=Nucleus and Vacuoles.=—There is a large nucleus called the macronucleus, and beside it a smaller one called the micronucleus. They are hard to see. About one third of the way from each end is a clear, pulsating space (bb. Fig. 15) called the pulsating vacuole. These spaces contract until they disappear, and then reappear, gradually expanding. Tubes lead from the vacuoles which probably serve to keep the contents of the cell in circulation. [Illustration: FIG. 16.—TWO PARAMECIA exchanging parts of their nuclei. ] =Feeding.=—A depression, or _groove_, is seen on one side; this serves as a mouth (Figs.). A _tube_ which serves as a gullet leads from the mouth-groove to the interior of the cell. The mouth-groove is lined with cilia which sweep food particles inward. The particles accumulate in a mass at the inner end of the gullet, become separated from it as a _food ball_ (Fig. 14), and sink into the soft protoplasm of the body. The food balls follow a circular course through the endoplasm, keeping near the ectoplasm. [Illustration: FIG. 17.—VORTICELLA (or bell animalcule), two extended, one withdrawn. ] =Reproduction.=—This, as in the amœba, is by division, the constriction being in the middle, and part of the nucleus going to each half. Sometimes two individuals come together with their mouth-grooves touching and exchange parts of their nuclei (Fig. 16). They then separate and each divides to form two new individuals. [Illustration: FIG. 18.—Euglena. ] We thus see that the paramecium, though of only one cell, _is a much more complex and advanced animal than the amœba_. The tiny paddles, or cilia, the mouth-groove, etc., have their special duties similar to the specialized organs of the many-celled animals to be studied later. [Illustration: FIG. 19.—SHELL OF A RADIOLARIAN. ] If time and circumstances allow a prolonged study, several additional facts may be observed by the pupil, _e.g._ Does the paramecium swim with the same end always foremost, and same side uppermost? Can it move backwards? Avoid obstacles? Change shape in a narrow passage? Does refuse matter leave the body at any particular place? Trace movement of the food particles. Draw the paramecium. Which has more permanent parts, the _amœba_ or _paramecium_? Name two anatomical similarities and three differences; four functional similarities and three differences. The amœba belongs in the class of protozoans called _Rhizopoda_ “root footed.” =Other classes of Protozoans= are the _Infusorians_ (in the broad sense of the term), which have many waving cilia (Fig. 17) or one whiplike flagellum (Fig. 18), and the _Foraminifers_, which possess a calcareous shell pierced with holes (Fig. 19). Much chalky limestone has been formed of their shells. To which class does the paramecium belong? Protozoans furnish a large amount of food to the higher animals. CHAPTER III SPONGES