io6 



SYSTEMATIC ZOOLOGY 



The subtype presents, in the case of some structures in the 

 body, a less highly developed condition than that which we find 

 in the Echinodermata, while on the other hand some organs are 

 more highly differentiated than in the latter group. On the 

 whole, zoologists consider the Platyhelminthes most closely 

 related in structure to the Coelenterata. Like them, thev have 

 no distinct body cavity, and the younger stages in the develop- 

 ment of some of the flatworms 

 are significantly like some of the 

 Coelenterata. But beyond these 

 points there' are great differences. 

 These worms always have a 

 dorsal and a ventral surface, and 

 an anterior and a posterior end. 

 Within the body are discrete 

 groups of nerve or ganglion cells, 

 lying for the most part near the 

 anterior end, where they form a 

 sort of brain, and such groups of 

 nerve-cells occur in none of the 

 Coelenterata or of the Echino- 

 dermata. In shape the Platy- 

 helminthes vary greatly ; many 

 are elliptical or oval, while others 

 are more or less elongated. They 

 are nearly all hermaphroditic, 

 and, like the worms in general, 

 are found in fresh or salt water, 

 in moist places, and as parasites 

 within or on the bodies of other 

 animals. Three classes are readily distinguished, strikingly 

 different in external appearance, but pretty closely related to 

 one another. 



i [i 



si/.' 



96. Various Turbellaria, natural 

 A, uta ; B, / 'orte v ; I 



Monotus ; I), V/n , E, Rhyncko- 



demiis : F, Bipalium ; 1 ',, Polycelis. ( After 



von Craft", from 1 

 Text-1 100k. ) 



CLASS I. TURBELLARIA 



The Turbellaria (Lat. turbo, a whirling) are for the most part 

 free-living Platyhelminthes. Thev derive their name from the 

 fact that the body is covered with cilia, so that as thev move 

 through the water one max see small plants and animals swept 



