112 



INVERTEBRATE ZOOLOGY 



65 A) possess a ciliated covering. The epithelium characteristic 

 of the body covering of the Turbellaria has become profoundly 

 modified in this group and the cells are scattered through the 

 underlying parenchyma. 



Most flukes are hermaphroditic and the conspicuous repro- 

 ductive organs are of prime importance in the classification of 

 the group (Fig. 66 A). A few, the Schistosomes (human blood 

 flukes), for example, are unisexual. Suckers, hooks, and spines 



h/foufh ' 



^foui^h 



Genifal pore 



Acehbulum 



Inkafine 



Vi'iellar/oi 

 Uferus 



Ovary 



Excretory 

 bladder 

 Testes 



Fig. 63 A. — GyrodactyJus clcgans, a 

 monogenetic trematode. {After Luhe) . 



Fig. 63 B. — Opisthorchis fcl incus, a 

 digenetic trematode. {After Stiles and 

 Hassall ) . 



are developed in varying combinations for securing attachment to 

 the host. The number and arrangement of these structures are 

 of particular significance in the grouping of the trematodes. Two 

 subclasses, Monogenea and Digenea, are recognized. 



Subclass Monogenea 



The monogenetic trematodes are usually external body para- 

 sites but in some instances they have migrated inward to loca- 

 tions such as the mouth cavity, the respiratory organs, the cloaca, 

 or the urinary bladder which are in direct communication with 

 the body surface. The chief organ of attachment (Fig. 63 A) is 



