f to another crab and begins to live at
the expense of its host. Then it commences to undergo remarkable changes
and finally becomes a mere sac-like organ with a number of long slender
root-like processes penetrating and taking nourishment from the body of
the unfortunate crab-host.

The worms furnish many well-known examples of parasites, whole groups of
them being especially adapted to parasitic life. The tapeworms, common
in many animals and often occurring in man, the roundworms of which the
trichina (Fig. 3) that causes "measly" pork is a representative, are
familiar examples. These and a host of others all show a very high
degree of specialization fitting them for their peculiar lives in their
hosts.

[Illustration: FIG. 1--A lamprey. (After Goode.)]

[Illustration: FIG. 2--_Sacculina_; _A_, parasite attached to a crab;
_B_, the active larval condition; _C_, the adult removed from its host.
(After Haeckel.)]

[Illustration: FIG. 3--_Trichina spiralis_ encysted in muscle of a pig.
(From Kellogg's Elementary Zooel.)]

[Illustration: FIG. 4--An external parasite, a bird-louse (_Lipeurus
ferox_).]

[Illustration: FIG. 5--A tachina fly (_Blepharipeza adusta_) the larva
of which is an internal parasite.]

[Illustration: FIG. 6--Work of an internal parasite, puss-moth larva
parasitized by a small ichneumon fly.]

From among the insects may be selected interesting examples of almost
all kinds and degrees of parasitism, temporary, permanent, external,
internal (Figs. 4, 5, 6). Among them is found, too, that curious
condition known as hyperparasitism where one animal, itself a parasite,
is preyed upon by a still smaller parasite.

"The larger fleas have smaller fleas
Upon their backs to bite um,
These little fleas still smaller fleas
And so _ad infinitum_."

Coming now to the minute, microscopic, one-celled animals, the Protozoa,
we find entire groups of them that are living parasitic lives, depending
wholly on one or more hosts for their existence. Many of these have a
very remarkable life-history, living part of the time in one host, part
in another. The malarial parasite and others that cause some of the
diseases of man and domestic animals are among the most important of
these.

PARASITISM

Among all these parasites, from the highest to the lowest the process
that has fitted them for a parasitic life has been one of degeneration.
While they may be specialized to an extreme degree in one direc