PLATYHELMINTHES 207 



dividing transversely. Within the sporocyst some of the 

 cells lining the cavity behave like parthenogenetic ova, 

 dividing to form a blastula, which invaginates to give rise 

 to a two-layered sac or gastrula. This grows into another 

 form of larva, the redia, which bursts out of the sporocyst 

 and migrates, usually into the liver of the snail. The redise 

 devour the tissues of the snail and finally kill it. Each 

 redia has an elongated body with an anterior mouth, a 

 muscular pharynx, and a short, sac-like gut. A little way 

 behind the pharynx the body-wall is thickened to form a 

 muscular collar, and not far from the hind end are two 

 blunt conical processes on one side. Posteriorly there is a 

 large body cavity lined by an epithelium like that of the 

 cavity in the sporocyst. Cells derived from the wall of the 

 body cavity develop in much the same way as in the sporo- 

 cyst and give rise to daughter rediae, which escape from the 

 parent by an opening behind the collar. Several generations 

 of rediae usually succeed one another thus, but eventually 

 they cease to produce daughters of their own kind, and give 

 birth instead to creatures known as cercaria, with a flat, 

 heart-shaped body, two suckers, a forked gut, and a tail. 

 The cercaria emerges from the redia, works its way out of 

 the snail, and swims by means of the tail. Soon it settles 

 upon a blade of grass, loses its tail, secretes around itself 

 a cyst by means of special cystogenous cells of the ectoderm, 

 and waits till the grass is eaten by a sheep. In the gut of 

 the latter the cyst is digested and the cercaria, making its 

 way up the bile duct, grows into an adult fluke. When the 

 generative organs are fully developed, the worms begin to 

 lay their eggs, and also to wander back to the duodenum 

 of the host. In the end they are cast out with the faeces, 

 and if the sheep survives till this happens it will usually 

 recover, though, owing to permanent damage to the liver, 

 the recovery is never complete. 



It will be seen that in this life-history we have a case of 

 alternation of generations far more complicated than that 

 of Obelia, and differing from the latter also in that not 

 sexual and truly asexual, but sexual and parthenogenetic 

 generations succeed one another. The former kind of 

 alternation of generations is known as metagenesis, the latter 

 as heterogamy. It should also be noticed that there are 



