CLASSIFICATION OF WORMS. 39 



times capable of asexual reproduction, when the metamorphosis 

 assumes a complicated alternation of asexual and sexual genera^ 

 tions. The Trematodes or Liver-flukes, and the Csfitodes or 

 Tapeworms, have this phenomenon in their life-cycle. 



The worms are divided into several classes. The four most 

 important here are : — 



(i) Platyhelminthes. (ii) Nemathelminthes. 



(iii) Glwdopoda. (iv) Hirudinea. 



(i) The Platyhelminthes are Flat -worms with elongated 

 bodies and cerebral ganglia ; they are provided with suckers 

 and hooks. Most are hermaphrodites. They are grouped in 

 five orders : — 



(a) Turhellaria (Planarians, &c.) These are of no agricul- 

 tural importance. 

 (h) Trematoda (Flukes). Parasitic, and produce various 

 diseases in animals. 



(c) Cestoda (Tapeworms). Purely parasitic, in man, animals, 



and birds, often with complicated life-history. 



(d) Ne7nertmea. Parasitic only in Mollusca and Crustacea. 



(e) Mesozoa. 



(ii) The Nemathelminthes are Eound-worms, with tubular, 

 cylindrical, or filiform bodies. The anterior or head region is 

 either armed with hooks or papillae, and there may be hooks on 

 the posterior end of the male. Sexes separated. There are three 

 sub-classes : — 



(a) Nematoda (Thread-worms). Many of these round thread- 

 worms are most injurious to animals, birds, and 

 man, living as parasites in various parts of the 

 body. 

 (6) Acanthocephala, a small group of Nemathelminthes found 



parasitic in man and some animals, 

 (c) NematopJiora, which are partially parasitic in insects, 

 (iii) The Chsetopoda are segmented worms with a so-called 

 " brain," an CBSOphageal nerve-ring and a ventral nerve-cord, 

 and a closed vascular or blood system. They form the highest 



