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

by Mary Somerville

Fig. 132. Foot of Naïs.

There is a colourless little fresh-water species of the genus Naïs, remarkable for the beauty of its bristled feet. There are two pairs on each ring of the worm, consisting of wart-like perforated protuberances, through which a number of microscopic bristles protrude, arranged in a radiating pencil like a fan. They are very slender, bent at the tip, and so transparent that they look like threads of spun glass; the worm thrusts them out and draws them in with extreme rapidity.

A blood-red Naïs lives in burrows in the mud at the bottom of springs and pools in immense multitudes; large tracts of the mud of the Thames are red with a species of them; half of their bodies stretched out of their burrows maintain a constant oscillating motion on its surface, but, like the earth-worm, they instantly shrink into their burrows on the least alarm. They have no respiratory organs; but their blood is aërated through their skin, which is so transparent that, with a microscope, the whole of the internal structure, the motions of the liquids, and the particles they contain are distinctly visible. The blood acts the part of internal gills, by aërating the colourless liquid contained in a set of vascular coils surrounding the organs of digestion.

The Tubicola are marine worms, forming the third order of Annelida, according to the system of M. Milne-Edwards. They live in tubes, either of a shelly calcareous substance, which forms naturally on the tenacious mucus of their skins, or in tubes artificially constructed by themselves of sand and particles of shell glued together. All the Tubicola can protrude their gills and the anterior part of their bodies, and some can leave their tube and return to it again. These worms, which form beautiful objects for the microscope, have ringed bodies with tubular bristled feet, and respiratory organs or gills fixed either on the head or near it. They have an alimentary canal loosely attached to the ventral wall of the body, and two systems of circulating liquids, one red, the other colourless. In the Tubular Annelids the principal organs of respiration are the contractile plumes on the head.

In the Terebella there are distinct organs for the aëration of both liquids, which form a beautiful plume when expanded, as in [fig. 133], which shows the animal when out of its tube. What may be called the head is fixed upon the first ring of the body. The mouth has a lip like a funnel-shaped cup with numerous long slender tubular tentacles; and two delicate arborescent branches or gills are fixed immediately behind the head. The colourless liquid which occupies the space between the alimentary canal and the ventral wall of the worm, is sent by the contractions of the body into the slender tubular filaments round the mouth, which are covered by cilia, whose action continually renews the stratum of water in contact with them. The blood in its usual course enters the capillary tubes of the arborescent gills, where it is oxygenized, and, after being distributed to the different parts of the body, returns to the heart and gills again.

Fig. 133. Terebella conchilega.—a, lip, surrounded by tentacles, b b, all placed upon the first segment of the body, c; the skin of the back, d, is laid open, exposing the circulatory system; e, pharynx; f, intestine; g, muscles of the belly; h, gland, supposed to be the liver; i, generative organs; j, feet; k k, gills; l, heart; m, dorso-intestinal vessel; n, intestinal vessel; n, venous sinus; o o, ventral trunk, branching into smaller veins, p.

The slender filaments which radiate from the head of the tubicular worms are flattened, sometimes tortuous, always ciliated, and are often barred and variegated by bright purple, green, and yellow tints, forming a rich and gorgeous crown.

The mucus, which cements together the particles of sand and shell for the artificial tubes of this kind of worms, is believed to be secreted from glands in the first segment of the body; but the long slender filaments of the head are the active agents in the structure. The tentacles are hollow bands with strong muscular edges, which the worm can bring together so as to form a cylinder, at any point of which it can take up a particle of sand, or a whole row of particles, and apply them to its glutinous body. The fibres at the free ends of the tentacles act both as muscular and suctorial organs; for when the worm is going to seize a particle of sand or food, the extremity of the tentacle is drawn in by the reflux of the colourless liquid in its interior, so that a cup-shaped cavity is formed in which the particle is secured by atmospheric pressure, aided by the power of the circular muscular fibres at the extremity of the tentacle.