596 TRANSACTIONS OF SECTION D. 



the genus Nummulites in the relative lengtli of life (as indicated by size) of their 

 sexually and asexually produced forms. In N. variolarius the life-cycle is appa- 

 rently equally divided between the two, while in N. complayiatus the small megalo- 

 spheric form (' iV. Tchihatcheffi') is almost as much dwarfed by the gigantic 

 microspheric form as, in the life-history of a fern, the prothalUis is by a member 

 of the sporophytic generation. 



The following i'aper and Reports were read : — 



1. The Life-cycle of Protozoa. By Professor Gary N. Calkins. 



The tendency in modern protozoa study is to regard all work as incomplete 

 which does not fill out the entire series of changes which an organism passes 

 through in its life-cycle, and the whole cycle is becoming more and more important 

 in the characterisation of genera and species, and should hereafter be the sole basis 

 of new species. The definition of protozoa should also convey some expression of 

 the importance of the life-cycle, and I would suggest something like the following: 

 Protozoa are animal organisms consisting of independent single cells, which repro- 

 duce by division or spore formation the progeny passing through various phases of 

 activity collectively known as the life-cycle and manifesting various degrees of 

 vitality with accompanying form changes. 



The need of some check to the undue multiplication of protozoan species is 

 evident to anyone who consults the annual literature lists, and the confusion that 

 has arisen in classical cases like that of Plasmodium and Polymitus, Coccidium and 

 Eimeria, Trypanosoma and Halteridium, Paramecium caudatum and P. aurelia, is 

 sufficient to justify more stringent rules. The case of Paramecium is particularly 

 interesting because of the wide distribution and the almost universal acceptance 

 of the two species caudatum and aurelia. The specific differences between them 

 have been emphasised by Maupas, Hertwig, and more recently by Simpson, and 

 are based upon minor difi'erences in form and upon the presence of two microuuclei. 

 In my cultures last year one ex-conjugantfrom a pair of conjugating P. cawrfa^iw?! 

 appeared with all the characteristics of P. aurelia, including the form of the 

 body, the double micronucleus, and the greater sluggishness which is supposed to 

 accompany this species. After forty-five generations in culture, in which the 

 organisms were watched day by day, the aurelia characters were lost, and by four 

 months after conjugation the entire race had become P. caudatum again, with but 

 one micronucleus and with pointed extremity, and with much greater vigour. 

 There is no doubt that we have to do with but one species in this case. 



In any such life-history the organisms pass through phases of vitality which 

 can be compared with the different age periods of the metazoa. Thus we recognise 

 a period of extreme vigour of cell multiplication corresponding to the period of 

 youth of a metazoon : we recognise a period of maturity or of adolescence charac- 

 terised by changes in the chemical and physical balance of the cell accompanied by 

 well-marked differences in size or in protoplasmic structure and leading to the 

 formation of conjugating individuals with or without sexual differentiation ; and, 

 finally, in forms which do not conjugate, we recognise a more or less definite period 

 of old age or senescence ending in death. In many forms, especially in those cases 

 where dimorphic gametes are formed, the period of sexual maturity leads directly 

 into that of old age, and gametes that fail to conjugate are destined to an early 

 death without further multiplication. Thus it is in the majority of sporozoa and 

 in many rhizopods. In ciliates, on the other hand, failui'e to conjugate, although 

 fatal in the long run, does not involve the immediate death of the organism, and 

 many generations may be formed after this period is passed. In such cases the 

 peculiar cytoplasmic changes which accompany protoplasmic senility can be easily 

 followed. 



In most cases the change from youth to middle age in a protozoan life-cycle is 

 too gradual to be made out. In some cases, however, as in Paramecium or Tetra- 

 mitus, or Cercomonas, &c., the body becomes plastic or amoeboid, thus indicating 

 some change in the physical make-up of the protoplasm. In other cases the change 



