UNINVITED GUESTS
FLATWORMS, FLUKES AND TAPEWORMS
The phylum Platyhelminthes follows the coelenterates in the ascending
series of zoölogical classification, and includes a baneful company of
creatures badly called "worms," which show none of the segmented or
ringlike form of body that characterizes the true worms of the phylum
Annulata to which we shall come presently. On the contrary, they are a
group of small, soft-bodied, flattened animals, which first show that
two-sided character, or bilateral symmetry, which has apparently been
absent from all the groups we have studied hitherto, whose members are
circular or globular in shape, and whose organs, in the adult, are
arranged radiately.
The simplest are the planarians (Turbellaria), which live a free
life, as a rule, although some are parasitic. They are little, thin,
leaf-shaped creatures that creep on the bottom of ponds and even of
deep lakes, or swim in the sea, and feed upon algæ and minute animals.
Similar to them in appearance are the flukes (Trematoda), of which the
best known of a large variety is that which infests sheep. Most of the
trematodes are parasitic.
The third class of flatworms is the Cestoda, the members of which are
universally parasitic, and are known principally as "tapeworms" in
reference to their form.
The phylum Nematothelminthes contains an assemblage of related worms,
some marine, but mostly living in fresh waters or on land, which are
eellike in form, very slender, and often have amazing length. The first
and lowest class is that of the nematodes, of which the minute "vinegar
eels" and "paste eels" are familiar examples. The remainder of the
nematodes are parasitic, and many of them are dangerous parasites.
In an allied family and genus (Trichina) is placed one of the most
dangerous of human parasites, the _Trichina spiralis_.
Here, too, comes that "hairworm" (Gordius), which most country folks
call "hair eel" or "hair snake." Many assert with the most positive
faith that if you will soak a horsehair in water it will "turn into a
snake," and will show you this long threadworm in a horse trough to
prove it. I never knew a cautiously made experiment in that direction
to succeed; nevertheless the fanciful error survives. The gordius,
which does look like a hair from a gray mare's tail, is somewhat
aquatic in its habits.