LIFE HISTORY OF SCHISTOSOMA 



303 



Recently Leiper has found cercariae showing the absence of a pharynx 

 (characteristic of the genus) in a Japanese mollusc. Such molluscs 

 were teased out in water and laboratory bred mice immersed therein. 

 One of these mice was killed a month later and adult schistosomes 

 were found in the portal vessels. Leiper has also found cercariae 

 showing absence of pharynx in four different species of molluscs in 

 Egypt. With such molluscs he was able to infect white rats and 

 other animals. He states that infection with these cercariee from the 

 mollusc host can bring about infection either by way of the mouth or 

 through the skin. Sodium bisulphate in a strength of i to 1000 killed 

 these cercariae almost immediately. 



It would therefore seem proven that all human schistosome in- 

 fections take place following cercarial and not miracidial development. 

 As proof that S. hcematobium and S. mansoni are different species, 

 Leiper notes that mice infected by molluscs of the genus Bullinus 

 showed schistosomes with terminal spined eggs, the ovary lying in the 

 lower half of the female. The male had four or five large testes. In 

 mice infected by molluscs of the genus Planorbis, the eggs were lateral 

 spined, the ovary was in the anterior half of the body and the male 



had eight small testicles. 







There is a view that the miracidium enters while bathing by the preputial channel, 

 hence the value of circumcision. 



If urine containing eggs is diluted with water the miracidium breaks out of the shell 

 and swims about as if in search of some desired object. 



The view is also entertained that the miracidium may gain access to the body 

 through the drinking water; there is much evidence against this. However access 

 to the body is gained, it is known that the larval forms make their way to the liver 

 where they develop. Arriving at maturity, the males and females become united 

 and proceed to the terminal branches of the portal vein, where the irritating eggs, 

 given off by the female, give rise to the symptoms. 



CESTODE OR TAPE-WORM INFECTIONS 



The cestodes and trematodes constitute the two great divisions of 

 the flat worms. Anatomically, a tape-worm may be considered as a 

 series of individual flukes united in one ribbon-like colony. The 

 cestode segments, or proglottides are covered by an elastic cuticle and 

 in their interior usually contain striated elliptical bodies composed of 

 calcium carbonate about 5 to 25^ according to the species in which they 

 are found. 



