2 Gary N. Calkins 



wig in 1889, discovered a difference in the two forms which appeared 

 to have specific value, and since then the two species in question, 

 caudaiiim and aurelia, have been generally accepted as "good" 

 species. 



Paramecium aurelia, according to Maupas, differs from P. cau- 

 datum in the following points: It is smaller (70 to 290 ft, as against 

 120 to 325 /i in P. caudatum); its posterior end is rounded, while 

 P. caudatum has an attenuated end (hence caudatum); it has two 

 small micronuclei (3 to 5 /a in diameter), while P. caudatum has 

 but one (8 to 10 /tt); in conjugation its macronucleus becomes "rib- 

 bon "-shaped at an earlier period than in P. caudatum; and after 

 conjugation its cleavage nucleus gives rise to four corpuscles, whereas 

 in P. caudatum there are eight. In deciding which of these forms to 

 call caudatum and which aurelia, Maupas could not determine which 

 type Miiller had seen, and went back therefore only to Ehrenberg, 

 who' in naming P. caudatum had noted the attenuated posterior 

 end. Hence it turns out that the more common form of Para- 

 mecium has become widely known as P. caudatum, while the less 

 common form bears the original name P. aurelia. If the two are 

 only variants of the same species, it follow^s from the rules of zo- 

 ological nomenclature that the common and well-known name Para- 

 mecium caudatum must be given up and P. aurelia substituted. 

 That this must be the case follows, as I believe, from the observa- 

 tions here described. In the following description the names P. 

 caudatum and P. aurelia will be used for those variants of the organ- 

 isms which agree with Maupas' specific characteristics. 



On March 11, 1905, four pairs of conjugating Paramecium 

 caudatum were isolated from a culture that had been running for 

 some weeks in the laboratory. Each pair was confined in a hollow 

 ground slide in a medium of hay infusion made the previous day 

 by boihng a small quantity of hay in tap water. The usual period 

 of conjugation is from 18 to 24 hours, and by the following day 

 all of the pairs had separated, and the different individuals were 

 swimming about freely in the hay infusion as apparently normal 

 ex-conjugants. Each individual was isolated and fed on hay infu- 

 sion, and each became the progenitor of a more or less extended Hne 

 of Paramecia, the method followed being the same as that described 



