220 UNSEGMENTED WORMS 



L. peregra ; in Victoria Bulimus tenuistriatus . This 

 diversity of host, also remarkable in the adult, is very 

 unusual. Within the snail, e.g. in the pulmonary chamber, 

 the miracidium settles down, loses its cilia, increases in 

 size, and becomes a sporocyst. The sporocyst is a hollow 

 sac, with a slightly muscular wall and with the beginnings 

 of an excretory system. Sometimes this sporocyst divides 

 transversely (Fig. ii8 (4)). 



Within the sporocyst a few cells behave like partheno- 

 genetic ova. Each segments into a ball of cells or morula, 

 which is invaginated into a gastrula, and grows into another 

 form of larva — the redia. These rediae burst out of the 

 sporocyst, and migrate into the liver or some other organ. 

 Each sporocyst usually forms at a time 5-8 rediae ; each 

 of these forms 8-12 more rediae ; and each of these forms 

 14-20 cercariae. In the winter a sporocyst may give rise 

 to cercariae directly. A redia is a cylindrical organism 

 with a short alimentary canal, excretory canals with " flame 

 cells," and a pair of blunt locomotor processes posteriorly. 

 A cercaria has a bifurcated gut, two suckers, a locomotor 

 tail, and the beginnings of gonads (Fig. 118 (6)). 



The cercariae emerge from the rediae, wriggle out of the 

 snail, pass into the water, and after swimming for a short 

 time, moor themselves to stems of damp grass. There 

 they lose their tails and become encysted. If the encysted 

 cercaria on the grass stem be eaten by a sheep, the cyst 

 is dissolved in the stomach, and the young fluke makes 

 its way up the bile duct and its tributaries. In about six 

 weeks it grows into the adult sexual fluke. 



It will be noted that the sporocyst is the modified embryo, but that 

 it has the power of giving rise asexually to redia?. These develop, 

 however, from special cells of the sporocyst, which we may compare to 

 spores or to precociously developed parthenogenetic ova. Though the 

 reproduction is asexual, it is not comparable to budding or division. 

 The same power is possessed by the rediae, and there are thus several 

 (at least two) asexual generations between the embryo and the 

 adult. 



The disease of liver -rot in sheep is common and disastrous. It 

 has been known to destroy a million sheep in one year in Britain 

 alone. 



Classification. — Order i. Heterocotylea, with a posterior ad- 

 hesive organ, often with a pair of accessory suckers beside the mouth. 



