The corona is much more complicated in the evertile type, characteristic of the stationary or sessile, and of the bdelloid rotifers, two groups very different otherwise, but alike in that they do not sally forth to seek their food, but wait for it to be brought to them by external currents or by those set up by themselves.
The sessile rotifers are unique among the class in having an immature stage, lasting some days. When hatched, the young ani mals, little resembling their parents, and having a very simple corona, swim about for a while. Having chosen a position they affix themselves (for life) by the foot, and as they grow, develop the evertile corona proper to the adult. They are mostly inde pendent, but certain of them form com munities by affixing themselves, when young, close to others. Sometimes such communities are attached to plant-stems, sometimes they are free, the animals radi ating from a common centre, and the com munity swimming through the water as a revolving sphere.
The ciliated area is mostly disposed as a band fringing a shallow disc-like ex pansion, rounded, elliptic, heart-shaped, or two-, four-, or eight lobed, into which the head opens as the corona unfolds. In one family the whole head opens out as a cup whose rim is drawn out into lobes beset with long hair-like setae arranged to form a living net, wherein the animals can draw their prey by the influence of cilia hidden in the depth of the cup. In one of the most beautiful of such forms, the "crown animalcule" (Stepha noceros fimbriatus), the rim is drawn out into erect arms with approximating tips and furnished with regularly placed tufts of cilia, closing the gaps between the arms and so forming a trap. Among the bdelloids the corona consists mainly of two discs usually distinct, surmounting short pedicels arising from the back of the gaping mouth. The discs can be employed for swimming and for feeding while swimming, but most species feed when anchored by the foot, and when they desire to travel usually creep in leech-like manner; some exceptional species however, swim continuously, and some cannot swim at all.
In the typical form of the evertile corona, the cilia of the band fringing the upper surface of the lobes or discs are conspicuous and constitute the trochus. Almost parallel with it is another band, of much shorter cilia, the cingulum. Among the sessile species it passes round the under edge of the lobes or discs, and in the Bdelloida, round the bases of the pedicels and so to and around the lower lip on the inside, merging into the cilia of the mouth. Particles floating within reach of the trochus-cilia are
struck by them within range of those of the cingulum, which in turn impel them to the mouth to be swallowed.
A curious illusory appearance of cogged wheels in rapid revolu tion, which greatly puzzled the early microscopists, is caused by the trochus-cilia. It happened that species showing this appear ance were among the first rotifers discovered and that a long period elapsed before it was satisfactorily explained. Meanwhile it had led to all the known species being called "wheel animal cules" and thus to the later name of Rotifera (wheel-bearers) here employed. It is now believed that a succession of nerve impulses, following each other at short and regular intervals, travel along the protoplasmic bases of the cilia, causing each of them, when reached, to lash violently downwards.
Among the many-segmented bdelloids, the first two segments form the rostrum, a structure peculiar to the group ; the rostral tip, specially adapted, is employed to affix the body when creeping, the mouth, on the third or oral segment, being then closed with the corona hidden within. When it is desired to feed or to swim, the mouth is opened, the corona pushed forth, and the rostrum, in a collapsed condition, is thrust to the back and kept there while the corona continues active.
Mastax.—The food of rotifers consists in most cases of float ing particles, excessively minute fragments of plant or animal tissues, bacteria, etc., but there are numerous exceptions. Many of the hunting rotifers will pounce upon weaker forms and gulp them down, tear them to shreds or suck out the soft interiors. Others successfully attack small Cladocera, such as Chydorus, and test-dwelling rhizopods are sometimes invaded and eaten. The contents of a water-snail's egg or of confervoid cells are obtained by piercing the investing shell or cell-wall. Diatoms, swallowed whole, are a favourite food of many forms and the smaller flagel lates are also in request. Among the sessile rotifers the trap-making species prey upon the lesser animalcules and also upon flag ellates. When secured, the food is passed down a short, distensible gullet to the mas tax, or jaws.