i 4 COMPARATIVE ANATOMY 



of the tissues of its host, and for a short time leads a free 

 existence, swimming in the water or wriggling among wet 

 grass and leaves. Soon, however, it ceases to move about, 

 comes to rest on the leaf of a water-plant or a blade of grass, 

 shakes off its tail, and proceeds to encyst. In this operation 

 the cystogenous cells secrete an abundant slimy material 

 which hardens immediately, and in a few minutes the animal 

 is covered over by a white hard case, and firmly fastened to 

 the leaf or grass on which it lies. The cercaria lives for some 

 time in the encysted condition, until the plant to which it 

 is attached is eaten by a sheep or other herbivorous animal. 

 Then, on reaching the stomach, its cyst-wall is dissolved by the 

 action of the gastric juice, and the cercaria makes its way into 

 the bile ducts, where it grows rapidly; its intestine becomes 

 hollow and branched, its generative organs are developed, and 

 in about six weeks time it becomes a sexually mature fluke. 



The life-history of Distomum hepaticum affords a very good 

 example of true alternation of generations and of adaptation in 

 connection with a parasitic habit of life. 



It was pointed out (vol. i. p. 252) that the so-called alternation 

 of generations in Obelia is not a true alternation, because the 

 asexually produced individuals are not developed from germ 

 cells, but originate, as do the members of plants, by a process 

 of vegetative multiplication. In Distomum, however, the case 

 is clearly different. The sexual generation is, of course, the 

 mature fluke, living in the bile ducts of the sheep. From its 

 eggs are formed miracidia, which, on finding their way into the 

 appropriate new host, develop by a process of simple growth 

 into the first asexual generation the sporocyst. The germinal 

 cells of the sporocyst, many of which, at least, are derived from 

 the germinal epithelium lining its body-cavity, behave and 

 develop like true ova, but they do not require fertilisation. 

 The sporocyst, then, is a parthenogenetic form, and its un- 

 fertilised ova give rise to the second asexual generation of 

 rediae. The germinal cells of a redia originate and develop in just 

 the same manner as those of the sporocyst, so the redia is also 

 parthenogenetic. After a larger or smaller number of generations 

 of rediae, the young flukes, called cercariae, are produced par- 

 thenogenetically by the rediae, and under appropriate conditions 

 they grow directly into the sexual form. Thus, if we use the 

 sign = to denote generation, whether sexual or asexual (par- 



