Make a drawing of a Paramœcium.
In comparing Paramœcium with Amœba it is apparent that the body of the first is less simple than that of the second. The definite opening for the ingress of food, the two nuclei, the fixed cilia, and the definite cell-wall giving a fixed shape to the body, are all specializations which make Paramœcium more complex than Amœba. But the whole body is still composed of a single cell, and there is, as in Amœba, no differentiation of the body-substance into different tissues, and no arrangement of body-parts as systems of organs.
Fig. 6.—Paramœcium sp.;
buccal groove at right. (From
life.)
Paramœcium may occasionally be found reproducing. This process takes place very much as in Amœba. The animal remains dormant for a while, the micronucleus then divides, the macronucleus elongates and finally divides in two, the protoplasm of the body becomes constricted into two parts, each part massing itself about the withdrawn halves of the macro- and micro-nuclei, and lastly the whole breaks into two smaller organisms which grow to be like the original. After multiplication or reproduction has gone on in this way for numerous generations (about one hundred), a fusion of two Paramœcia seems necessary before further divisions take place. This process of fusion, called conjugation, may be noted at some seasons. Two Paramœcia unite with their buccal grooves together, part of the macronucleus and micronucleus of each passes over to the other, and the mixed elements fuse together to form a new macro- and micronucleus in each half. The conjugating Paramœcia now separate, and each divides to form two new individuals.