The simplest of the worms are those forming the class _Turbellaria_, so designated on account of the commotion they produce in the water surrounding them by means of the vibratile cilia that fringe their bodies--a characteristic that is also expressed by their popular name of Whirl Worms. They are usually small creatures, with soft, flattened, unsegmented bodies, though some of the larger species are really wormlike in form, and are more or less distinctly divided into a chain of segments. Many of them are marine, and may be seen gliding over stones left uncovered by the receding tide with a smooth slug-like motion, and when disturbed in a rock pool, occasionally swimming with a similar smooth motion by the aid of their cilia. They avoid bright light, and are consequently generally found on the under surfaces of stones, especially in rather muddy situations, and where the stones are covered with a slimy deposit of low forms of life. In these turbellarians the mouth is situated on the under surface, thus enabling the animal to obtain its nourishment from the slimy surface over which it moves, and it is also provided with an extensile proboscis that aids it in the collection of its food. The digestive tube is generally very complex in form, extending its branches into every part of the soft body; and, there being no special organs of respiration, the animal derives all the oxygen required by direct absorption from the water through the soft integument. When searching for turbellarians on the sea shore one must be prepared to meet with interesting examples of protective colouring that will render a close examination of rocks and stones absolutely necessary. Some of these worms are of a dull greyish or brownish colour, so closely resembling that of the surface over which they glide that they are not easily distinguished; and the thin bodies of others are so transparent that the colour of the stone beneath is visible through them, thus preventing them from being clearly observed. Overturned stones should be examined for their flattened bodies gliding along rapidly in close contact with the surface. They may be removed without injury by placing a wet frond of a sea weed close to the stone, in front of one end of the body, and then urging them to glide on to it by gently touching the opposite end. Sometimes, however, the turbellarians remain perfectly still when exposed to the light, in which case they are even more difficult to detect, but a little practice will soon enable one to distinguish them with readiness.