KEY TO CLASSES AND ORDERS OF ANIMALS 13 



11(12) Tentacles from the margin of the umbrella; gonads on 

 the radial canals; margin entire. (Pratt, 117.) 



Order Trachymedusae 



12(11) Tentacles arise aborally from the exumbrella; gonads 

 on the manubrium; margin scalloped; hydroid 

 unknown. (Pratt, 120.) Order Narcomedusae 



Phylum PLATYHELMINTHES 



1(2) Cilia present at least on the ventral surface; almost 

 all free living. Class Turbellaria, p. 13 



2(1) Surface of body of adult not ciliated; parasitic. 3 



3(4) Body usually flattened, rarely cylindrical, mouth and 

 alimentary canal present, the latter forked. Sucker 

 or suckers and hooks serving for attachment, on 

 ventral surface, or terminal, or both; ecto- or endo- 

 parasitic. Class Trematoda, p. 14 



4(3) Body of adult worm usually divided into segments 

 (proglottids) without mouth or alimentary canal. 

 Organs for attachment at the anterior end; endo- 

 parasitic, adult in alimentary canal,- young imbedded 

 in tissues. Class Cestoda, p. 14 



Class TURBELLARIA 



I (4) Alimentary canal not highly branched. 2 



2(3) Without a distinct alimentary canal; digestion occurs 

 in spaces in the parenchyma; mouth ventral, opening 

 into a short pharynx; marine. (Pratt, 158.) 



Order Acoela 



3(2) Alimentary canal present; alimentary tract a single 

 blind tube, rodlike, saclike, or slightly lobed. (Pratt, 

 160; W. and W., 333.) Order Rhabdocoelida 



4(1) Alimentary canal highly branched. 5 



