TREMATODES I CLASSIFICATION 147 



CLASSIFICATION OF THE TREMATODES. 



Order I, Heterocotylea, Montic. Exclusively ectoparasitic species, with strongly 

 developed clinging or clasping organs, characterised by excretory 

 organs discharging on the dorsal surface at the anterior end, develop- 

 ment direct ; live mostly on the body or on the gills of fresh or salt- 

 water fish, but may also live in the urinary bladder of amphibians and 

 in the oesophagus of tortoises. 



Order II. Aspidocotylea, Montic. Trematodes with a low organisation 

 living endoparasitically ; they are distinguished by a large ventral sucker, 

 the excretory organs discharge through a posterior pore, development 

 direct ; parasitic in the intestine or gall-bladder of tortoises and 

 marine fishes, as well as in a few shell-fish, especially in their excretory 

 organs. 



Order III. Malacocotylea, Montic. Trematodes, the clinging apparatus of 

 which as a rule only consists of one or two sucking discs (oral and 

 ventral suckers) ; accessory suckers are rare, and in only one family 

 the Holostomidea a strongly developed clinging ( apparatus is present 

 at the anterior end ; chitinous clasping organs are always lacking. The 

 intestine is usually forked ; the oral orifice (with the exception of 

 Gasterotomum) is situated at the front. Both sexual organs are almost 

 always combined in the same individual ; the genital pore is usually 

 on the ventral surface. The excretory organs discharge through a pore 

 at the posterior end. Always endoparasitic, and living almost without 

 exception in vertebrate animals, particularly in the intestine. 



This order is divided into two groups, ACCORDING TO THE DEVELOPMENT : 



A. Metastatica, Lkt. Development without alternation of generations, 



yet with the formation of two larval forms and a change of hosts. 

 Fam. Holostomidea, with several sub-families and the genera Hemi- 

 stomum, Holostomum, Diplostomum and Polycotyle. 



B. Digenea s. str., Lkt. Development complicated by the interpolation 



of generations that reproduce asexually (sporocysts, redia) and by 

 one or two changes of host. 



Fam. Paramphistomidtz, ventral sucker at the posterior end, the 

 excretory bladder discharges on the dorsal surface, above the 

 acetabulum. The genital pore, situated on the median line of 

 the ventral surface is at about a third of the body from the 

 anterior end ; the intestinal fork always without appendages ; 

 the pharynx is very near the front, and is usually termed the 

 oral sucker ; hermaphrodites. 

 Divided again into several sub-families and genera (Paramphi- 



stomum, Gastrothylax, Gastrodiscus, &c.). 



Fam. FasciolidcB, with oral and ventral suckers, excretory pores dis- 

 charging at the posterior border ; genital pore on the ventral 

 surface, or situated on the lateral or posterior border. The 

 intestinal fork is generally without appendages ; hermaphrodites. 

 Divided into numerous sub-families and genera, of which only 

 Fasciola, Fasciolopsis, Paragonimus, Opisthorchis, Cotylogonimus 

 and Dicroccelium interest us here. 



