It will be seen that the characters of the gizzard are very useful for classification, only breaking down indeed in the Ploima; for though the majority of these present one or other of the four varieties of the malleate type, Triarthra and Pterodina (but not the other genera of their respective families) have the gizzard malleoramate.
The oesophagus is, when present, a contractile ciliated tube in which the food makes no sojourn on its way to the stomach.
The stomach may be nearly spherical, ovoid, or elongated and cylindrical. Its walls are formed of large cells, often granular and sometimes brownish, whence a hepatic function has been assigned to them. Its apertures are both surrounded by constricting muscular fibres. The intestine may be simple or divided by a similar constriction into intestine proper and rectum. The whole of the alimentary tract, with the exception of the mastax, is richly ciliated within. The rectum opens into the slender non-ciliated cloaca. The intestine is sharply bent upwards and towards the back in the tubicolous forms, but is nearly straight elsewhere; in Trochosphaera and Apsilus it is bent ventrally. In Asplanchnaceae and in Paraseison there is no rectum, the stomach being a blind sac.
The so-called salivary glands, usually two in number, open into the pharynx or mastax; and the paired gastric glands (Fig. 106, gg) open into the oesophagus or stomach. While the prehension of food is usually accomplished by the ciliary current of the disc and pharynx, we have seen that a more active swallowing action takes place in Flosculariaceae and Asplanchnidae, which devour whole Algae, Infusoria, and even other Rotifers, the long spines of Triarthra not availing as a protection. Many Ploima put out the tips of their trophi to nibble at débris, or, in the case of Diglena and Distemma, to attack Desmids, or the Infusorian Stentor. But this use of the trophi is most efficient in Ploesoma. Bilfinger[[258]] writes: "It has the courage to attack larger Rotifers; thus I was able to observe under the microscope how it fell upon a Rattulus but little smaller than itself and destroyed it. First it plunged the sharp prongs of its mastax deep into the tender frontal area of its unhappy victim; then followed a pumping action of the gizzard, and stroke by stroke the whole contents of the victim's body passed into the brigand's stomach." From this it is an easy transition to the ectoparasitism of Drilophagus, Balatro, and some species of Albertia, which cling to their host by the exserted trophi.
Renal Organs.—The kidneys consist of a pair of convoluted tubes, formed of a succession of perforated, so-called "drainpipe" cells (Fig. 106, k); they open directly or indirectly into the cloaca. Their walls are thin in the straight parts, but thick and glandular in the coils which occur at intervals. These tubes bear little tag-like appendages, hanging freely into the body-cavity, often widening towards the free end, and flattened or circular in section (Fig. 106, ns). They show during life a peculiar flickering motion in their interior, like the equivalent "flame-cells" of many Platyhelminthes (see p. [25]), and are in function the representatives of the multicellular renal funnels of Annelids. On one side, especially on the edge of the flattened tags, the appearance is as of a tapering whip-like lash, attached by its base to the free end of the tag and waving in its cavity; but the side view of the flattened tags shows an appearance of successive transverse or oblique waves. In many if not all cases the free end of the tag is closed by a vacuolated plug of protoplasm, which sometimes at least bears two flagella waving freely in the body-cavity. The probable explanation of the two distinct wave appearances within the tag is that the protoplasmic plug bears on its inner face a row or tuft of long cilia hanging down into the cavity of the tag. The tags probably keep up a current of liquid through the kidneys, while the contents of the body-cavity are constantly replenished by osmosis.
The two renal tubes may end blindly below the disc, or else join by a short transverse dorsal communication in front of the brain, as in Stephanoceros, Atrochus (Fig. 112, C), and Apsilus among Flosculariaceae, Lacinularia among Melicertidae, and Hydatina among the Illoricate Ploima (Fig. 106, rc). In some species of Asplanchna, if not all, a recurrent branch occurs opening at either end into the main tube of its own side.
The kidneys unite to discharge into the cloaca near its orifice, and on its distal (primitively ventral) side in many Melicertidae. In Bdelloida the common duct formed by their fusion opens into the ventral side of a dilated bladder-like section of the cloaca (Fig. 109, A, bl), which contracts rhythmically to discharge the liquid; while in the majority of the class they open singly or by a common duct into a separate contractile vesicle or bladder, which also discharges at regular intervals into the cloaca on its ventral or distal side (Figs. 106, bl and 112).
Fig. 112.—Apsilidae: A, Apsilus lentiformis, ♀, dorsal view (after Metschnikoff); the square brain is seen with nerves to the lateral antennae; B, larva of A. lentiformis (?), showing the paired eyes and ciliated cupped foot; C, adult of Atrochus appendiculatus, ♂ (after Wierzejski). al, Lateral antennae; am, median antenna (just in front is seen the renal commissure); an, anus; br, brain, below which the paired eyes are seen; c, cloaca; em, embryo; em', em', em''', three successive stages of embryos in the uterus of C; k, kidney. The coarser muscles are striated.
This bladder may reach when expanded one-third the diameter of the whole animal, and contract as often as three times per minute; so that in a period of nine minutes a bulk of water equal to that of the animal must have diffused through the body-wall, to be removed by the kidneys. It is obvious that while the function of the kidneys is primitively excretory, the passage of the water through the body must bring in the oxygen dissolved in the external medium, and carry off the carbonic acid formed in the tissues, and so fulfil the act of respiration. This mechanism is physiologically comparable with that of the contractile vacuole of fresh-water Protozoa. In a few genera (Conochilus, Lacinularia, Pterodina) the kidneys open separately after a slight dilatation into the cloaca.