The phylum Platyhelminthes follows the cœlenterates 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 areknown 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.