The flatworms are the first animals to assume pronounced bilateral symmetry. They are soft-bodied, having no supporting skeleton, and they have no segments, or divisions, such as are found in higher types, as in /Annulata/. They have no body-cavity, the space between the organs and the body-wall being filled with tissue. The alimentary canal has no anal aperture, the excretions being carried off by a water-vascular system consisting of branching vessels which end in minute bundles of vibrating cilia, called ciliary flames. These flames communicate with the exterior through small pores or flame-cells. Their sense-organs are eyes [pg165] and otocysts. The latter are sacs containing crystals of carbonate of lime, and their function is supposed to be that of hearing. The flatworms are hermaphroditic, and their propagation is by means of eggs. They are numerous and very generally distributed, occurring in fresh and salt water, on land and shore, on the surface and in the depths of the sea. A vast number also are parasites and infest, internally or externally, nearly every living creature. The parasitic forms differ anatomically from the free-living worms. The internal parasites, living by absorption of the digested food of their hosts, have no digestive organs; they are also devoid of organs of sense and of defense. The external parasites, being carried about by their hosts, are without organs of locomotion. CLASS =TURBELLARIA= The turbellarians are the simplest group of bilateral animals and occupy the lowest place among worms. They owe their name to the fact that they are covered with cilia, which are constantly in motion and cause a slight turbulence in the water around them. ORDER =POLYCLADIDA= ("/Many-branched/") The polyclads are found below half-tide mark on the under side of stones and on seaweeds. Sometimes they are found swimming about at night. They are leaf-like in form, one inch to two inches in length, and are very thin and delicate. They adapt themselves to the inequalities of the surfaces to which they attach themselves, and often are of the same color, and therefore are so inconspicuous that they are apt to escape notice. They move with a gliding motion over surfaces, and often swim when in search of food. The intestine is much branched and has no anal aperture. From the mouth, which is placed in the center of the ventral surface, a pharynx, or muscular fold, is protruded in some species, which enables the worm to attach itself to and consume prey of considerable size. After digestion has taken place, the fecal [pg166] matter collects in the main intestine and is forcibly ejected by the pharynx. Numerous eyes are collected in groups on the anterior dorsal surface, or arranged around the margin of the body. The eggs are laid in shell-like cases, and cemented together in plate-like masses or in spirals, and attached to shells or stones. (Plate XLVIII.) GENUS ^Planocera^