PLATYHELxMINTHES 77 



hermaphroditic gonads. Its digestive system consists of a mouth, 

 pharynx, oesophagus, intestine and non-anastomosing caecal tubes. 

 The intestine is usually darkened by the blood and bile used as food. 

 The excretory syste?n consists of a main duct with four anterior ducts, 

 a dorsal and a ventral one on either side. The nervous system 

 consists of a collar around the pharynx; two lateral ganglia and one 

 ventral median ganglion. The male reproductive system consists of a 

 pair of testes, vasa deferentia near the seminal vesicle, the ejacu- 

 latory duct and the penis or cirrus with its sac. Tho. female repro- 

 ductive system consists of the ovary, oviduct, yolk glands, shell gland, 

 vitellarian ducts and the vagina or uterus, opening at the genital pore. 



Trematode Parasites.— Some of the trematodes infesting other 

 mammalian hosts are likely to infest man occasionally. For exam- 

 ple, Fasciola hepatica has been found in the human liver. Some 

 are normally parasites of man. 



The blood fluke Schistosojna (Bilharzia) haojiatobium is a human 

 parasite found in Egypt involving large numbers of troops during 

 the recent war. The cercaria enter the blood stream through the 

 skin, finding their way to the small veins of the bladder and colon 

 and becoming mature in the submucous tissue. They require as an 

 intermediate host the non-operculate fresh water snail Bulinus or 

 Planorbis. Developing in the liver of the snail into tubular sporo- 

 cysts, germ cells develop within the sporocysts into cercariae with 

 forked tails. These later escape from the snail and in the free state 

 these cercaria must find a mammalian host in forty hours or die. 

 When they come in contact with mammalian skin they cast off their 

 tails, dissolve the cells of the skin, and work their way into a lym- 

 phatic or the blood stream and thence to the liver where they 

 mature and eventually lodge in the mesenteric veins. There was 

 very good scientific basis for the blind worship of the Nile ibis, an 

 important enemy of snails. (See p. 154.) 



Schistosoma japonicumy found in the Orient, is somewhat smaller 

 than Bilharzia. It requires as its intermediate host the snail 

 Katayama nosophora in Japan and Oncomelania hupensis in the 

 Yangtze Valley. It is quite possible that several snails found in the 

 United States might act as carriers for Schistosoyna japonicum but 

 none as yet have been found. Schistosoma cause inflammation of 

 the rectum and bladder. Other species of Schistosoma ^ are found 

 in tropical America, the West Indies and the Philippines. 



3 Cort, W. W. 1928. Schistosome dermatitis in the United States (Michigan). 

 Jour. Amer. Med. Assoc, vol. 90, p. 1027. 



