INVERTEBBATA. 487 



two ovaries, numerous testes, and numerous yolk-glands ; 

 the genital aperture is single. In Rhabdocoelida there 

 are one or two ovaries, two or many testes and yolk 

 glands may be present or absent. The nervous system 

 consists of a large cerebral ganglion divided into two lobes 

 from which pass off a pair of long posterior nerve cords 

 and smaller nerves to the anterior end. A pair of eyes are 

 usually present at the anterior end. 



CLASS : TREMATODA. 



Either internal or external parasites provided with 

 suckers, with mouth at anterior end, and usually bifurcate 

 intestine. No external cilia as a rule. The reproductive 

 system is similar to that of Turbellaria and is exemplified 

 by that of the Liver-fluke, but a special feature is the 

 presence of one or two canals leading from pores on the 

 dorsal surface to the shell gland. These are called 

 Laurels or Laurer-Stieda canals and serve to conduct the 

 sperms from another individual to the shell gland, where 

 fertilisation takes place. 



The Trematoda are divided into two orders: the 

 Digenea, resembling Fasciola Tiepatica, which exhibit 

 alternation of generations and pass from an invertebrate 

 to a vertebrate host in the course of their life-history ; 

 and the Monogenea, which develop directly without 

 alternation of generations and without distinct meta- 

 morphosis, and which are attached throughout life to 

 one host. The Monogenea are usually external parasites, 

 they are common on the gills and other parts of fishes ; 

 but Polystomum lives in the bladder of the frog and 

 Aspidogaster concharum is another example living in the 

 pericardium of Anodon. The suckers vary in number, 

 and are situated on a special disc at the posterior end of 

 the body. Hooks are often present as well as suckers. 

 The Digenea mostly have two suckers only, like the Liver- 

 fluke, which belongs to the family Distomidae, while the 

 Monogenea have numerous suckers, and the typical forms 

 belong to the family Polystomidae. 



CLASS : CESTODA. 



Endoparasitic Platyhelmia without cilia and without 



