166 FRESH- WATER RHIZOPODS OF NORTH AMERICA. 



The sarcode is colorless, except that the endosarc may be more or less 

 colored by the presence of food, as seen in fig. 12. The form, arrange- 

 ment, and mode of attachment of the sarcode mass are the same as in the 

 species of Nebela, etc. The pseudopods are many, and variable like those 

 of E. pida. 



In the encysted state of H. petricola, the sarcode forms a compressed 

 oval or spherical body, nearly colorless or pale yellowish, with a granular 

 constitution, often mingled with globules of various sizes of an oleaginous 

 appearance. The mouth of the shell is closed by a laminated epiphragm. 



The shell of Heleopera petricola ranges from ^^th to jl^th of an inch in 

 length, ith to ^^th of an inch in the greater breadth, and gith to ^^th of ^^ 

 inch in the less breadth. The mouth is from ^-ith to ^th of an inch broad 

 by j^th of an inch in the opposite diameter. 



AEOELLA. 



Diminutive of the Latin, area, an ark. 



Shell composed of chitinoid membrane, with a minutely hexagonal 

 cancellated structure, translucent and commonly of a brown color, variable 

 in shape, but usually more or less campanulate, with a circular base con- 

 cavely infundibuliform and convex at the border, and with the mouth 

 central. Sarcode occupying the central portion of the shell, connected 

 with the mouth by a cylindrical neck, and by means of threads of ectosarc 

 with the dome of the shell. Pseudopods few', digitate, blunt, simple or 

 branching. Commonly two nuclei, situated one on each side of the sar- 

 code mass. Contractile vesicles several, occupying the periphery of the 

 upper part of the latter. 



The genus Arcella was discovered and first described by Ehrenberg 

 in 1830. In its varied forms, it is among the commonest of the shell-bearing 

 fresh-water rhizopods. Ehrenberg and others have described, figured, and 

 named a number of varieties as so many dififerent species, which, however, 

 appear to graduate into one anotlier by those of intermediate or transi- 

 tional character. Claparfede and Lachmann say they have proved that an 

 Arcella, of the form described by Elarenberg as A. vulgaris, sometimes con- 

 structs for itself a new shell of one of the forms to which Perty gives the 

 names of ^. angulosa, A. dentata, and A. Okeni. They add, that there can 



