PLANARIAN WORMS 445 



passes on into the small intestine, whence it travels up the bile- 

 duct into the liver, there becoming adult. 



Class III.— PLANARIAN WORMS (Turbellaria) 



These are widely - distributed flat -worms, differing from the 

 members of the two. preceding classes in being as a rule free-living, 

 though a few are parasitic. All are carnivorous. Planarians are 

 found abundantly in the shallower parts of the sea and in fresh 

 water, while others are terres- 

 trial, and are to be seen on 

 damp earth and vegetation. 



One of the commonest British 



species is a fresh-water form, mouth 



Planaria lactea (^^. 274). It Fig. 274.— P/aMawW ^wifea (enlarged), with pharynx 



/* ^ ' ' / protruded to exterior 



is a flattened whitish creature 



of elongated oval shape, and an inch or less in length, which 

 may be found gliding along over water-plants and stones. These 

 movements are effected by a uniform covering of cilia with which 

 the soft skin is clothed, and the presence of which constitutes one 

 of the important differences from the Tape- Worms and Flukes. 

 The ciliary action sets up minute currents and whirlpools in the 

 surrounding water, which can easily be seen with a lens, and have 

 given rise to the name Turbellaria, from the Latin turbellce, a 

 word vulgarly employed to denote the bustling of a crowd. 



The front end of the body is comparatively broad, while the 

 hind end is pointed. Upon the under side of the body, nearer 

 the hind than the front end, is situated the mouth, which leads 

 into a muscular pharynx continuous with a three-branched intestine, 

 of which one section is directed forwards and the others backwards, 

 while all three are beset with numerous branches that end blindly, 

 reminding one of the condition of the digestive organs in a Liver- 

 Fluke (p. 444). When the animal is in pursuit of prey, the pharynx 

 is protruded from the mouth and used as an organ of capture, 

 the food being taken directly into it, so, paradoxical as it seems, the 

 mouth is not used as a mouth. As in Flukes and Tape- Worms there 

 are muscular layers beneath the skin, and the spaces between the 

 complex internal organs are filled up with packing material. As, 

 too, in those forms there are no circulatory or respiratory organs, 

 and the excretory structures are much of the same kind. The 



