THE BIOLOGY OF ARCELLA. 



BY A. G. PAPWORTH. 



Since the discovery and description of the genus by Elirenberg in 1830, 

 Arcella has been a favorite subject for microscopic investigations. It 

 is, without a doubt, our most common fresh water shell-bearing rhizopod, 

 and may be found in a large proportion of cultures taken from ponds 

 and streams. Combining amcjeboid simplicity of organization with a 

 complicated system of rej)r()duction, and added to this a shell rivalling 

 those of the Foraminifera, it is scant wonder that for the past half 

 century the literature on the subject has continued to accumulate. Many 

 and diverse conclusions have been reached by separate workers. It is 

 in an attempt to embody the most conservative of these in a connected 

 account, together with personal observations, that this article is written. 



GENERAL DESCRIPTION. 



In appearance, Arcella is distinctive. In spite of marked variations in 

 form within the genus, it is not liable to confusion with any other com- 

 mon Protozoon. The comparison that inevitably presents itself on seeing 

 the organism for the first time, is that it looks like a little doughnut. The 

 yelloM' or brown color, circular shape, and small central opening, favor 

 the simile. 



From the side, however, the shell is seen to be campanulate in shape, 

 broadest at the bottom, and wider than high. Across the bottom of the 

 inverted bowl extends a continuation of the shell, covering the base 

 except for a round central opening, one-fourth to one-third the diameter 

 of the shell. The surface of the dome is generally smoothly convex, 

 but may be pitted or faceted. The color varies from faintest yellow 

 through brown, almost to black. Average dimensions are .1mm wide, 

 .05mm higii, M-ith a mouth oldening of .03mm. In all cases except very 

 young individuals, a minute configuration or cancellation of the shell is 

 visible. Unlike the related Difftugias, Arcella never has sand or other 

 foreign particles incorporated in the shell. 



Within the shell can be seen a mass of clear protoplasm of irregular 

 outline, usually filling most of the shell cavity. Two nuclei are commonly 

 visible, but there may be only one or many. Contractile vacuoles, gas 

 vacuoles and food balls are features usual Iv distinguishable without 



IStli Midi. Acad. Sci. Kept., nil 7, 



