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Species. They contain from 8 to 30 and 40 globules within the they meet with. Notwithstanding their extreme mi- Species. Infusoria. membrane.

100

Monas,

82. MONAS.

nuteness, they know how to avoid obstacles, or to pre- Infusoria.
vent any interference with one another in their mo-
tions sometimes they will suddenly change the direc-

Worm invisible to the naked eye; most simple, pellu- tion in which they move, and take an opposite one ;
cid, resembling a point.

There are five species; viz. *atomus, punctum, *mica, lens, and termo.

*

This order contains 15 genera and 229 species. How many kinds of these invisibles there may be (says Mr Adams), is yet unknown; as they are discerned of all sizes, from those which are barely invisible to the naked eye, to such as resist the force of the microscope as the fixed stars do that of the telescope, and with the greatest powers hitherto invented, appear only as so many moving points. The smallest living creatures our instruments can shew, are those which inhabit the waters; and though animalcula equally minute may fly in the air, or creep upon the earth, it is scarce possible to get a view of them; but as water is transparent, by confining the creatures within it we can more easily observe them by applying a drop of it to the glasses.

Animalcules in general are observed to move in all directions with equal ease and rapidity;, sometimes obliquely, sometimes straight forward; sometimes moving in a circular direction, or rolling upon one another, removing backwards and forwards through the whole extent of the drop, as if diverting themselves; at other times greedily attacking the little parcels of matter

EXPLANATION OF FIGURES.

Fig. 1. Ascaris Lumbricoides, entire, and nearly of its natural size; a, the head; b, the tail; c, the depressed band; d, the punctiform aperture; e, the line extending from the head to the tail;f, the gyrated apparatus as it appears through the skin of the worms.

Fig. 2. Represents the viscera of the worm in their natural situation; a, the head; b, the gullet; c, the intestinal canal; d, the lines of the body of the worm; e, the uterus, and its convoluted apparatus.

Fig. 3. Ascaris Vermicularis of its natural size.

Fig. 4. The same viewed by the microscope; a, the head; b, the tail; e, the pistilliform stomach; d, a convoluted apparatus surrounding the intestinal canal; e, an orifice which is probably the anus; f, the external part of the organs of generation.

Fig. 5. Trichuris Hominis, of its natural size. Fig. 6. The same considerably magnified; a a, the head; b, the tail; c, the proboscis; dd, the intestinal canal; a hollow tube; X the ovaria.

Fig. 7. A portion of the Tania Solium, of its natural size, and usual appearance; a a, the marginal oseula.

and by inclining the glass on which the drop of water is, as it can be made to move in any direction, so the animalcules appear to move as easily against the stream as with it. When the water begins to evaporate, they flock towards the place where the fluid is, and shew a great anxiety and uncommon agitation of the organs with which they draw in the water. These motions grow languid as the water fails, and at last cease altogether, without a possibility of renewal if they be left dry for a short time. They sustain a great degree of cold as well as insects, and will perish in much the same degree of heat that destroys insects. Some animalcules are produced in water at the freezing point, and some insects live in snow. By mixing the least drop of urine with the water in which they swim, they instantly fall into convulsions and die.

The same rule seems to hold good in these minute creatures, which is observable in the larger animals, viz. that the larger kinds are less numerous than such as are smaller; while the smallest of all are found in such multitudes, that there seem to be myriads for one of the others. They increase in size, like other animals, from their birth until they have attained their full growth; and when deprived of proper nourishment, they in like manner grow thin and perish.

Fig. 8. a, the head enlarged by the microscope; &, a full view of the head when very much magnified; a, the oscula at the base; c, the mouth.

Fig. 9. Sipunculus Saccatus, of its natural appear

ance.

Fig. 10. Laplysia Depilans, or Sea-hare.
Fig. 11. Holothuria Tremula.

Fig. 12. Asterias Caput Medusa, or Arborescent

Sea-star.

Fig. 13. Echinus Esculentus, or common Sea-urchin. Fig. 14. Tubularia Magnifica, as it proceeds from its native rocks; a, the animal with its tentacula fully expanded from the tube; b, another with the tentacula collapsed; c, one with the tentacula nearly withdrawn into the tube; d, the animal with the tentacula entirely withdrawn; e, e, e, probably acti

niæ.

Fig. 15. Hydra Grisea, or Fresh-water Polype, magnified: a, the mouth; b, the attached part; c, c, c,., &c. the arms; e, the transparent body.

Fig. 16. Vorticella Polypina, magnified.

INDEX.

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