In some parts of the world these are found in great masses; yet each of these minute and scarcely visible shells is the production of an animal which has passed away, and left behind but this tiny memorial of its existence.

CLASS II.-INTERNAL PARASITES.

ENTOZOA.

"Verily, for mine owne part, the more I looke into Nature's workes, the sooner am I induced to beleeve of her, even those things that seem incredible."-HOLLAND'S PLINY.

THE body of every vertebrate animal forms the abode of many other animals that live within it. These creatures constitute the class Entozoa, a word which simply means "within an animal," and is very appropriate to the internal parasites, which constitute the present group.

With this class we are as yet imperfectly acquainted; but some idea of its numbers may be formed from the fact, that no species of animal is supposed to be exempt from their attacks, and that the human body is infested with no

less than eighteen species. It is stated that every animal has one

α

b

*Fig. 5, a, Cyst magnified; b, head still further enlarged.

transparent description, and, after having subjected them to a fast for some time, he put into it an infusion of indigo or carmine, which tinged the water. When

they began to feed, he found, as the stomachs filled, they became visible by the blue or red particles shining through the transparent skins.

Those of the second order have rounded projections or "lobes" about the head (Fig. 3); and these, when seen under the microscope, appear like wheels in rapid motion. Hence these creatures were called "wheel-animalcules," and the order itself wheel-bearing" (Rotifera). The parts do not in reality move like wheels, but the movements of the hair-like organs with which they are fringed make them seem to do so; while, at the same time, their action causes currents in the water, which bring to the little beings their supply of food.

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Fig. 4.-SHELLS OF INFUSORIA.

Some animalcules, notwithstanding their minuteness,are furnished with flinty shells, which exhibit a beautiful variety both in their shapes

and in the patterns, that seem as if carved upon them (Fig. 4).

In some parts of the world these are found in great masses; yet each of these minute and scarcely visible shells is the production of an animal which has passed away, and left behind but this tiny memorial of its existence.

CLASS II.-INTERNAL PARASITES.

ENTOZOA.

"Verily, for mine owne part, the more I looke into Nature's workes, the sooner am I induced to beleeve of her, even those things that seem incredible."-HOLLAND'S PLINY.

THE body of every vertebrate animal forms the abode of many other animals that live within it. These creatures constitute the class Entozoa, a word which simply means "within an animal," and is very appropriate to the internal parasites, which constitute the present group.

With this class we are as yet imperfectly acquainted; but some idea of its numbers may be formed from the fact, that no species of animal is supposed to be exempt from their attacks, and that the human body is infested with no less than eighteen species. It is stated that every animal has one

α

b

Fig. 5, a, Cyst magnified; b, head still further enlarged.

or more species peculiar to itself. If so, the number of species among the Entozoa must exceed that of all other animals existing in the world.

One

These singular beings differ widely in their structure. Some resemble delicate transparent membranes filled with water. It is one of these (Fig. 5) that gives to the flesh of the pig the appearance termed measly. The farmer is well acquainted with two other kindsone found in the brain, and another in the liver of the sheep. species that infests the human body is the common Tapeworm (Fig. 6), which is occasionally found several yards in length. Its head is furnished with suckers and bristles, by means of which it can fix itself securely. Each joint of the body has the power of producing thousands of ova or eggs. When these come to maturity, the lower portion of the body separates from the upper; so that the Tapeworm may, from this peculiarity, be com

pared to trees or plants which fling off their seeds when ripe.

CLASS III.-POLYPES.

ZOOPHYTA.

"Here, too, were living flowers,
Which, like a bud comparted,

Their purple cups contracted;
And now, in open blossom spread,

Stretched like green anthers many a seeking head."

THE animals belonging to this class were formerly regarded

as vegetables. They

were afterwards considered to be partly of an animal and partly of a vegetable nature, which idea is still conveyed in the term Zoophyte, a word derived from the Greek, and literally meaning "animal plant."

In the two former classes, the exterior of the body did not exhibit any radiated structure; in the present class it is seen, not in the form of the body, but in the arrangement of the

parts surrounding the mouth,