OCCURRENCE OF INFUSORIA 105 



acquired many of the characteristics of the higher animals 

 and plants while remaining unicellular. The study of several 

 of these more or less highly differentiated though unicellular 

 forms will occupy the next seven Lessons. 



It was mentioned above that, in the earlier stages of the 

 putrefaction of an organic infusion, bacteria only were 

 found, and that later, monads made their appearance. Still 

 later organisms much larger than monads are seen, generally 

 of an ovoidal form, moving about very quickly, and seen by 

 the use of a high power to be covered with innumerable fine 

 cilia. These are called dilate Infusoria, in contradistinction 

 to monads, which are often known as flagellate Infusoria : 

 many kinds are common in putrefying infusions, some occur 

 in the intestines of the higher animals, while others are 

 among the commonest inhabitants of both fresh and salt 

 water. Five genera of these infusoria will form the subjects 

 of this and the four following Lessons. 



A very common ciliate infusor is the beautiful " slipper- 

 animalcule," Paramoedum aurelia, which from its compara- 

 tively large size and from the ease with which all essential 

 points of its organization can be made out is a very con- 

 venient 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. 21 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 



