l82 TROPICAL MEDICINE AND HYGIENE 



without any further change, perforating the skin of the 

 host in order to do so. 



In Strongyloides iniestinalis the embryos have escaped 

 from the egg-capsule before they leave man. After the 

 faeces have been passed the larvae in a few days become 

 sexually mature, copulate, and produce rhabditiform larvae, 

 which at the first moult become filariform larvae and are 

 capable of becoming parasitic. 



The Trematodes and Cestodes require for their develop- 

 ment an intermediate host before they can attain sexual 

 maturity in man. In many of them not only develop- 

 ment, but an asexual multiplication, takes place in this 

 intermediate host. 



In the Trematodes, both Fasciolidae and probably 

 Paramphistomidae require a mollusc or fish as the inter- 

 mediate host. The eggs, having escaped, develop. The 

 ciliated larvae the miracidium having escaped from the 

 egg has a short free life in water, and then entering the 

 mollusc becomes a sporocyst in which several rediae 

 form, in each of which many cercariae develop. These 

 cercariae are either deposited encysted on herbage or pass 

 into water as free-swimming cercariae, or enter another 

 host which is subsequently devoured. We do not know 

 what becomes of the miracidium in Schistosomidae ; prob- 

 ably they are capable of perforating the skin and do not 

 require an intermediate host. 



In the Cestodes either an embryo develops by the time 

 the eggs are passed, as in the Taeniidae, or it develops 

 subsequently, as in the Dibothriocephaloidae. 



In either case the oncosphere, i.e., embryo in its em- 

 bryonic capsule, is swallowed by the intermediate host 

 and the embryo, set free, passes into the muscle, liver, or 

 other part of its intermediate host, becoming a cysticercus 

 from which numerous scolices are produced asexual 

 multiplication or only a few, or in some cases one only. 

 In the Dibothriocephaloidae the larva, having escaped 

 from the oncosphere in the intermediate host, becomes an 

 elongated larva plerocercoid larva no asexual multi- 



