On Molecular and Microscopic Science, Volume 2 (of 2)

by Mary Somerville

The ferments are the least in size and lowest in organization of all the Infusoria. The higher group which abounds in stagnant pools and ditches are exceedingly numerous, and their forms are varied beyond description. They are globular, ovoid, long and slender, short and thick, many have tails, one species is exactly like a swan with a long bending neck, but whatever the form may be, all have a mouth and gullet. Although the skin of the Infusoria is generally a mere pellicle, that of the red Paramœcium and some others resembles the cellulose covering of a vegetable cell, engraved with a pattern; but in all cases respiration is performed through the skin.

Whatever form the cell which constitutes the body of the Infusoria may have, the highly contractile diaphanous pellicle on its exterior is drawn out into minute slender cilia which are the locomotive organs of these creatures. Vibrating cilia form a circlet round the mouth of some of these animalcules, a group of very long ones are placed like whiskers on each side of it, as in the Paramœcium caudatum, and in some cases there is a bunch of bristles in front. Certain Infusoria have cilia in longitudinal rows, and in many the whole body is either partially or entirely covered with short ones. In some Infusoria their vibrations are constant, in others interrupted, and so rapid that the cilia are invisible. These delicate fibres which vary from the ¹⁄₅₀₀th to the ¹⁄_13,000th part of an inch in length, move simultaneously or consecutively in the same direction and back again, as when a fitful breeze passes over a field of corn. These animalcules seize their prey with their cilia, and swim in the infusions or stagnant pools, in which they abound, in the most varied and fantastic manner; darting like an arrow in a straight line, making curious leaps and gyrations, or fixing themselves to an object by one of their cilia and spinning round it with great velocity, while some only creep. These motions, which bring the animalcules into fresh portions of the liquid, are probably excited by the desire for food and respiration.

Fig. 104. Paramœcium caudatum. a a, contractile vesicles; b, mouth.

Fig. 105. Kerona silurus.—a, contractile vesicle; b, mouth; c c, animalcules which have been swallowed by the Kerona.

None of the Infusoria have regular jointed limbs, but certain families of the higher genera have peculiar and powerful organs of locomotion partly consisting of strong ciliary bristles placed on the anterior in rows, used for crawling or climbing, and partly consisting of groups of strong processes which serve as traction feet, generally trailing behind the animal while swimming, or used to push it forward. When the bristles or cilia of this high group of Infusoria are used for crawling their motions may be traced to the contraction of the skin; but in the Infusoria that are never fatigued though their cilia vibrate incessantly night and day, it may be presumed that these motions are altogether independent of the will of the animal, in as much as there are innumerable cilia in the human frame that are never at rest during the whole course of our existence, nor do their vibrations cease till a considerable time after death—a striking instance of unconscious and involuntary motion.

The cell which constitutes the body of the Infusoria is filled with sarcode, which is the receptacle of the food, and in that substance all the internal organs of the animalcule are imbedded. In the higher genera it is full of granular particles of different sizes and forms, and it contains a nucleus in its centre, characteristic of cellular protozoa generally. The nucleus is of a dull yellow colour, and is enclosed in a transparent capsule, which in the smaller Infusoria reflects light brilliantly. It is generally of an ovoid form and single, but in several species the nucleus is double, and in others there are several nuclei.

The Infusoria have a distinct mouth and gullet, and for the most part another aperture for ejecting the indigestible part of their food, though some discharge it by the mouth, others through any part of their surface. A few of the larger Infusoria devour the smaller; others feed on minute vegetable particles, chiefly diatoms. Solid substances that are swallowed are collected into little masses mixed with water, and enter into clear spherical spaces called vacuoles in various parts of the sarcode, where they are partially digested. When the animal has not had food for some time, clear spaces only filled with a very transparent fluid are seen, variable both in size and number. It was on account of the digestive vacuoles that the Infusoria were called Polygastria by Ehrenberg.

Transparent contractile vesicles of a totally different nature from the vacuoles are peculiarly characteristic of such Infusoria as have a digestive cavity. They exist either singly or in even numbers, from 2 to 16, according to the species, and never change their places; but they dilate and contract rhythmically at pretty regular intervals. When dilated, they are filled with a clear, colourless fluid, the product of the digestive process which they are supposed to diffuse through the body of the animal.