112 PLATYHELMINTHES [CH. V 



Sub-order 2. Alloiocoela. 



The alimentary canal is lobed : the testes are scattered. 

 Sub- order 3. Rhabdocoela. 



The alimentary canal is rod-shaped, the testes are com- 

 pact lobed organs. 



Order 2. Dendrocoelida. 



Turbellaria with a branched alimentary canal. 



Sub-order 1. Triclada. 



The alimentary canal has three main branches, one 

 running forward and two running back. Male and female 

 openings united. 



Sub-order 2. Polyclada. 



The alimentary canal has many branches sometimes 

 uniting with one another in a net-like manner, i.e. " anasto- 

 mosing." Male and female openings as a rule distinct. 

 Marine. 



Class II. TREMATODA. 



Parasitic Platyhelminthes with usually leaf-like, rarely cylin- 

 drical bodies : no cilia on ectoderm : forked alimentary canal : 

 ventral sucker or suckers and hooks often present : not segmented. 



Order 1. Monogenea. 



Trematodes with a sucker or suckers at the posterior end 

 and in some cases another anteriorly. Often external parasites 

 or infesting the mouth, nose or branchial cavities of their 

 hosts. Development direct. 



Order 2. Digenea. 



Trematodes with a sucker at the anterior end and another 

 on the ventral surface or posteriorly. Internal parasites and 

 their organs of adhesion more weakly developed than in the 

 Monogenea. Development with an alternation of generations. 



Class III. CESTODA. 



Parasitic Platyhelminthes with elongated and usually segmented 

 body : no mouth or alimentary canal : suckers and hooks present 

 but on the head only : the segments break off when ripe. 



