262 PARAMCECIUM chap, m 



while others are among the commonest inhabitants of both 

 fresh and salt water. 



A very common ciliate infusor is the beautiful " slipper 

 animalcule,'' Paramecium, which from its comparatively large 

 size and from the ease with which all essential points of its 

 organization 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 \ — \ mm. (200 — 260/u.) in length ; in 

 fact it is just visible to the naked eye as a minute whitish 

 speck. 



Its form (Fig. 70, 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 of the body, where it ends in a tolerably 

 deep depression. 



The groove is called the buccal groove (a & b, buc. gr) : 

 at the narrow end is a small aperture the mouth (ntth), which, 

 like the mouth of Euglena (Fig. 69), leads into the soft 

 internal protoplasm of the body. 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 of the 

 body, and gradually curves over to the middle of the 

 ventral surface. 



