172 TREMATODA. 



special opening, not far from the mouth, by which they pass out. 

 There is no such opening in Sporocysts, but the Cercaria3 bore their 

 way through the walls. 



After leaving their parent the Cercarise pass into the external 

 medium, and for a short period have a free existence. They soon how- 

 ever enter a new host, making their way into its body by a process of 

 boring, which is effected by the head (especially when armed with 

 chitinous processes) assisted by movements of the tail. 



The second host is usually some Invertebrate (Mollusc, Worm, 

 Crustacean, Insect larva, &c.), but occasionally a Fish or Amphibian 

 or even a vegetable. The tail is very often lost as the Cercaria bores 

 its way into its host, but whether it is so or not, the Cercaria, after it 

 has once reached a suitable post in its new host, assumes a quiescent 

 condition, and surrounds itself with a many-layered capsule. The 

 cephalic armature and tail (if still present) are then exuviated, and 

 the generative organs gradually become apparent though very small. 

 In other respects the organization is not much altered. 



Though an encysted Cercaria may remain some months without 

 further change, it eventually dies unless it be introduced into its 

 permanent vertebrate host, an act which is usually effected by the 

 host in which it is encysted being devoured. It then becomes freed 

 from its capsule as a fully formed Trematode, in which the generative 

 organs rapidly complete their development. 



In some cases the Redise or Sporocysts do not give rise to tailed 

 Cercaria?, but to tailless forms. In such cases, as a rule, the encyst- 

 ment takes place in the host of the Redia or Sporocyst, but the tail- 

 less larva? sometimes pass through a free stage like the Cercaria;. In 

 the case of Distomum cygnoides, parasitic in the bladder of the Frog, 

 the Cercaria passes directly into the adult host without the interven- 

 tion of an intermediate host. 



The life history of a typical entoparasitic Trematode is shortly as 

 follows. 



(1) 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 numerous 

 tailed larvas, known as Cercariaa, are developed by a process of internal 

 gemmation. 



(4) The Cercarise 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 



