* VERMES 10- 



away by the currents which the motion of the cilia produi 

 The}' occur in fresh and salt water, while a few are parasitic, 

 and some live in moist places on the land. The fresh-water 

 species are mostly small, many of them microscopic; in salt 

 water we rind species which attain a length of from two to five 

 centimeters! Fig. 96), and among the land Turbellaria individuals 

 have been found in the tropics, thirty-five centimeters in length; 

 hut most of the representatives of this class are small. The 

 anterior end of the body is not sharply marked off from the 

 rest to form a head, but contains the brain, and is the most 

 sensitive region. On its dorsal surface are 

 the eyes, usually two lying near the brain, 

 but in some cases there are more. In some 

 Turbellaria a single otocyst is present, and 

 sometimes there is a pair of short tentacles. - 



( >n the ventral side of the body is the mouth 

 (Fig. 97), which lies in the median longitu- 

 dinal axis of the body, but in some cases 

 near the anterior end, in some near the >.j 



middle, and in others near the posterior end. 

 In the Turbellaria. as in all the other Platv- 

 helminthes, an anus is absent. Posterior to 

 the mouth there may be a single genital 



, .1 , . r FlG. 97. Yungia auranti- 



opening, common to the two sets of repro- aai , [Ui rbeIladan 



ductive organs, or there may be two open- from Naples, ventral as- 



,- , , . . pect; may attain a length 



mgs, one for each sex, one being anterior to o) seven cen(ime) 



the Other. orange colored. (Drawn 



,-^r ■< ■ 1 /-r-- o 1 from a preserved speci- 



Ut the internal organs (rig. 98), the men ) 

 most important for the further classification 



of the Turbellaria is the digestive system. This exhibits in 

 general three types. In its most elemental'}- condition, which 

 occurs in some of the small marine species, there is no ali- 

 mentary canal bounded by a definite wall, but the mouth opens 

 more or less directly into a mass of loosely arranged cells, which 

 constitute the so-called parenchyma, and there digestion takes 

 place. In other species the mouth opens into a straight tube 

 lying in the median longitudinal axis of the body, and extending 

 both anterior and posterior to tne mouth opening ; and in still 

 other cases the alimentary canal is greatly branched throughout 



