WORMS 165 
and otocysts. The latter are sacs containing crystals of ecarbo- 
nate 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 distrib- 
uted, 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 con- 
stantly 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 them- 
selves, and often are of the same color, and therefore are so incon- 
spicuous 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 sur- 
facé, 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 
