POLYGASTRIC ANIMALCULES; MONADS. 549 



days. But even this is as nothing to the more recent state- 

 ment of Ehrenberg, that some species are sufficiently fertile to 

 produce in four days 140,000,000 from a single germ ; a number 

 sufficient to compose, by the aggregation of their minute siliceous 

 shields, two cubic feet of solid rock like the Polierschiefer of 

 Bilin ( 1132). 



1127. The classification of the Polygastrica proposed by 

 Ehrenberg is principally founded on his views in regard to the 

 alimentary canal ; and, until these shall have been established, it 

 cannot be regarded as valid. For this reason it seems undesir- 

 able to introduce its details in this place ; but a few of the more 

 important groups, which are separated from one another by 

 evident characters, may be here noticed. It is interesting to 

 remark, however, that nearly every one of the subdivisions 

 founded upon the arrangement of the digestive organs, may be 

 further divided into two groups ; one characterised by the pre- 

 sence, and the other by the absence, of a firm envelope or sheath, 

 the existence of which becomes evident when the animalcule is 

 compressed between two pieces of glass. 



1128. The Monads constitute the smallest, and perhaps the 

 simplest, of the Polygastric Infusoria. They usually present 

 a spherical 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 purpose, 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 condition. 

 It is often very difficult to distinguish the Monads from the 

 young of other species, which they much resemble. Where a 

 number of minute gelatinous points are seen in the neighbour- 

 hood of clusters of larger forms, they are probably to be regarded 



