92 



PROTOZOA. 



" micronucleus " lying by the side of the larger "macro- 

 nucleus." Food vacuoles occur as in the Anmba. There 

 are two contractile vacuoles, from which fine canals radiate 



'/> 2 



Fig. 21. — Conjugation ai Paramaciitm aurelia — four 

 stages. (After IMaupas.) 



T. Shows macronucleus (^V), and two micronuclei («) in each of 

 the two conjugates. 



2. Shows breaking up of macronucleus, and multiphcation of 

 micronuclei to eight. 



3. Shows the fertilisation in progress; the macronucleus is 

 vanishing. 



4. Shows a single (fertilised) micronucleus in each conjugate. 



into the surrounding protoplasm ; these discharge into the 

 vacuole, which then bursts to the exterior. 



Life History. — Growth is followed by obliquely transverse 

 division into two (Fig 20, <//».). One-half includes the 

 "mouth," the other has to make one. As well as this 

 simple fission, a process of transient conjugation also occurs. 



Fig. 22. — Diagrammatic expression of process 

 of conjugation in Paraniixciuni aurelia. {After 

 Maupas.) 



A. The two micronuclei enlarge. 



B. Each divides into two. 



C. Eight micronuclei result. 



D. Seven disappear ; one (darkened) divides into two. 



E. .'\n interchange and fusion occurs, and the conjugates 

 separate. 



F. The fertilised micronucleus divides into two. 



G. Each conjugate begins to divide, the micronucleus 

 of each half dividing into two, one of which becomes 

 macronucleus, while the others form the two normal 

 micronuclei. The top line represents four individuals, each 

 with a macronucleus and tu'o micronuclei. 



Two individuals approach one another closely, the two 

 nuclei of each break up, an exchange of pieces of the micro- 

 nucleus takes place ; the two then separate, each to recon- 

 struct its two nuclei. This process is necessary for the 

 continued health of the species. 



