RHIZOPODS. 127 



seems like a disk with a pale circular spot in the mid- 

 dle. When seen in side view it has a flat lower 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 very young specimens 

 the shell is often nearly colorless. It is generally trans- 

 parent. The mouth of the shell, in the centre of the 

 flat surface, is circular and smooth. The body of the 

 animal is colorless, and is attached to its home by fine 

 threads of its own substance. There are several species, 

 recognizable by the form of the shell. 



1. Margin of the shell smoothly circular. Common 



everywhere. Arcella, vulgdris, Fig. 100. 



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



sembles, when seen from above or below, a wheel 

 with pointed cogs. Not as common as the preced- 

 ing. A. dentdta, Fig. 101. 



3. Shell somewhat balloon - shaped when seen in side 



view ; higher than wide, the sides often depressed 

 in wide facets. Not rare. A. mitrdta. 



Fig. 101. Arcel- 

 Fig. 100. Arcella vulgaris. la dentata. Fig. 102. Tnuema Suchelys. 



9. TRINEMA ENCHELYS (Fig. 102). 

 This shell is pouch-shaped, colorless, small, and in- 



