INFUSORIAL ANIMALCULES ; MONADS.' 553 



in their interior. By these methods, sometimes employed singly, 

 occasionally in combination, a single individual may soon become 

 the parent of an immense population. Thus the Paramecium 

 aurelia, if well supplied by food, has been observed to divide 

 itself every twenty-four hours ; so that in a fortnight, allowing 

 the product of each division to multiply at the same rate, 16,384 

 animalcules would be produced from the same stock; and in 

 four weeks, 268,435,456 new beings would result from a con- 

 tinued repetition of the process. But this animalcule has occa- 

 sionally been observed to increase with much greater rapidity ; 

 the first million being produced (according to calculation based 

 on fair data) within seven days. 



1215. In some Infusoria no mouth has yet been detected, and 

 it is supposed that they must derive their nourishment by simple 

 absorption through their external membrane. Of these the 

 Euglence are a very abundant form, occurring frequently in such 

 numbers in stagnant waters, as to give these a green or red 

 colour. They possess a considerable power of changing their 

 form, and are usually provided with a red eye-spot. The 

 Peridinium, another form of mouthless animalcule, is enclosed in 

 a curious silicious carapace, which is usually furnished with pro- 

 jecting spines and curved processes. The Opalince are flat, 

 elliptical creatures, with the surface covered with oblique lines of 

 cilia. They are found in the intestines of Frogs and Worms. 



1216. The Monads constitute the smallest of the Infusoria. 

 They usually present an oval form ; and seem like moving 

 points of gelatinous matter. They swim freely and with activity, 

 apparently by the vibrations of a sort of filament, which is said to 

 be tubular. The number of coloured particles seen in their bodies, 

 when the fluid in which they live has been tinged for the pur- 

 pose, is usually small ; in many species never above four or six. 

 They only seem to possess one orifice to the digestive cavity; 

 and from their extreme minuteness no cilia can be detected in 

 its neighbourhood, though it is probable that such exist there. 

 Of the animalcules included in this family, some have a tendency 

 to aggregate themselves in clusters ; and it is probable that those 

 which thus unite pass the earlier part of their lives in a separate 



