When the fore-part of this creature is first seen to open or divide, the parts, which when fully protruded resemble wheels, seem only like a couple of semicircles, the edges of which are set with little spiculæ, having a nimble, and continually vibrating motion upwards and downwards, for the purpose of agitating the water, each wheel being in this case doubled, or like a round piece of paper folded in the middle.
When the wheels are in motion, the head appears very large in proportion to the size of the animal; and though it is then everywhere transparent, yet a ring or circle, more particularly distinguished by its brightness, may be perceived about the middle of the forehead, from whence many vessels are seen to originate.
The thorax or breast is united to the head by a short annular circle or neck; the size of the thorax is nearly one-sixth part of the whole animalculum. In it the heart is distinctly seen; being placed nearly in the center, the diastole and systole cannot fail to attract the eye of every attentive observer; the alternate dilatation and contraction is very perceptible through the back of the animal, being performed with great strength and vigour. It appears to be composed of two semilunar parts, which in the time of contraction approach each other laterally, and form between them a figure somewhat like a horse-shoe, whose upper side is flat, the under one convex. In the diastole, these two parts separate; the separation begins exactly in the middle of the lower part next the tail. In each of the semilunar parts there is a cavity, which closes when they come together; and opens when they separate.
The motion of the heart is communicated to all the other parts of the thorax, and indeed through the whole animal. It is necessary however to remark, that this motion is sometimes suspended, or at least quite imperceptible, for two or three minutes, after which it re-commences, and goes on with the same vigour and regularity as before. From the under part of the thorax a small transparent horn proceeds, which cannot be seen unless the insect turns on its back or side.
Below the thorax there is an annular circle that joins the thorax to the abdomen; this is considerably the largest part of the animal, and contains the stomach and viscera. When full of food, the intestines are opake, and of a crimson colour, extending from the thorax quite through the abdomen and a great part of the tail, exhibiting a fine view of the peristaltic motion, or those gradual contractions and dilatations of the intestines, which propel their contents downwards. Numerous ramifications of vessels, both longitudinal and transverse, surround the intestines. The abdomen is not only capable of contraction, but also admits of such a degree of extension, as to form a case for all the other parts of the body. The tail extends from a joint at the lower part of the belly to the posterior extremity; it is of a tapering form, and consists generally of three joints; when the animal is inclined to fix itself to any thing by the tail, it thrusts out four, sometimes six, little hooks from the extreme part; these are placed in pairs, one at the very extremity itself, the other two a little way up the sides; the three pair are seldom seen at the same time. The wheels appear to be the organs used by the animal to assist it in swimming.
All the actions of this creature seem to imply sagacity and quickness of sensation; at the least touch or motion in the water, they instantly draw in their wheels. Baker conjectures that they have eyes lodged near the wheels, because while they are in the globular or maggot state, their motions are slow and stumbling; but after the wheels are protruded, they are performed with great regularity, swiftness, and steadiness. Can we sufficiently admire the wonderful contrivance in the apparatus of this animal? a being so diminutive, as not to exceed in size a grain of sand!
[Plate XXVI.] Fig. 17, represents the wheel animal in what Baker calls the maggot state; while in this form small spiculæ are seen to dart out near the anterior part; the snout is sometimes more, at other times less acute than in this delineation. a, a small horn near the thorax.
Fig. 15 represents its manner of moving from place to place, while in the maggot state. a, the projecting horn.
Fig. 12 exhibits it with the two semicircular parts, a a, protruded, and in the posture in which it places itself, when preparing to swim about, or going to set its wheels in motion.
Fig. 1 shews the head at its full extent, and a couple of small bodies, a a, on the top of it, armed with small teeth, b, like those of the balance-wheel of a watch.