296 ESSENTIALS OF ZOOLOGY 



Digestion, Assimilation, Respiration, and Catabolism or Dissimi- 

 lation all occur in a manner very similar to that described for 

 Amoeba. Egestion occurs at a definite anus. 



Excretion of the waste products of metabolism in solution is by 

 means of the alternate filling and expelling of fluid by the two con- 

 tractile vacuoles, or it may occur to some extent by diffusion 

 through the entire cell membrane. 



Growth occurs as it does in Amoeba and in all other organisms. 

 Under favorable conditions the storage of nutrient materials, like 

 starch and fats, occurs in the cytosome. Nutrition in this animal is 

 holozoic, and its living process is essentially like that of all higher 

 forms of animal life. 



Reproduction and Life History 



The actual reproduction is by transverse binary fission which in 

 itself is asexual. The cell divides transversely into individuals, and 

 this is repeated for long series of generations, one after another. 

 During this division process in P. caudatum, both the macronucleus 

 and the micronucleus divide, the old gullet divides into two, and two 

 new contractile vacuoles are formed by division of the old ones. The 

 micronucleus divides by mitosis, but the division of the macronucleus 

 is not distinctly so. The time required for the completion of a divi- 

 sion ranges between thirty minutes and two hours, depending on en- 

 vironmental conditions. Division is repeated at least once each 

 twenty-four hours and under especially favorable conditions, twice 

 a day. It has been estimated that if all survived and reproduced at 

 a normal rate, the descendants of one individual over a month's time 

 would number 265,000,000 individual Paramecia. 



P. caudatum is a conjugating form of Paramecium, while P. aurelia 

 and others seem not to conjugate. Conjugation is a temporary union 

 of two individuals with exchange of nuclear material. Calkins car- 

 ried some cultures of P. caudatum through a long series of genera- 

 tions and observed that conjugation occurs at intervals of approxi- 

 mately every two hundred generations. When two Paramecia are 

 ready to conjugate, they come in contact, with their oral surfaces 

 together, and adhere in this position (as A and B in Fig. 100). A 

 protoplasmic bridge is formed between the two individuals. This 

 union resembles a sexual act and has recently been described as 

 such. The conjugants are usually small, rather unhealthy appearing 

 individuals. Shortly after the adherence of the conjugants the 



