The various internal organs of a Rotifer are readily seen through its transparent skin (Fig. 34, A). It has a nervous system, many bands of contractile muscles and a pair of little tubular kidneys or nephridia, besides reproductive germs (the eggs). I have here sketched only the digestive canal. The mouth leads through a gullet to a very curious organ called the "gizzard," marked G. All the wheel animalcules have this gizzard, but its teeth, shown as two oval bodies in the drawing, differ a great deal in shape and complexity in the different kinds. Whilst the Rotifer is feeding by bringing currents of water to its mouth, the two halves of the gizzard are kept in rapid movement by muscles, causing them to rub against one another and to grind up the food particles which reach them through the gullet. The gizzard (G) is followed by the digestive stomach (St), and that by the intestine (Int), which opens at the vent (V). The side (or three-quarter profile) view of a similar specimen (Fig. 34, B) shows only the surface of the little animal, and is intended to show especially the snout-like head-lobe (S), with its two eye-spots, which are red in colour. Standing out backwards from this is a finger-like process (T), which is called the spur, or tentacle. It has hairs at its tip, and is a sensory organ.
Fig. 35.—The Rotifer Pedalion mirum—seen from the right side, magnified 180 diameters. w.a., wheel apparatus or "ciliated" margin of the cephalic disc. r.e., right side eye-spot. m., mouth. p., tactile process. d.l., median dorsal limb (as it is seen in profile, only three of the fringed hairs at its extremity are seen). v.l., the great ventral limb (only five of its fan of eight fringed hairs are seen). l.l.1, dorso-lateral, and l.l.2, ventro-lateral limbs of the right side: they show the complete fans of eight fringed hairs. x., the pair of posterior processes tipped with vibratile cilia, better seen in Fig. 36.
Fig. 36.—The Rotifer Pedalion mirum—seen from the ventral surface. Letters as in Fig. 35. The complete fan of eight fringed hairs terminating the great ventral limb are seen, and the three spine-like processes on each side of it. The fringed hairs of the two ventro-lateral limbs, l.l.2, are omitted; they are fully shown in Fig. 35, and are the same in number and disposition as those forming the "fan" of the great ventral limb. Compare these hairs with those of the "Nauplius" Crustacean larva drawn as a tail-piece to Chapter XIII.
In some wheel animalcules there is a pair of these spurs, and the very remarkable wheel animalcule drawn in Figs. 35 and 36 has six large processes which, though much bigger, appear to be of the same nature. Of these four are seen in Fig. 35, namely, d.l., the dorsal limb, v.l., the great ventral limb, and l.l.1 and l.l.2, the two lateral limbs of the right side, all of them carrying fan-like groups of fringed hairs. They are moved by very powerful muscles, and strike the water with energetic strokes, so as to cause the little owner to dart through it. This jumping or darting wheel animalcule is called "Pedalion," and was discovered and described by Dr. Hudson. It is so astonishing and wonderful a little beast, that when Dr. Hudson sent me some alive in a tube by post in 1872, soon after he had discovered it, I could not believe my eyes, and thought I must be dreaming. It is very like the young form of Crustaceans known as a "Nauplius" (see tail-piece to the present chapter) in having (what no other wheel animalcule has) great hollow paired limbs moved by striated muscular fibre, carrying fringed hairs only known before in Crustaceans (crabs, shrimps and water fleas), and striking the water violently just as do those of the Nauplius. And yet all the while it has on its head a pair of large ciliated wheels which serve it just as do those of the common Rotifer. No Crustacean, young or old, has this "wheel-apparatus" nor any vibratile "cilia" on the surface of its body. Pedalion possesses an astounding "blend" of characters. Fig. 35 shows, besides the "paddles" or "legs" (of which two on the other side of the animal are not seen), the broad and large wheel-apparatus W (within which the right eye-spot r.e. is seen), and a little lobe (p) called the "chin" lying just below the mouth (m). The big leg (v.l.) and the pair on each side (l.l.1 and l.l.2), of which that on the right side only is seen, end in beautiful fringed hairs, which are only seen elsewhere in the Crustacea (water-fleas and others). Those on the lateral limbs and the great ventral limb (Fig. 36) are set in two groups of four on each side of the free end of the limb, whilst those on the dorsal leg (d.l.) are apparently not so numerous. I have corrected the drawings, Figs. 35 and 36, by reference to actual specimens kindly given to me by Mr. Rousselet.