126 MICROSCOPY FOR BEGINNERS. 



7. CENTROPYXIS ACULEATA (Fig. 99). 



The shell of this Rhizopod is usually formed of sand- 

 grains, and is brown in color, but sometimes it consists 

 of a brown membrane with scattered adherent sand- 

 grains. I have also met with shells formed entirely of 

 small diatoms fitted together as beautifully and accu- 

 rately as the sand-grains of Difflugia. These diatom 

 shells were found in an aquarium, and were probably 

 built of these plants because suitable sand was not 

 to be had. Centropyxis, when seen in side view, ap- 

 pears as if it had once been a hemi- 

 sphere with the mouth near one side of 

 the flat surface, but that while it was soft 

 the convex part had in some way been 

 pushed over towards one side, thus leav- 

 ing the shell oblique or inclined, the back 

 part being much thicker than the front, 

 the upper surface sloping down from the deeper rear 

 to the thin front margin, the circular or oval mouth 

 remaining nearer the thin border. The figure shows 

 the under part of a shell, which, in this position, ap- 

 peal's almost circular. The spines on the thick part are 

 usually sharp-pointed, and vary in number from one to 

 nine. The body of the animal is colorless, and the 

 pseudopodia are blunt and finger-like. This is the only 

 known species. Common. 



8. ARCELLA (Figs. 100, 101). 

 "When seen from above or below, the shell of Arcella, 



