GENERAL ZOOLOGY 



Mouth 



Seminal 

 receptacle 



Pharynx 

 Genital atrium 

 j)\ — Contractile bladder 

 — Egg in uterus 

 Ovary 



Yolk duct 

 - Testis 

 Yolk glands 



Gut 



Fig. 11.13. Order Monogenea; anatomy 

 oi Sphyranura osleri, a monoe;enetic trema- 

 tode parasitic on external surfaces of the 

 amphibian, .\ecturus. (Redrawn from 

 R. R. Wris;ht and A. B. Macallum, 1897, 

 Journal oj Morphology, vol. 1.) 



Hook 



THE CLASS TREMATODA 



The class Trematoda includes the flukeworms, all of which are parasites. 

 Although these worms show some structural modifications in adaptation to 

 parasitism, notably the replacement of the epidermal layer by a cuticle and 

 the development of hooks and suckers for attachment to the host, the trem- 

 atodes have not undergone such extreme modification as the cestodes. The 

 flattened body of trematodes is characteristic of the Platyhelminthes, and the 

 internal structure, which includes an enteron, shows the same general rela- 

 tion of parts found in Turbellaria. The Trematoda are divided into the 

 orders Monogenea ("one kind") and Digenea ("two kinds"), according to 

 their parasitic habits and related modifications of their structure and life 

 cycle. 



Monogenea. The monogenetic trematodes are external parasites upon 

 the body surfaces of aquatic animals, mostly vertebrates, or in cavities near 

 the surface, such as the mouth and urinary bladder. Monogenetic parasites 

 do not alternate between host species in the course of the life cycle. The 

 structure of a typical monogenean is shown in Figure 11.13. In correlation 

 with their attachment in relatively exposed positions, such trematodes have 

 well-developed hooks and adhesive suckers. The animals are monoecious, 



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