PLATYELMINTHES. 209 



tailed Cercariae, but to tailless forms. In such cases, as a rule, 

 the encystment takes place in the host of the Redia or Sporocyst, 

 but the tailless larvae sometimes pass through a free stage like 

 the Cercariae. In the case of Distomum cygnoides, parasitic in 

 the bladder of the Frog, the Cercaria passes directly into the 

 adult host without the intervention of an intermediate host. 



The life history of a typical entoparasitic Trematode is shortly 

 as follows : 



1 i ) It leaves the egg as a ciliated or non- ciliated free larva. 



(2) This larva makes its way on to the gills or into the 

 body of some Mollusc or other host, throws off its epidermis and 

 becomes a Redia or Sporocyst 



(3) In the body cavity of the Redia or Sporocyst nume- 

 rous tailed larvae, known as Cercariae, are developed by a process 

 of internal gemmation. 



(4) The Cercariae pass out of the body of their parent, 

 and out of their host, and become for a short time free. They 

 then pass into a second, usually invertebrate host, and encyst. 



(5) If their second host is swallowed by the vertebrate 

 host of the adult of the species, the encysted forms become free, 

 and attain to sexual maturity. 



The majority of these stages are simply parts of a complicated 

 metamorphosis, but in the coexistence of larval budding (giving 

 rise to Cercariae or fresh Rediae) with true sexual reproduction 

 there is in addition a true alternation of generations. 



Polystomeae. The ova of the Polystomeae are usually large 

 and not very numerous, and they are in most cases provided 

 with some process for attachment. Some species of Polystomeae, 

 e.g. Gyrodactylus, are however viviparous. The young leave the 

 egg in a nearly perfect state, and at the utmost undergo a slight 

 metamorphosis and no alternations of generations. Some how- 

 ever (Polystomum, Diplozoon) are provided with temporary cilia, 

 but the number investigated is too small to determine whether 

 ciliation is the rule or the exception. The ciliated larvae have a 

 short free existence. The cilia are developed on special cells 

 which may be arranged in transverse bands in the same way as 

 in the larvae of many Chaetopods, but are not, in the larvae at 

 present known, distributed uniformly. When the free larvae 

 become parasitic the cells with cilia shrink up. 



B. II. 14 



