the middle, where it 

 ends in a tolerably deep depression. 



The groove is called the buccal groove (A and B, 

 buc. gr) : at the narrow end, as in Euglena (Fig. 69), 

 is a small aperture, the mouth (mth), which, however, 

 leads directly into the soft internal protoplasm. The 

 surface of the creature on which the groove is placed 

 is distinguished as the ventral surface, the opposite 

 surface being upper or dorsal ; the broad end is 

 anterior, the narrow end posterior, the former being 

 directed forwards as the animalcule swims. These 

 descriptive terms being decided upon, it will be seen 

 that the buccal groove begins on the left side, and 

 gradually curves over to the middle of the ventral 

 surface. 



