Flat worms have a flat, generally leaf-like body, and usually no
body-cavity, the space between the internal organs and the muscular
body-wall being filled up with connective tissue.
I will only deal here with the orders of Tapeworms (_Cestoda_) and
Flukes (_Trematoda_).
ORDER: =Cestoda= (TAPEWORMS).
[Illustration:
FIG. 135.—_Tænia saginata_: head, and a number of joints, represented
natural size.
]
[Illustration:
FIG. 136.—The Common Tapeworm (_Tænia officinalis_): _a_, head and
neck, strongly magnified; _b_, joints, natural size; _c_, eggs,
strongly magnified.
]
[Illustration:
FIG. 137.—Tapeworm Larva (of _Tænia solium_), much enlarged.
]
[Illustration:
FIG. 138.—The three types of Bladder-worm, diagrammatically
represented.
]
[Illustration:
FIG. 139.—Measle of _T. solium_, with head thrust out (× 6).
]
The external characters of an ordinary tapeworm may be gathered from
Figs. 135 and 136. A very small “head” continued behind into a small
“neck” can be distinguished, and a very large number of joints, of which
the first are very small and short, while those which follow get larger
and larger the further they are from the head. The tapeworm at first
consists only of a head which holds on to the lining of the intestine by
means of suckers or similar structures. The hinder part of the head next
elongates to form the so-called “neck,” and a cross-wall is quickly
developed a little way in front of the hind end of this neck. By this
division is formed the first joint, which, though forthwith tolerably
independent, remains attached to the neck of the tapeworm head, until a
new joint is separated off from the hinder part of the neck, immediately
in front of the first joint. Every new joint is similarly formed by the
constriction of the hinder part of the neck. Each joint possesses a
complete set of hermaphrodite reproductive organs; it becomes sexually
mature without reference to other joints, breaks off, and creeps about,
even for considerable distances, and finally ruptures, so that the
numerous eggs are liberated. Each joint may therefore be regarded as an
individual, though the tapeworm, considered as a whole, also possesses a
certain individuality. The tapeworm is a species of animal which exists
in two forms. One of these is the sexually reproducing joint, which
leaves the original host, and from the eggs of which are developed
larvæ, which become, in another host, _bladder-worms_, or _measles_.
These develop a _head_, which is the asexual second form of the
tapeworm. This, therefore, is a case of metagenesis (p. 16). It appears,
from what has been said, that the head can in no sense be compared to
the head of an ordinary animal; it has neither mouth nor jaws, for
tapeworms do not possess a gut. The head and joints take up osmotically
through their walls the nutritive matter found in the intestine of their
host. There are suckers and, in several species, a circlet of hooks on
the head (Fig. 136, _a_), by which the animal holds fast to the
intestinal wall. Thousands, or even tens of thousands, of eggs may be
produced by a single joint. The joint is liberated when the eggs begin
to ripen, often before. In some species several adjacent joints are
pinched off at the same time. The tapeworm joints leave the intestine of
their host either alone or with the dung. After falling to the ground
they creep along like snails, and attach themselves to various plants.
It is obvious that a tapeworm joint or a number of the eggs produced by
it may easily be taken up by a grazing animal. The eggs of a particular
kind of tapeworm are only able to develop further if they reach the
intestine of a particular kind of host. It is undoubtedly true that, as
a rule, only a relatively small number of the eggs produced by a
tapeworm develop further, but the great powers of reproduction prevent
the species from dying out. If, now, either a joint or the eggs from one
are taken into the gut of a suitable host, the joint and the egg-shells,
or the latter only, as the case may be, are digested in the stomach or
intestine, so that the larvæ are set free. These (Fig. 137) are
spherical, glass-like, of microscopic size, as might be expected, and
bear three pairs of hooklets, by means of which they quickly perforate
the wall of the gut, and make a way through the tissues of the host.
They may also be carried further by the blood-stream. The larva of a
particular species settles down finally, not only in some particular
kind of host, but also in some definite organ or tissue. It then loses
its hooks, and is rapidly transformed into a hollow _bladder-worm_
(measle), which grows till it reaches a definite size, depending on the
species. Meanwhile it gradually develops one (_Cysticercus_) or several
(_Cœnurus_) ingrowths, or (in _Echinococcus_, Fig. 138) several new
bladders are developed inside the bladder-worm, and ingrowths are
usually formed within these, similar to those of the simpler kinds of
bladder-worm. These ingrowths resemble tapeworm heads in all respects,
possessing suckers, and, it may be, a circlet of hooks, but these are
_inside_, and not _outside_. Later—either in the animal originally
inhabited by the bladder-worm, or after it has been transferred in the
flesh of this to the gut of some carnivorous animal—the bladder-worm
contracts, so that it can no longer hold the fluid which is present, and
the ingrowth is turned inside out, the suckers thus becoming external.
The tapeworm head is formed in this way, but the bladder still remains
attached to its hinder end (Fig. 139). If a host inhabited by
bladder-worms is not devoured by another animal, the bladder-worms,
after reaching their full size, remain for a long time in the same stage
of development without being able to develop further. But if the host is
devoured by another animal, the bladder-worms enter its stomach and
intestine, where the bladder is digested, while the tapeworm head
remains uninjured. This thrusts out its suckers and attaches itself by
means of them to the wall of the intestine, the process of
joint-formation already described beginning soon afterwards.
The following tapeworms, which are important to the owner of stock, will
be briefly dealt with.
The =Common= or =Armed Tapeworm of Man= (_Tænia solium_).
[Illustration:
FIG. 140.—Measles in Pork; the white spot is the head (natural size).
]
Six to ten feet long, with seven to eight hundred joints, and a circlet
of hooks (Fig. 136), lives in the human intestine. Several of its
sexually mature joints are commonly pinched off at a time, and if these
are picked up from a field or dung-heap by a pig, the contained eggs
develop into “pork measles” (_Cysticercus_, Fig. 140), which are
especially abundant in the connective tissue between the muscles, and
make the meat “measly.” They may also occur in the connective tissue of
other parts (fat, brain, spinal cord). Young pigs not more than six
months old are most in danger of becoming measly. A tolerably large
number of measles can be borne very well, but if very abundant they
cause emaciation, weakness, lameness, and poverty of blood. The flesh of
an animal badly infested with measles may be pale, watery, or even
greasy, while pork not so much diseased may seem all right, though it is
really unfit for food, since if eaten in an insufficiently cooked state
by human beings it would produce tapeworms. The measles are most
abundant in the muscles of the breast and neck, and next to them in
those of the hams and shoulders. From twelve to twenty thousand measles
may be present in a single pig.