172 FRESHWATER RHIZOPODS OF NORTH AMERICA. 



or brown, but is found from a colorless transparent condition through every 

 tint of raw and burnt sienna to dark umber brown, and so opaque as to 

 obscure the contents from view. 



The sarcode of Arcella vulgaris is colorless, independently of the color 

 imparted to the endosarc by the presence of food. In the side view of 

 the animal, it visually appears as an oblate spheroidal ball resting on the 

 inverted funnel of the mouth, and attached by a variable number of diver- 

 gent threads of ectosarc to the inside of the dome of the shell. 



Generally I have been able to detect two nuclei, and only two, in the 

 sarcode, one situated on each side. A number of contractile vesicles 

 also are visible at the periphery of the sarcode mass. Among the food 

 contents of the endosarc, yellowish-bi'own and green food-balls are to be 

 detected, and sometimes with these small diatoms and desmids. 



Sometimes individuals are seen with the sarcode detached from the 

 mouth and retracted to the top or to one side of the fundus of the shell. 

 Sometimes the sarcode mass assumes an irregular form, as represented in 

 fig. 14, pi. XXVII. In this individual, the sarcode subsequently assumed 

 the ordinary appearance, as seen in most of the accompanying figures. Its 

 shell was so delicate and flexible, that the summit would become depressed 

 with the protrusion of the pseudopods, and would again assume an even 

 convex condition on their retraction. 



Occasionally I have found Arcella vulgaris with the sarcode in an 

 encysted condition, presenting the appearance of a spheroid ball, resting 

 upon the position of the mouth, as seen in fig. 33. In this individual, the 

 ball was surrounded with a flocculent granular matter, probably excrement, 

 discharged from the sarcode as it assumed the quiescent state. The ball 

 was white and granular, and enclosed in a membrane. 



Sometimes I have observed the encysted ball of sarcode so large that 

 it produced a complete eversion of the base of the shell, as represented in 

 fig. 3, pi. XXVIII. 



Frequently I have observed pairs of individuals of Arcella vulgaris 

 applied with their bases together, and with the shells alike in form, though 

 often differing in color, and sometimes slightly in size. The couples have 

 exhibited various degrees of difference in hue, and sometimes, while one 

 shell is colored, the other is completely colorless, or presents only the 

 faintest straw-yellow tint. See figs. 17, 24, pi. XXVII; fig. 1, pi. XXVIII. 



