40 TEEMATODA. 



group of worms, and are, as a rule, free living ; some are par- 

 tially parasitic. There are three sub-classes : — 

 (a) Piili/cJiii'tdj. Marine worms onl^'. 

 (&) OUgoohoito;. Earth-worms, &c. 

 (c) Myzostomaria. 

 (iv) Hirudinea (Leeches). These are partially parasitic ; 

 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 and horticulture — are the Trematodes or Liver- 

 flukes, the Cestodes or Tapeworms and cysts, the Nematoda or 

 Thread and Whip worms, and the cdlied Aeanthoeephalce, the 

 Oligoclia'tdi or Earth-worms, and the ecto-parasitic Hirudinea or 

 Leeches. 



Platyhelminthbs 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, ilost 



