A nervous system is well developed. Within the head lies a large ganglion, or brain, which is mostly of flattened form. From thence nerve-threads pass down each side and to the eyes, the antennae and other sense-organs. The bdelloids and many of the hunting rotifers have a single antenna behind the head, sometimes of conspicuous length, but frequently very minute. Others of the latter group and many of the sessile forms have two antennae on the head, and certain loricated species have one antenna on the head, and two more to right and left further to the rear. Finally, several largish species are furnished with four distinct antennae. There are of ten two eyes, either in front of the head or behind the brain, but a single eye is more usual, and while there are occasionally three, very many species have none. When examined the eyes show mostly a crystalline particle backed by ruby-red pigment. Among secondary sense-organs frequently present may be mentioned the trochal setae of the bdelloids, and the tactile setae of the Synchaetidae and allied forms. In many species, notably in the great family Notommatidae, there is con spicuous in close contact with the brain, the so-called retro-cere bral organ, of three principal parts, viz.—a central pendulous sac stretching some way behind the ganglion, having its interior filled with coarsely granular matter usually somewhat opaque, and flanked by two glands, sometimes larger, sometimes smaller, than the central sac. In many of the swimming species the organ has not been detected, and its function is not yet certain.
An excretory system is repre sented by a very slender, much convoluted tube, which passes down each side of the body from the head rearwards. To the tubes are attached at intervals by short stalks a series of minute "flame cells," tag-like in form, hollow and closed at the free end and enclosing a pulsating cilium. The tags, which usually number five to each tube, but in certain Asplauchnae are greatly in excess of this, are believed to draw out from the body the effete fluids, which are carried either to the cloaca or to a collapsible bladder near, whence they are discharged at short intervals.
Besides the special muscles, which operate the motions of the mastax, there are very numerous sinews, which pass freely through the body, each having its own course and office, and operate the movements of the several parts of the rotifer, apart altogether from those arising from ciliary action. The illoricated species, bdelloids and others, have also an exceptionally interesting system of muscles, nestling close under the skin, and somewhat difficult to see, but controlling the skin tension.
When there is no lorica, the skin-pores of the trunk exude a secretion, frequently so viscid that debris, etc., readily adhere to it. Sometimes this becomes a close-fitting coat, but it may be made into a loose case, often of flask-like shape, enclosing the roti fer, which can protrude its head from the open end and feed when it pleases. Among the sessile forms slightly conical cases are made,
often of gelatinous substance, sometimes hardened. In one well known species the skin secretion is not employed, but by a special organ connected with the corona, the rotifer prepares small pellets of unswallowed particles, and with these builds, brick by brick, a more permanent dwelling.
This same pore-secretion has been said to be the means em ployed by most bdelloid rotifers, when fearing desiccation, to encase their bodies, which they have retracted to the smallest possible compass, but doubt has again been cast on the accuracy of the usually accepted explanation, which in its day put an end to many long-winded discussions on the revivification of rotifers, etc. It seems more important to ascertain which species have the power of so avoiding sudden death, and it is certain that it is almost limited to bdelloid forms and is probably by no means universal among them.
More than four-fifths of the known species of Rotifera are represented only by females. The males of the others are in most cases extremely rare, much smaller and somewhat unlike their own females. They attract attention by their restless, rapid and seemingly aimless swimming. If one be examined, it will generally be found minus jaws, alimentary canal or bladder, but having a very simplified corona of rather long cilia, and as sex organs, a great sperm-sac, occupying much of the body, a seminal duct and mostly a protrusible penis. They sur vive a very few days. No male has yet been found among Bdel loida. As sex organs the females have only the two-fold ovary, usually conspicuous in the trunk. The larger part, the yolk-mass, contains generally eight large cells, which produce yolk material; the smaller and separated part, the germ-mass, containing germ cells. Among the bdelloids and a small series of marine parasitic forms, there are two such ovaries; in all others one only. The combined organ is usually of ovoid shape, rarely elongate and band-like. From the ovary a long, collapsible tube leads to the cloaca.
Reproduction is in general oviparous, sometimes ovoviviparous, the eggs being retained until the embryos are well advanced. Three kinds of eggs are produced, always by different females, (I) Unfertilized or parthenogenetic, hatching in few days, having a thin shell, and producing females (see PARTHENOGENESIS) (2) Male, much smaller, also thin-shelled and parthenogenetic, hatch ing promptly and producing males; (3) Resting, as large as the unfertilized eggs but having a stouter shell, requiring fertilization and not hatching for a protracted period and then producing fe males, which later develop the ordinary unfertilized eggs. By means of the resting eggs, the species is carried over a danger period. Fertilization is internal; the males of certain species possess intromittent organs, but in other cases, the body-wall of the female is penetrated. The spermatozoa may be very large and in some species superficially resemble trypanosomes.