184 



THE THEOKY OF EVOLUTION 



there are, however, true Turbellaria is shown by their 

 different embryonic development from that of the 

 Tapeworms. There would therefore also have been 



in the Tapeworms a marked 

 alteration in the genesis of 

 the embryo ; the adult forms 

 of a Tapeworm and of such 

 parasitic Turbellaria would by 

 themselves form no reason for 

 establishing different classes. 



As between Tapeworms and 

 Turbellaria so is the separation 

 between Turbellaria and Dis- 

 toma (sucking worms) not 

 always to be clearly followed. 

 There are free-living Turbellaria 

 with suction apparatus but no 

 hooks, which are only service- 

 able to parasites. Such Tur- 



FIG. 32. PLANARIA. in- IT M ^ 



, , , ,. . . bellana could easily become 



Plan of a free-living Planaria. J 



an, eye ; ci, cilia ; D, front Parasites (Ekto-parasites) if 



main intestine; da, branch 



of same ; Di, left rear, and they could thereby obtain 



l)r, right rear main intes 



tine ; g, brain ; ge, ger nourishment more easily. We 



minal gland ; m, mouth _ . . , 



ph, anus ; T, feelers ; te na ve namely supporting e vidence 



testicle bladder ; u, ovarium; -, -, . i i i i 



vi, yolk gland. by observation which shows how 



'(After von Graff.) opportunity alone may render 



an animal a facultative parasite whether the oppor- 

 tunity be artificially or spontaneously provided. 



' Thus can animals, which live normally in the 

 excreta of man, also develop themselves inside the 



