128 



AQUATIC MICROSCOPY FOR BEGINNERS. 



ferior surface, and a more or less strongly convex or 

 elevated upper surface. In color it is usually some 

 shade of brown, .but may be almost black. In young 



Fig. loo, — Arc^lja vulgaris. 



Fig. loi. — Arcella dentata. 



specimens the shell is often nearly colorless. It is 

 generally transparent. The mouth of the shell, in the 

 center of the flat lower surface, is circular and smooth. 

 The body of the animal is colorless, and is attached 

 to its home by fine, radiating threads of its own sub- 

 stance. There are several species, recognized by the 

 form of the shell. 



I. Margin of the shell smoothly circular. Common 

 everywhere. Arcdla vulgaris, Fig. ioo, A being, 

 a side view of an empty shell to show its flattened 

 dome-like form, and the manner in which the 

 margin of the orifice is turned in to produce an 

 internal mouth to the shell. This, is one of the 

 characteristics of all Arcellse. 

 Margin of the shell with several teeth, so that it re- 

 sembles, when seen from above or from below, 

 a wheel with pointed cogs. Nut so common as 

 the preceding. A. dentata. Fig. loi. 

 Shell somewhat balloon-shaped when seen in side 

 view, higher than wide, the sides often depressed 

 in wide facets. Not rare. A. mitrdta. 



2. 



