These arms support tentacles on each side, and leave a gap between, so that the whole pattern is crescentic, or crescent-shaped, and not circular. Extending as far as the points of the arms, and carried all round the crescent, is an extremely delicate membrane, like the finest gauze, which unites all the tentacles by their basal portions, and makes an elegant retreating curve between every two. Each tentacle exhibits two rows of cilia, which scintillate as their vibrations cause them to catch the light. The motion of the cilia is invariably down one side and up the other, the current or pattern being carried on from one tentacle to the other, all through the series. This characteristic, and the facility with which each cilium can be distinguished, gives great interest and beauty to the spectacle of this wonderful apparatus, by which water-currents are made to bathe the tentacles, and assist respiration, and also to carry food towards the mouth, over which a sort of finger or tongue is stretched to guard the way, and exercise some choice as to what particles shall be permitted to pass on. This organ is called the epistome, from two Greek words, signifying "upon the mouth."
If the cell is an old one, it may be covered with so much extraneous matter as to obscure the economy within; but we are fortunate in having a transparent specimen before us, through which we can see all that goes on. The alimentary tube, after forming a capacious cavity, much longer than it is broad, turns round and terminates in an orifice near the mouth, and just below the integuments. When refuse has to be discharged, this orifice is protruded; and after the operation is over, it draws back as before. Long muscles, composed of separate threads or fibres, pull the creature in and out of its cell, and at the part where the stomach ends, and the intestine turns round, is attached a long flexible rope, called the funiculus, which goes to the bottom of the cell. The passage of the food down to the stomach, its digestion, and the eviction of the residue, can all be watched; and when a large morsel is swallowed, the spectacle is curious in the extreme.
One day a polyzoon caught a large rotifer, (R. vulgaris,) which, with several others of its tribe, had been walking over the cœnœcium, and swimming amongst the tentacles, as if unconscious of danger. All of a sudden it went down the whirlpool leading to the mouth, was rolled up by a process that could not be traced, and without an instant's loss of time, was seen shooting down in rapid descent to the gulf below, where it looked a potato-shaped mass, utterly destitute of its characteristic living form. Having been made into a bolus, the unhappy rotifer, who never gave the faintest sign of vitality, was tossed up and down from the top to the bottom of the stomach, just as a billiard-ball might be thrown from the top to the bottom of a stocking. This process went on for hours, the ball gradually diminishing in size, until at last it was lost in the general brown mass with which the stomach was filled. The bottom of the stomach seems well supplied with muscular fibres, to cause the constrictions by which this work is chiefly performed, and by keeping a colony for a month or two, I had many opportunities of seeing my Polyzoa at their meals.
When alarmed the tentacles were quickly retracted, but although these creatures are said to dislike the light, and usually keep away from it in their native haunts, my specimens had no objection to come out in a strong illumination, and seemed perfectly at their ease. They were indeed most amiable creatures, and never failed to display their charms to admiring visitors, who rewarded them with unmeasured praise. Twice I had an opportunity of observing an action I cannot explain, except by supposing either that the tentacles of the Plumatella have some poisonous action, or that rotifers are susceptible of fear. On these occasions the common rotifer was the subject of the experiment. First one and then another got among the tentacles, and on escaping seemed very poorly. One fellow was, to borrow a phrase from Professor Thomas Sayers, "completely doubled up," and two or three seconds—long periods in a rotifer's life—elapsed before he came to himself again.
By keeping a colony of the Plumatella for a few weeks in a glass trough, and occasionally supplying them with fresh water from an aquarium, containing the animalcules, they are easily preserved in good health, and as they develop fresh cells, the process of growth may be readily watched. This production of fresh individuals enlarges the parent colony, but could not be the means of founding a new one, which is accomplished by two other modes. A little way down the cells Professor Allman discovered an ovary attached to the internal tube by a short peduncle, or foot stalk, while a testis or male generative organ is attached to the funiculus, or "little rope," we have already described.
July and August are the best times for observing the ovaries, and they are most conspicuous in the genera Alcyonella and Paludicella. True eggs are developed in the ovaries in a manner resembling this mode of multiplication in other animals; but there is another kind of egg, or, perhaps to speak more properly, a variety of bud, which is extremely curious. In looking at our specimens we noticed brown oval bodies in the cells; these, on careful examination, presented the appearance of the sketch. The centre is dark, covered with a network, which is more conspicuous in the lighter coloured and more transparent margins. These curious bodies are produced from the funiculus, and act as reserves of propagative force, as they are not hatched or developed until they get out and find themselves exposed to appropriate circumstances. Professor Allman names them Statoblasts, or stationary germs, and they bear some resemblance to what are called the "winter eggs" of some other creatures. The Professor was never able to discover any mode by which they were permitted to escape from the cells, and in our colonies none were allowed to leave their homes until the death of their parent, and the decomposition of its cell had taken place; a process which went on contemporaneously with the growth of new cells, until the plant on which the cœnœcium was situated, rotted away, and then unfortunately the whole concern went to pieces.
Plumatella repens on a leaf.
The tubes of the Plumatella, and of most other Polyzoa, are composed of two coats, called respectively endocyst and ectocyst, that is, "inner case" and "outer case." The first is vitally endowed, and exhibits vessels and muscular fibres. The second or outer case is thrown off by the first. It is a parchment-like substance, strengthened by the adhesion of dirt particles, and does not appear to exercise any vital functions, but to be merely a covering for protection. The inner layer terminates in the neck of the bag before described, as exserted when the polypide comes out, and inverted when it goes in. This mode of making a case or sheath by inversion of a bag is technically called invagination, and is readily seen in new and transparent cells.
The movement of eversion, or coming out, is chiefly produced by the contraction of the endocyst; while the inversion, or getting in again, is performed by the long muscles, which, when the animal is extended, are seen attached to it like ropes. Upon these muscles Professor Allman remarks that they are "especially interesting in a physiological point of view, as they seem to present us with an example of true muscular tissue, reduced to its simplest and essential form. A muscle may here be viewed as a beautiful dissection far surpassing the most refined preparation of the dissecting needle, for it is composed of a bundle of elementary fibres, totally separate from one another through their entire course." He further adds, "The fibres of the great retractor muscle are distinctly marked by transverse striæ;—a condition, however, which is not at all times equally perceptible, and some of our best observers have denied to the Polyzoon the existence of striated fibre."