Page:A Text-book of Animal Physiology.djvu/50

 of the struggle for existence and the survival of the fittest. If the cells of an organism are mightier than the bacteria, the latter are overwhelmed; but if the bacteria are too great in numbers or more vigorous, the cells must yield; the battle may waver—now dangerous disease, now improvement—but in the end the strongest in this, as in other instances, prevail. UNICELLULAR ANIMALS WITH DIFFERENTIATION OF STRUCTURE.

(Vorticella). Amœba is an example of a one-celled animal with little perceptible differentiation of structure or corresponding division of physiological labor. This is not, however, the case with all unicellular animals, and we proceed to study one of these with considerable development of both. The Bell-animalcule is found in both fresh and salt water, either single or in groups. It is anchored to some object by a rope-like stalk of clear protoplasm, that has a spiral appearance when contracted; and which, with a certain degree of regularity, shortens and lengthens alternately, suggesting that more definite movement (contraction) of the form of protoplasm known as muscle, to be studied later.

The body of the creature is bell-shaped, hence its name; the bell being provided with a thick everted lip (peristome), covered with bristle-like extensions of the protoplasm (cilia), which are in almost constant rhythmical motion. Covering the mouth of the bell is a lid, attached by a hinge of protoplasm to the body, which may be raised or lowered. A wide, funnel-like depression (œsophagus) leads into the softer substance within which it ends blindly. The outer part of the animal (cuticula) is denser and more transparent than any other part of the whole creature; next to this is a portion more granular and of intermediate transparency between the external and innermost portions (cortical layer). Below the disk is a space (contractile vesicle) filled with a thin, clear fluid, which may be seen to enlarge slowly and then to collapse suddenly. When the Vorticella is feeding, these vesicles may contain food-particles, and in the former, apparently, digestion goes on. Such food vacuoles (vesicles) may circulate up one side of the body of the animal and down the other. Their exact significance is not known, but it would appear as if digestion went on within them; and