Paramecium calkinsi sp. n

There are at present four well-established species of Paramecium (P. aurelia, P. caudatum, P. bursaria, and P. putrinum) which fall naturally into two quite clearly defined groups. One group comprises Paramecium aurelia and Paramecium caudatum which are characterized by a relatively long spindle-shaped body. The other group includes Paramecium bursaria and Paramecium putrinum which exhibit a somewhat shorter and broader form, with a tendency toward a dorso-ventral flattening. All the species have a single micronucleus except Paramecium aurelia, which has two micronuclei each showing characteristic ‘endosome’ structure. The purpose of the present paper is to record the discovery in January, 1920, of a hitherto undescribed form of Paramecium. It has now been extensively studied in pedigree cultures for more than a year and during this time it has bred true. I therefore definitely designate it a new species, Paramecium calkinsi, in recognition of the fact that Professor G. N. Calkins of Columbia University introduced students of the Infusoria to exact, daily isolation, pedigree culture methods. The general body form of the new species places it at once in the bursaria-putrinum group, but its micronuclei both in structure and number are identical with those of Paramecium aurelia. In brief, Paramecium calkinsi represents the ‘aurelia’ type of micronuclear complex in the ‘bursaria’ group of species. Details of the structure and life history of Paramecium calkinsi will appear in the Biological Bulletin.