48 Animal Life and Intelligence. 



darts off again. But if that object be the minute water- 

 snail, Limneeus truncatulus (Fig. 10, B., natural size), instead 

 of darting off, the embryo bores its way into the tissues 

 until it reaches the pulmonary chamber, or more rarely 

 the body-cavity. Here its activity ceases. It passes into 

 a quiescent state, and is now known as a sporocyst (Fig. 10, 

 E.). The active embryo has degenerated into a mere 

 brood-sac, in which the next generation is to be produced. 

 For within the sporocyst special cells undergo division, and 

 become converted into embryos of a new type, which are 

 known as redia (F.), and which, so soon as they are suffi- 

 ciently developed, break through the wall of the sporocyst. 

 They then increase rapidly in size, and browse on the 

 digestive gland of the water-snail (known as the intermediate 

 host), to which congenial spot they have in the mean time 

 migrated. The series of developmental changes is even 

 yet not complete. For within the redise (besides, at times, 

 daughter rediae) embryos of yet another type are produced 

 by a process of cell-division. These are known as cercariee 

 (Fig. 10, G.). Each has a long tail, by means of which 

 it can swim freely in water. It leaves the intermediate 

 host, and, after leading a short, active life, becomes encysted 

 on blades of grass. The cyst is formed by a special larval 

 organ, and is glistening snowy white. Within the cyst lies 

 the transparent embryonic liver-fluke, which has lost its 

 tail in the process of encystment. 



The last chapter in this life-history is that in which the 

 sheep crops the blade of grass on which the parasite lies 

 encysted ; whereupon the cyst is dissolved in the stomach 

 of the host, the little liver-fluke becomes active, passes 

 through the bile-duct into the liver of the sheep, and there, 

 growing rapidly, reaches sexual maturity, and lays its 

 thousands of eggs, from each of which a -fresh cycle may 

 take its origin. The sequence of phenomena is charac- 

 terized by discontinuity of development. Instead of the 

 embryo growing up continuously into the adult, with only 

 the atrophy of provisional organs (e.g. the gills and tail of 

 the tadpole, or embryo frog), it produces germs from which 



