Common objects of the microscope

by J. G. Wood

CHAPTER IX

Fresh-water Worms—Planarians—Hydra—Polyzoa—Rotifers—Chætonotus—Water-Bears.

The fresh-water worms form a large and well-defined group, and a few words regarding them may be useful.

They are very common, and very difficult to find information about, most of the work relating to them having been done in Germany. At the same time, they are so highly organised and so transparent that the process of their life-history may be easily followed.

One large group has the peculiarity of multiplying by division, the last joints or segments being devoted to the formation of the new individual. At one time of the year the ordinary sexual process of reproduction takes the place of this method, and each worm is then surrounded by a belt such as may be seen in the common earthworm under similar conditions. Further information on this subject is greatly needed.

The type is the common Naïs, which has a body of thirty segments or more, two eye-specks on the head, and a double row of bristles along the back; whilst below, each segment carries strong hooked bristles, nearly buried in the body, by means of which the worm crawls. Inside the mouth is a large proboscis, which can be protruded, and this leads into a stomach which is merely an enlargement of the intestine which succeeds it. The circulation of the blood (which is colourless) can be easily watched. It begins at the tail with a contraction of the dorsal vessel, passes up to the head, and then down below the intestine to the tail again. The intestine is ciliated inside, and it is by a current of water carried into the intestine by these cilia that the blood is aërated.

In the next genus, Dero, this is clearly seen, for the tail (Plate XIV. Fig. [1]) is opened out into a wide shield, from which rise four, six, or even eight finger-like processes. These parts are all ciliated, and contain a network of blood-vessels. The worm lives in a case which it builds in the mud, and the way to find it is to put some of the mud into a glass beaker with water, and allow it to stand. If there be members of this family in it, their tails will be seen protruding above the water. Pour out the mud sharply, fill up with water, and allow the dirt to subside, and the worms may then be made to leave their cases by pressure by a camel hair pencil on the lower end of the tube, and may be caught with the dipping tube and placed in the live-box. They have no eyes, otherwise the general outline of the body closely resembles that of Naïs.

Slavína (Fig. [2]) has a row of touch-organs, like pimples, round each segment, and is a dirty looking creature, with an inordinately long first pair of bristles, but this reaches its acme in Pristina (Fig. [3]) (sometimes, though wrongly, called Stylaria) parasita, which has three long sets of bristles upon the back, and keeps these in constant wing-like motion. The true Stylaria has a long trunk, set right in the head, and tubular (Fig. [6]); it grows to a considerable length, and when in the stage of fission it is very funny to see the two proboscides waving about, one on the middle, as well as the original one at the head. There is also a form with a shorter proboscis of the same kind.

Bohemilla has a tremendous array of saw-like bristles upon the back, whilst Chætogaster has none at all in this position, and few below. Æolosoma has merely tufts of hair instead of bristles, and swims freely. It is easily recognised by the red, yellow, or green pigment spots in its skin, and by the ciliated head. Rarest of all the family is the one which connects it with the ordinary Tubifex, the red worm which lives in masses in the mud of brooks and ponds, the waving tails protruding above the water, and being instantly withdrawn when a foot is stamped upon the bank. Their Naid cousin is Naidium, and has red blood, but multiplies by fission, which Tubifex does not.