Fig. 110.—Philodina roseola. (After Baker.) A, B, Crawling, with extended proboscis, and showing antenna; C, D, E, attached, with "wheels" extended for catching food; F, attached, with anterior end retracted.
"If the Water standing in Gutters of Lead, or the slimy Sediment it leaves behind, has any Thing of a red Colour, one may be almost certain of finding them therein,[[255]] and, if in Summer, when all the Water is dried away, and nothing but Dust remains, that Dust appears red, or of a dark brown, one shall seldom fail, on putting it into Water, to discover Multitudes of minute reddish Globules, which are indeed the Animals, and will soon change their Appearance, in the Manner just now mentioned....
"A Couple of circular Bodies, armed with small Teeth like those of the Balance-Wheel of a Watch, appear projecting forwards beyond the Head, and extending sideways somewhat wider than the Diameter thereof. They have very much the Similitude of Wheels, and seem to turn round with a considerable Degree of Velocity, by which Means a pretty rapid Current of Water is brought from a great Distance to the very Mouth of the Creature, who is thereby supplied with many little Animalcules and various Particles of Matter that the Waters are furnished with.
"As these Wheels (for so from their Appearance I shall beg Leave to call them) are every where excessively transparent, except about their circular Rim or Edge on which the Cogs or Teeth appear, it is very difficult to determine by what Contrivance they are turned about, or what their real Figure is, though they seem exactly to resemble Wheels moving round upon an Axis....
"As the Animal is capable of thrusting these Parts out, or drawing them in, somewhat in the Way that Snails do their Horns, the Figure of them is different in their several Degrees of Extension and Contraction, or according to their Position to the Eye of the Observer, whereby they not only appear in all the various Forms before represented, but seem at certain Times as if the circular Rim of the Wheel or Funnel were of some Thickness, and had two Rows of Cogs or Teeth, one above and the other below that Rim."
Digestive Organs.—The pharynx is usually a narrow ciliated tube, which varies in length from genus to genus, but in no other important point, save in Flosculariidae, where it assumes the form of a crop, into which the mouth hangs freely down as a narrow ciliated tube. At its lower end is an enlargement, the mastax or gizzard.[[256]] This is a strong muscular sac containing the trophi or hard chitinous chewing organs, with an antero-ventral inlet from the pharynx, and a postero-dorsal outlet through which the food passes into the stomach either directly or through a slender gullet (Fig. 106, oe). In the ventral wall of the gizzard of most Ploima is a median piece, the fulcrum, from which run forwards and upwards two pieces, the rami, which are hinged on the fulcrum. The Y-shaped structure formed of these three pieces is called the incus (anvil). At either side of the gizzard and at a higher level is a paired piece, the malleus, so called from its resemblance to a hammer, of which the manubrium (handle) looks backwards, and is embedded in the side walls of the mastax, while the toothed claw or uncus looks forwards and inwards, and is hinged at its inner side with the tip of the ramus. As the unci and rami are usually strongly toothed, this gizzard forms a very efficient apparatus for chewing. In some cases, when the pharynx is short and dilatable, the points of the unci and rami may be protruded for biting, for clinging to the host (in the parasitic genera Albertia and Drilophagus), or for the prehension of food (Rattulidae, etc.).
Fig. 111.—Diagram of trophi. (After Hudson.) A, Malleate; B, submalleate; C, virgate; D, forcipate; E, malleoramate (Melicerta); F, incudate (Asplanchna); G, uncinate (Stephanoceros); H, ramate (Rotifer). f, Fulcrum; i, incus; ma, manubrium (malleus in G); r, ramus; un, uncus.
The type we have just described is termed the "malleate" type (Fig. 111, A). If all the trophi are slender and scarcely toothed, we have the "virgate" type (C), which is frequently asymmetrical. In the "submalleate" type (B) the mallei only are slender; in the "forcipate" type (D) both the unci and rami are slender and sharply pointed.[[257]] In the "malleoramate" type (E) the manubrium is a curious looped structure, while the uncus is formed of a number of parallel slender elongated teeth; this characterises the family Melicertidae, and the genera Triarthra, Pterodina, and Pedalion. In the "uncinate" type (G) the mallei are simply incurved hooks with a few teeth at the free end, the rami are simple or absent, and there is no fulcrum; this type occurs in Flosculariaceae only. In Asplanchnidae the rami are large and hooked, constituting the "incudate" mastax (F); but here reduced mallei are often present, and in Asplanchnopus they are almost as well developed as in Melicertidae, affording a transition to the malleoramate type. In this group too the mastax has a very peculiar form; it is divided into two chambers, dorsal and ventral. The dorsal chamber forms a great purse-like sac or crop, with a framework of four longitudinal bars: into this the gullet and pharynx open. The ventral pouch is much smaller, and in its base the large rami are inserted, so that they can be protruded into the crop. This ventral sac with the rami may even be everted through the crop and the mouth, to swallow the small Rotifers and Entomostraca which form the food of this group, or to eject the undigested remains of the food. Two lateral sacs open at the junction of the ventral pouch and the crop, but whether they play a part in the deglutition of food or in the disgorging of faeces is uncertain. The fact that the whole of this apparatus is lined by a non-ciliated chitinous cuticle justifies our view that it is simply an enlargement and specialisation of the mastax.
The trophi in Bdelloids also are only represented by the rami, which have the form of segments of a sphere, excavated on the curved sides for the attachment of muscles, and transversely ridged on the two flat sides; the gizzard is here called "ramate" (H).