280 ZOOLOGY SECT. 



strict sense of the term, living on the surface of the " host " 

 animal, depositing their eggs there, and being carried about 

 by it, but subsisting on minute living animals captured in the 

 water. 



^ The Digenetic Trematodes are all internal parasites, and in the 

 adult condition inhabit, in nearly all cases, the alimentary canal, 

 liver, or lungs of some vertebrate animal, swallowing the 

 digested food or various secretions of their host. But, as mentioned 

 before in the account given of their development, they are internal 

 parasites, not only in the adult condition, but throughout the 

 greater part of their life. After a short period of freedom as 

 ciliated larvae, they again enter into a state of parasitism as sporo- 

 cysts or rediae in a second host ; and, after a second free interval as 

 cercariae, may enter the body of a third host to become encysted. 

 The second host is, very generally, a Mollusc, and the cercaria 

 may become encysted in the same animal. 



The Cestodes are, of all the Platyhelminthes, those that are most 

 modified in accordance with the condition of internal parasitism 

 in which they remain throughout life. The adult Cestode is 

 almost always an inhabitant of the alimentary canal of a verte- 

 brate. The intermediate host is frequently also a vertebrate 

 commonly of a kind which is liable to become the prey of the 

 final host. In the case of Tcenia crassicollis of the intestine of 

 the domestic Cat, for example, the cysticercus stage occurs in the 

 livers of Rats and Mice ; the cysticercus of Tcenia serrata of the 

 Dog is found in Hares and Rabbits. But in many cases the inter- 

 mediate host is an invertebrate. In either case the passage from 

 one host to another is a passive translation, not an active 

 migration as in the Trematodes. 



A few human parasites belong to the Trematoda, but none 

 that are of very common occurrence among Europeans. Fasciola 

 hepatica has occasionally been found in the human liver ; Dis- 

 tomum rathousii is a common intestinal parasite in China ; 

 Opisthorchis sinensis occurs in the liver of Man in China and Japan ; 

 Dicroc&lium lanceatum and various other species of the genus 

 occasionally occur in the human subject. Schistosomum hcema- 

 tobium and S. japonicum (Fig. 231), otherwise known as Bilharzia 

 hcematohia, and B. japonica, which differ from most other Trematodes 

 in being unisexual, are found in pairs in the sub-mucous tissue of 

 the human urinary bladder or rectum in various parts of Africa 

 and South America, in Arabia, the Philippines, and Japan. Eggs 

 with contained larvae are voided with the urine, and if they reach 

 water, the ciliated larvae or miracidia penetrate into the interior 

 of a fresh- water snail and give rise to sporocysts from which cercariae 

 are developed. The latter become free, and, if the opportunity 

 presents itself, may enter the human body through the skin or the 

 mucous membrane of the mouth, and, entering the venous system, 



