EDMONDSON — PROTOZOA OF IOWA. 1 7 



Arcella vulgaris Ehr. 



Shell usually some shade of yellow or brown, about one-half as 

 high as broad, the character of the surface varied. Mouth circu- 

 lar, in the center of the ventral surface. 



Diameter of shell, 75 microns. 



Arcella vulgaris is one of the most common of fresh-water 

 Rhizopods, being found nearly everywhere in small streams, lakes 

 and ponds. 



The shell varies in color from pale yellow, which indicates a 

 young individual, to dark brown, indicating advanced age. The 

 species also varies greatly in the character of the markings of its 

 surface; commonly the shell presents a cancellated appearance, 

 the punctae being arranged with regularity. In older individuals, 

 however, this character may disappear, and no trace of the punctae 

 be found. Sometimes the convex surface is ornamented by shal- 

 low concavities arranged with more or less regularity. 



Other markings of the surface may also occur. 



Figure 20, Plate III, illustrates a common variety, the punctae, 

 however, not appearing in this individual. Fig. 21 shows a some- 

 what rarer variety, the surface being marked with shallow con- 

 cavities. 



Conjugation has often been observed to take place by the tem- 

 porary union of the ventral surfaces of the two individuals, as 

 represented by Fig. 22. During the fall of 1903 many individ- 

 uals taken from the pond water in Johnson county were seen to 

 undergo reproduction, a portion of the protoplasmic mass first 

 being pushed out from the individual's mouth and resting upon 

 it in spherical form, yet connected with the protoplasm within 

 the shell. (See Fig. 23, PI. IV.) 



From the extruded portion another shell is secreted, very trans- 

 parent at first, the two individuals finally separating. Fig. 24 

 represents an advanced stage in the process, the shells to the left 

 and right being the mother and daughter Arcella respectively, the 

 latter drawing into itself a sufficient quantity of protoplasm be- 

 fore separating from the parent. 



Sometimes compact masses comprising a score or more of indi- 

 viduals of this species may be seen adhering to each other, float- 

 ing in the water. 



In members of this genus the protoplasmic body of the individ- 



[Proc. D. a. S., Vol. XI. J 3 [May 2, 1906.] 



