24 INVERTEBRATE ZOOLOGY. 



Order 1. Monogenea (Polystomeae) Ecto-parasites upon gills, integu- 

 ment, bladder, etc., of aquatic verte- 

 brates. Sucking-discs, three or more. 

 Development direct. -^ Polystominn. 

 Order 2. Digenea (Distomeae). . . .Ento-parasites with never more than 



two sucking-discs. Development by het- 

 erogony. through forms known as Hporo- 

 cyst, Redia, Cercaria, etc., which inhabit 

 several hosts, the first being a pond snail, 

 or some allied form. 

 ■if Phasciola, (Distomuni). 



Class III. Cestodes Ento-parasitic Platodes without intestine. 



Adult form generally composed of ' 'links'" 

 {Prnglottids), which may be considered 

 reduplicated abdomens, or as attaining 

 the value of individuals of a colony. 

 Larval form (Cyst ice reus) provided with 

 sucking-discs, and often loops. In the 

 typical forms the cysticercus encj^sts it- 

 self in the muscles or internal organs of 

 its first host, and develops into the chain- 

 form only when swallowed by some spe- 

 cial animal, which thus serves as its sec- 

 ond host, if Taenia. 



Cladus II. Vermes. 



Class I. Rotifer A Minute aquatic forms, which may be 



considered as modified Trochozoa. The 

 prae-oral band of cilia persists, often 

 modified to form a pair of contractile 

 organs resembling rotating wheels. Pro- 

 tocoelom spacious. Alimentary canal 

 differentiated into a stomach, containing 

 chitinous teeth, an intestine and a cloaca 

 into which the protonephridia and rejiro- 

 ductive ducts empty. Generally a jointed 

 forked appendage or "foot," projecting 

 posteriorily and serving for temporary 

 attachment. Bisexual, the males rare, 

 and much reduced. 

 if Brachionus, Rotifer. 



