TKEMATODA. 35 



sitic ; they have no parapodia, and are armed with a 

 ventral terminal sucker for attachment, and also with an 

 oral sucker. They are hermaphrodites like the Earth- 

 worms. 

 The worms for us to consider — that is, those having a bearing 

 on agriculture — are the Trematodes or Liver-flukes, the Cestodes 

 or Tapeworms and cysts, the Neiiiatoda or Thread and Whip 

 worms, and the allied Acanthocephcdce, the OUgochcetce or Earth- 

 worms, and the ecto-parasitic Hirudinea or Leeches. 



Platyhelminthes OB Flat-worms. 



The Flat-worms are mostly parasitic, such as the Flukes and 

 Tapeworms. They may or may not be segmented, and are 

 usually hermaphrodites. Their bodies are flattened dorso-ven- 

 trally ; in very few is an anus present. Two hosts are generally 

 necessary to complete their life-cycle, and the sexual form may 

 attain a great size. Both sexual and asexual reproduction take 

 place, the development often being accompanied by a compli- 

 cated alternation of generations. The parasitic species are all 

 provided with suckers, and most of them with hooks for attach- 

 ment to their host. They are mostly oviparous. 



The Trematoda or Flukes. 



The Flukes or Trematodes are parasitic worms with unseg- 

 mented, usually leaf-shaped, flat bodies, resembling- somewhat 

 a " Flat-fish " in form. They possess a mouth surrounded by 

 an oral sucker, and also a ventral sucker, but no anus. Most 

 Trematode worms go through a kind of alternation of genera- 

 tions, and live in two distinct hosts during their changes. 



We can distinguish two groups of Flukes — (1) the Distomata, 

 which have two suckers and no hooks, and present an alter- 

 nation of sexual and asexual generations, — the asexual phase 

 always taking place in some mollusc. (2) The Polystomata, 

 which have two small lateral anterior suckers and one or two 

 posterior suckers. Hooks, usually two in number, are also 



