RHIZOPODS. 127 



7. Centropyxis aculeAta (Fig. 99). 



The shell of this Rhizopodis usually formed of sand- 

 grains, and is brown in color, but sometimes it con- 

 sists of a brown membrane with only scattered sand- 

 grains adherent. I have also met with 

 shells formed entirely of small diatoms 

 fitted together as beautifully and as 

 accurately as are the sand-grains of 

 Difflugia. These diatomaceous dwell- 

 ings were found in an aquarium, 

 and were probably built of these Fig. 99.— Centropyxis 



, , - -11 1 aciileAta. 



plants because suitable sand was not 

 to be had. 



Centropyxis, when seen in side view, appears as if it had 

 once been a hemisphere 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 leaving the shell oblique or inclined, the 

 back part being miich thicker than the front, the 

 upper surface sloping down from the higher rear to 

 the shallower anterior margin, the circular or oval 

 mouth remaining nearer the thin border. 



The figure shows the lower aspect of a shell, which, 

 in this position, appears 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. It is common. 



8. Arc^lla (Figs. 100, loi). 



When seen from above or from below, the shell of 

 ArcMa seems like a disk with a pale circular spot in 

 the center. When seen in side view it has a flat, in- 



