19 ZOOLOGY. 



there is frequently a most complicated metamorphosis coupled with an 

 alternation of sexual and non-sexual generation (see 218). A 

 Liver-fluke (Distomum hepaticum} is found in the bile ducts of the liver 

 of the sheep, where it gives rise to a much-dreaded disease " liver rot." 

 The eggs which are formed, fertilized and pass through the early stages 

 of cleavage here, pass out of the bile ducts to the intestine and thence to 

 the exterior. If the larva reaches water it develops into a free-swim- 

 ming larva (Fig. 90, C.), which to insure further development must bore 

 into the tissues of a particular pond-snail (Limnaa truncatuld). It there 

 develops into a kind of sac (sporocyst} from the inner cells of which 

 special cells are budded (Fig. 90, ). These cells have the power of 

 developing into embryos of a second generation by cell division that 

 is to say, non-sexually. Several such non-sexual reproductions may occur 

 in the body of the snail (Fig. go, -f-). These later generations 'of larvae 

 pass, often by the death of the snail, into the water, whence they may 

 enter the alimentary tract of the sheep in drinking. The larvae find their 

 way to the liver and develop there again into the adult fluke. It is evi- 

 dent that such a form must have immense powers of reproduction, when 

 it is considered that the reproduction takes place at several points in the 

 life cycle (Fig. 90, -f *) This may be seen to be a necessity to compensate 

 for the great loss of life involved in changing from host to host. It is 

 said that a single fluke may produce half a million eggs. Each of these 

 which succeeds in reaching the host snail may produce hundreds of the 

 last generation of asexual individuals. The disease is prevalent only in 

 those countries where this species of Limnaa occurs. It is much worse 

 in wet years. Millions of sheep have died in England alone, in a single 

 year, from the attacks of this parasite. Trematode parasites are common 

 among the vertebrates and frequent most diverse organs. 



Class 3. Cestodes (Tape-worm, etc.}. The Cestodes are internal 

 parasites having a complicated life history usually involving two hosts. 

 In the tissues of the first host occurs the " bladder- worm," Cysticercus, 

 or embryonic stage (Fig. 91, A}; in the intestine of a second host the 

 strobila or adult tapeworm (Fig. 91, C) is found. The adult form has 

 no mouth or digestive tract, the animal taking its food by absorption of 

 the digested material in which it is bathed. The anterior end is supplied 

 with hooks or suckers by means of which it attaches itself to the intes- 

 tinal wall. Just behind this "head" is a region in which transverse 

 division (Fig. 91, z; and 122) is continually going on. This results 

 in the continuous formation of new segments or proglottides, the older 

 ones being pushed further from the head by those newly formed. Each 

 proglottis becomes in time a sexually mature hermaphrodite individual. 

 All stages of sexual maturity are found in one strobila or colony, the 

 posterior individuals being most mature. At the posterior end of an old 

 colony the proglottides (Figs. 91, 92) are filled with the developing embryos, 

 and on breaking away from the chain these brood cases pass with the 

 faecal matter from the intestine. In this way it becomes possible for the 

 embryos to find the way into a new host. On being swallowed by some 



