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

by Mary Somerville

From the anterior rim of the shelly cup, the Brachion protrudes a waved outline of limpid flesh which, as soon as it rises above the level of the sharp-pointed spines, spreads out into three broad flattish muscular lobes. On the edges of the middle one there are very strong cilia like stiff bristles, which do not vibrate, but are either erect or converge to a point, whereas the edges of the other two lobes are thickly fringed with long stout cilia, which, by striking the water in perpetual rapid succession, each cilium bending and rising again, produce the appearance of two circles of dark spots in rapid horizontal rotation, like wheels on their axis. It is merely an optical deception, for both the animal and its lobes may be at rest. The vibrations of the cilia can be instantaneously arrested, and the whole apparatus drawn out of sight, and as instantaneously protruded and set in motion.

In the flesh, on the ventral side of the Brachion, there is a deep cleft, the edges of which as well as the whole interior of a tube of which it is the orifice, are thickly covered with vibratile cilia. This tube leads to a mouth with powerful jaws of unwonted structure, which is so deeply sunk in the tissues of the body, that it never comes into contact with the water. It opens into a gullet leading to a stomach, intestine, and vent, at the posterior end of the body.

The vibrations of the cilia on the lobes of the animal’s head form two circular currents in the water, like whirlpools, which draw all floating particles into their vortices, and the streams from the two whirlpools uniting into one current, flow off horizontally and pass immediately over the slit on the ventral side of the animal. Some of the floating particles are arrested by the cilia on the edges of the slit, and are drawn into the sunken mouth by the vibrations of the cilia in the tube. The edges of the slit act like lips, and seem to possess the sense of taste, or of some modification of touch, which enables them to select from the particles presented to them, such as are fit for food; these are admitted into the mouth, where they are bruised by the powerful jaws. The mouth or masticating apparatus is the most extraordinary and complex part of this animal. It consists of two horny toothed jaws, acting like hammers upon an anvil. The two hammers, which approach each other from the dorsal sides of the body, are each formed of two parts united by a hinge; the first parts correspond to the handles; the second parts, which are bent at right angles to the first, resemble hands with five or six finger-shaped teeth united by a thin membrane. The teeth are parallel to one another when they meet on the anvil, and are seen through the transparent mass tearing the food into fragments. Some of the Rotifers resemble the Errant Annelids in being able to turn this complicated machine inside-out through the ciliated tube and slit, so as to bring it into contact with the water. When the food has been masticated it is sent into the stomach, where it is digested. The whole of this process is seen through the transparent and colourless body of the Brachion, because its favourite food is the Syncryn velox, a minute bright green plant, which from its active motions was at one time believed to be an animal.

The Brachion has four longitudinal muscular bands transversely striated, which move the ciliated lobes of the head, push them out and draw them in. From these muscular threads are sent to the different parts of the body, to the mouth especially, two strong bands, which bend and unbend the joints of the hammer-like jaws. The vigorous motions of the long serpentine foot and the firm hold of its anchors are owing to muscular bands fixed high up on the interior wall of the body, which extend throughout the whole length of the flexible organ. As long as the Brachion is fixed, the vibrations of the cilia on its lobes only produce whirlpools in the water, but the moment that it lets go its hold, these vibrations, in consequence of the reaction of the water, give the animal both a smooth progressive motion and a rotation round its axis.

Minute as the Brachionus pala is, it has several organs of sense. A sparkling, ruby-coloured, square eye-speck with a crystalline lens and crimson pigment layer is placed on a wart-like prominence on its back, and this prominence Mr. Gosse believes to be the brain of the animal. In the cleft between the spines and close to the eye-speck are two tubes, one within the other. The innermost tube, which can be protruded and withdrawn, has a bunch of bristles at its extremity that have the sensibility of antennæ. Nerves from the brain pass into these, to the various organs of the body, and to the lobes on the head.

The Brachion has no propelling vessel or heart to maintain the circulation of its liquids, but, like the Annelids, a colourless liquid occupies the general cavity between the alimentary canal and the internal wall of the body. It is believed to be connected with nutrition, and is furnished with oxygen by a complicated organism, and is kept in motion by the vibrations of long cilia. The determination of the whole structure and motions of a creature barely visible to the naked eye, is a wonderful instance of microscopic research, and of the perfection of the mechanism exhibited in the most minute objects of creation.

Fig. 137. Common Rotifer:—a, mouth; b, eye-spots; c, wheels; d, probably antenna; e, jaws and teeth; f, alimentary canal; g, glandular mass enclosing it; h, longitudinal muscles; i, tubes of water-vascular system; k, young animal; l, cloaca.

[Fig. 137] represents the common Rotifer when its wheels are expanded and when they are retracted. The body is slender and flexible, it is stretched out by longitudinal muscles, and its girth is diminished by circular ones. The internal structure is similar to that of the Brachion, but there is a prominence or head between the wheels on which there are two crimson eye-specks, and the foot terminates in three concentric movable tubes that can be protruded and drawn in like the tubes of a telescope; each has a pair of claspers to enable the Rotifer to fix itself to any object.

The Rotifers are male and female, but, like the greater number of Infusoria, the males are only produced at intervals. The female Rotifers have their perfect form when they leave the egg: they even come out of the egg while it is attached to the tail of the mother, as in the Brachionus pala ([fig. 136]). The males, when hatched, have neither spines nor mouth, yet, during their short lives, their motions are very fleet on account of the vibrations of long cilia round their front.