GENUS ARCELLA. 109 



yellowish. The former is the old shell; the latter, the new one. The 

 Arcella passes alternately from one shell to the other, leaving always, 

 however, a part of its body within the old shell. Finally, when the new 

 shell has assumed due consistence, the Arcella passes entirely into it, and in 

 the violent separation which occurs at the moment between the two shells, 

 the old one is split.* 



Individuals of Arcella are often met with in pairs applied base to base, 

 and, in common witlr other rhizopods in a similar position, are generally 

 supposed to be in conjugation, that is to say, in a condition akin to sexual 

 intercourse. Sometimes the two individuals present the same appearance 

 of form, size, and color, but frequently exhibit marked difference in color, 

 while the size and shape are the same or very nearly so. Often the one 

 shell is colorless or faintly yellowish, while the other exhibits a more or less 

 decided yellow or brown color. Between these extremes, couples are met 

 with exhibiting differences of color of every shade of those just mentioned. 

 Frequently the sarcode of the conjoined Arcellas may be observed to form a 

 single mass, rapidly streaming to and fro between the shells. Subsequently 

 the mass separates into two portions, which retire into their respective shells. 

 These finally separate, and the animals move away without showing any- 

 thing conspicuously different from what is ordinarily observed in them. 



Many of the instances of paired Arcellas I have looked upon as prob- 

 able cases of conjugation ; but others, especially those presenting wide 

 differences in the color of the shells, I have suspected to be cases of repro- 

 duction, in which the individual with the colorless or nearly colorless shell 

 was the offspring derived by division from the sarcode mass of the individual 

 with the colored shell. This view accords with that of Hertwig and Lesser, f 

 as I understand them, in relation. to the same kind of cases. I have never 

 observed the final result in the manner described by Claparede and Lach- 

 mann, that is to say, the occupation of the apparently new shell by all the 

 sarcode mass of the old one, and the rupture of this. The new shell, instead 

 of being larger than the parent shell, is often slightly smaller, but is more 

 delicate and flexible, and perhaps, therefore, also more capable of expansion. 

 It assumes color and becomes darker and stronger with age. At first, the 

 cancellated structure is barely visible, but soon becomes more distinct. 



* Etudes sur lea Infusoires et lea Ehizopodes, i, 445. 



t Archiv f. niLkroskopische Anatomic, x, 1874, Supplement, p. 97. 



