146 THE ANIMAL PARASITES OF MAN 



in the liver, or in its bile ducts, and in the gall-bladder ; other 

 accessory organs* of the intestine, such as the pancreas, bursa 

 Fabricii (of birds), are only attacked by a few species. 

 Others inhabit the lungs, or the air sacks in fowls, a few the 

 trachea. Trematodes have also been known to occur in the 

 urinary bladder, the urethra and the kidneys of all classes of 

 vertebrates ; they are also present in the vascular system of a 

 few tortoises, birds and mammals ; in birds they even penetrate 

 into the oviducts, and are occasionally found enclosed in the 

 deposited eggs ; one species is known to occur in the cavum 

 tympani and in the Eustachian tube of a mammal (Halicore), 

 another in the frontal sinus of the polecat ; several kindred 

 species settle in the conjunctival sac under the membrana nic- 

 titans of birds, one species even lives in cysts of the skin of singing 

 birds. In an analogous manner the monogenetic 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 

 urinary bladder, in the oesophagus, and in sharks also 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 ; these double-tenanted cysts are also observed in 

 other situations, and, though they form the rule in species sexually 

 distinct, they are 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. 



