230 THE ANIMAL PARASITES OF MAN 



conjunctival sac under the membrana nictitans of birds, one species 

 even lives in cysts in the skin of song-birds. In an analogous manner 

 the ectoparasitic Trematodes are not entirely confined to the surface of 

 the body or the trachea of the lower vertebrate animals ; a few species 

 appear exclusively in the urinafy bladder, in the oesophagus, and in the 

 case of sharks in an accessory gland of the rectum. 



Trematodes live free and active within the organs attacked, 

 though they may attach themselves by suction for a longer or shorter 

 period ; in other cases, however, they bore more or less deeply 

 into the intestinal wall with their anterior end, or lie in cysts 

 of the intestinal wall which only communicate with the lumen 

 through a small opening ; in those species living in the lungs of 

 mammals the host likewise produces a cyst, which usually encloses 

 two specimens; such association of a pair is also observed in 

 other situations, and, though this is the rule in species sexually 

 distinct, it is not entirely confined to these. 



As regards the AGE attained by endoparasitic Trematodes, there 

 are but few reliable records, and these differ considerably ; the 

 overwhelming majority of species certainly live about a year, or 

 perhaps a little longer, but there are some whose term of life 

 extends to several or many years. 



Trematodes are but rarely found encysted in the higher verte- 

 brate animals ; the condition, however, is more frequent in amphi- 

 bians, and especially in fishes, as well as in numerous invertebrate 

 animals. 



CLASSIFICATION OF THE TREMATODES OF MAN. 



The following classification, partly artificial, partly natural, 

 embraces only the flukes found in man : 



Order. Digenea, v. Beneden, 1858. 



Anterior sucker single and median, present. Eggs few. The (specialized) 

 terminal portion of the uterus serves as a vagina. Development indirect, i.e., an 

 intermediate host is required. 



Sub-order. Prostomata, Odhner, 1905. 

 Mouth surrounded by the anterior sucker. 



Group. Amphistomata, Rudolphi, 1801, ep., Nitzsch, 1819. 



Gut forked, two suckers, the posterior sucker (acetabulum) terminal or ventro- 

 terminal behind the genitalia, or at most embraced by the vitellaria. Skin with no 

 spines. Excretory bladder a simple sac opening dorsally near hind end. Testes in 



