Arcellina.] THE INFUSORIA. ' 169 



on the back. Processes transparent, slender, and small. 

 A lateral aperture is seen in this species, which is the 

 smallest of the genus. Found in stagnant water. Size 

 l-550th. 



Genus XLII. Arcella. The capsule Animalcules. — 

 The characteristics of this genus are, its being possessed 

 of numerous variable processes, or single processes, cleft 

 into many, and spread abroad, and its being furnished with 

 a flattened shield-like lorica. The structure of the lorica, 

 as to details, is very diff'erent in the different species. 

 For instance, in A. vulgaris it exhibits regular and delicate 

 facets. In A. dentata, the facets are large and crystal- 

 line. In A. aculeata, it is beset with spiculi ; and in 

 A. hyalina, it is homogeneous and clear. The organs of 

 locomotion are evidently extensile and retractile processes, 

 radiant and variable. The digestive cells are readily filled 

 with coloured vegetable substances. In A. vulgaris, a con- 

 tractile vesicle has been perceived. 



156. Arcella vulgaris. The common Arcella. — Lorica 

 round andbell-shaped, with an hemispherical or turgid back ; 

 smooth, and composed of rows of minute granules; colour 

 yellow or reddish-brown. Found abundantly amongst 

 lemna and aquatic plants. Size l-570th to l-240th. 



157. Arcella aculeata. The spinous Arcella. — Lorica 

 hemispherical, though of ten mis-shapen and spinous at the 

 margin. It is formed of short spiculi, and is of a yellowish 

 colour. The spines sometimes issue from only one-half 

 of the margin of the shell, or shield-like lorica ; the shell 

 is not readily destroyed by heat. Fig. 92 represents one 

 of these creatures. The projecting spines, and the large 

 rounU opening in the lorica, are visible. Fig. 93 represents 



