//. TYPES IN DETAIL IVITH THEIR SUBORDINATE GROUPS. 13 



Order 2. Digenea Ento-parasites with never more 



than two sucking-discs. Develop- 

 ment by heterogony, through forms 

 known as sporocyst, redia, cercaria, 

 etc., which inhabit several hosts, 

 the first being a pond-snail or some 

 allied form. 



X Fasciola {Disiomum). 



Sub-class III. Cestodes Ento-parasitic Platyhelminthes 



without intestine. Adult form 



generally composed of links [pro- 



gloUids). Larval form (cysticercus) 



provided with sucking-discs, and 



often hooks. In the typical forms 



the cysticercus encysts itself in the 



muscles or internal organs of its 



first host, and develops into the 



link-form only when swallowed 



by some special animal, which 



thus serves as its second host. 



X Tcenia. 



Supplementary group : Nemertea or Nemertini. A group of free- 

 living worms, mostly marine, with a few representatives in fresh water 

 and in damp earth. Their position in the system is uncertain, but they 

 seem in most characteristics to be allied to the Platyhelminthes. 

 Like them they are dorso-ventrally flattened, and possess lateral excre- 

 tory canals and diverticula to the intestine ; their nervous system is also 

 similar. They differ, however, in the possession of a body cavity, a set 

 of blood-vessels, and a complete alimentary canal with anus. At their 

 anterier end they possess a long retractile proboscis. The more typical 

 marine forms have a minute ciliated larva called a. pilidium. Some are 

 small, a few marine forms attain a great length (30-40 feet). They are 

 divided into three orders, as follows : 



Order i. Hoplonemertini Proboscis armed with bristles. 



X Tetrastetnma. 



Order 2. Schizonemertini Proboscis unarmed, head nearly 



divided by deep, longitudinal fis- 

 sures. X Cerebratulus. 



Order 3. PALiEONEMERTlNI Proboscis unarmed. Head nearly 



entire. x Cephalothrix. 



Class II. NEMATHELMINTHES . . .Cylindrical worm-shaped Vermes, 



possessing a body cavity. Intes- 



