In the remainder of this book the classification of animals is always kept in sight, and the student will see the terms species, genus, family, order, etc., practically used. In it all should be kept constantly in mind the significance of classification, that is, the existence of actual relationships among animals through descent. FOOTNOTES: [4] The lesser group called _variety_, or subspecies, we may leave out of consideration for the present. [5] Some species of animals are not represented by male individuals: and in some all the individuals are hermaphrodites, as explained in chapter XIV. [6] Each of these higher groups has a proper name composed of a single word. In the case of no group except the species is a name-word ever duplicated. Each genus, family, order, or higher group has a name-word peculiar to it, and belonging to it alone. CHAPTER XV BRANCH PROTOZOA: THE ONE-CELLED ANIMALS Of this group the structure and life-history of the Amœba (_Amœba_ sp.) and the Slipper Animalcule (_Paramœcium_ sp.) have already been treated in Chapter VI. Another example is the BELL ANIMALCULE (_Vorticella_ sp.) TECHNICAL NOTE.--Specimens of _Vorticella_ may usually be found in the same water with _Amœba_ and _Paramœcium_. The individuals live together in colonies, a single colony appearing to the naked eye as a tiny whitish mould-like tuft or spot on the surface of some leaf or stem or root in the water. Touch such a spot with a needle, and if it is a Vorticellid colony it will contract instantly. Bring bits of leaves, stems, etc., bearing Vorticellid colonies into the laboratory and keep in a small stagnant-water aquarium (a battery-jar of pond-water will do). Examine a colony of _Vorticella_ in a watch-glass of water or in a drop of water on a glass slide under the microscope. Note the stemmed bell-shaped bodies which compose the colony. Each bell and stem together form an individual _Vorticella_ (fig. 8.) How are the members of the colony fastened together? Tap the slide and note the sudden contraction of the animals; also the details of contraction in the case of an individual. Watch the colony expand; note the details of this movement in the case of an individual. Make drawings showing the colony expanded and contracted.