The class includes two or three other orders of coral--polyps that grow in a solitary way or in groups, forming those elegant objects called sea fans, sea pens, and so forth, which can be referred to only briefly. One of these is the order Alcyonaria, in which some are soft-bodied, others are strengthened by a network of spicules. A very beautiful one is the "sea pen," which takes the shape of an ostrich plume; another is the strange mass of parallel tubes called organ-pipe coral; and some of them are very large, the great tree coral of the eastern Atlantic depths being sometimes as tall as a man, while it looks like a sturdy, leafless tree. As in all the others, however, it is covered by a living fleshy coat of protoplasmic substance studded with polyps whose gay colors and waving tentacles give it the appearance of being clothed with minute sessile blossoms. The best known of this group, probably, is the red coral of commerce, which is the scarlet, ivorylike interior stem of a branching alcyonarian colony. This coral has from the earliest time been cut into cameos by lapidaries, as well as used for making necklaces and other toilet ornaments. CHAPTER VI 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.