GEimS AEOELLA. 167 



therefore be no doubt as to the specific identity of these different forms. 

 Hertwig and Lesser further remark, that they were able to distinguish only 

 a single species to the genus. 



It has, nevertheless, appeared to me convenient to designate those 

 which are widely different by separate names, as is done with other organic 

 forms more positively regarded as distinct species. In this view I have 

 described the more conspicuous forms, and have adopted the names applied 

 to them, grouping with each those of intermediate characters which most 

 nearly approximate them. 



The shell of Arcella is usually more or less campanulate, and varies 

 greatly in the proportion of height and breadth, — sometimes being so low as 

 to appear shield-like, sometimes so high and expanded as to be balloon-like. 

 Usually it is widest at the circular base, but often is widest near or at the 

 middle. It has an even, convex dome, or this may be divided into facets 

 defined by more or less prominent folds, or else it is impressed with rows of 

 concave pits. The base is convex at the border, and forms an inverted 

 concave funnel, with the circular mouth in the centre. 



The shell is usually of some shade of brown: pale or darker raw 

 sienna, to burnt sienna or deep brown, or even almost black. The younger 

 the shell apparently the paler it is, and in the earliest condition is colorless 

 and transparent. 



The shell is composed of a more or less translucent or transparent 

 chitinoid membrane, with a minutely hexagonal cancellated structure. It 

 is intrinsic to the animal, and never has incorporated with it quartz-sand, 

 diatom shells, or other extraneous matters. 



The hexagonal cancelli are about jisoo*^ ^^ ^^ ^"^^^ ^^ diameter. 

 According to Hertwig and Lesser they are hollow. This was proved by 

 an ingenious experiment. The shell, after treatment with sodium carbonate, 

 was treated with acetic acid, when the cancelli became filled with minute 

 bubbles of gas. 



The sarcode mass occupies the interior of the dome of the shell cen- 

 trally, and is ordinarily attached to the mouth by a short neck. The 

 periphery of the mass is attached by divergent threads of ectosarc to the 

 interior of the dome and base of the shell. 



The pseudopods are usually few, simple, cylindrical, and rounded at 

 the end. 



