208 TREMATODA. 



The contractile trunks unite posteriorly, but instead of opening 

 directly to the exterior are prolonged into a vessel which 

 traverses the substance of the tail, and after a longer or shorter 

 course bifurcates into two branches which open laterally. 



The tail is provided with an axial rod of hyaline connective 

 tissue, like the notochord of the tail of a larval Ascidian, and is 

 frequently provided with membranous expansions. It is used as 

 a swimming organ. Beneath the epidermis are layers of circular 

 and longitudinal muscular fibres, the latter arranged in the tail 

 as two bands. 



The Cercarias when fully developed leave the Sporocyst or 

 Redia, and then their host, and become free. In most Rediae 

 there is a special opening, not far from the mouth, by which they 

 pass out. There is no such opening in Sporocysts, but the 

 Cercariae bore their way through the walls. 



After leaving their parent the Cercariae pass into the external 

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

 soon however 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 Trema- 

 tode, in which the generative organs rapidly complete their 

 development. 



In some cases the Rediae or Sporocysts do not give rise to 



