Il6 TlMEHRI. 
hairs, when they are termed cilia ; or of long whip-like 
threads, known as flagella ; or again of small threads 
terminated by suckers ; and the various organisms are 
termed accordingly as Ciliate, Flagellate and Su6lorial 
Infusoria. By means of these processes the organisms 
move rapidly through the water, either steadily by means 
of cilia, or plunging along by means of the flagella ; and 
these same processes serve to procure food. In the 
ciliate and suftorial forms, the processes are always very 
numerous; in the flagellate forms, usually only one flagel- 
lum is present, and this has given rise to the term monad, 
applied to such forms. In all these forms, the outer part of 
the body consists of a layer which is firm and dense as 
compared with the central soft substance of the cell, and 
the cilia and flagella are processes of this firmer layer. 
Except in the parasitic, suftorial forms, a distinct mouth is 
present, and a short gullet leads into the soft central 
protoplasm — stru6lures that present the rudiments of a 
digestive tra6l. Rudiments of muscular structures are 
also present, seen most clearly in the contra6lile, stalked 
forms. A large nucleus, and one contra6lile vesicle, or 
several, are also present; but there is no shell or skeleton. 
The particles of food-matter taken into the soft central mass 
are surrounded with minute drops of water taken in with 
them, and these give the appearance of being minute 
stomachs, and as such, indeed, they were considered by the 
old writers. Reproduction takesplace certainlybybudding 
and by fission, with or without conjugation or encys- 
tation ; and although several other processes have been 
described, they have been more or less discredited by 
recent research. A very remarkable feature is to be 
found in the fact that the minute germs of many monads 
