ACTION OF THE DIGESTIVE JUICES. 



75 



Entozoa we need hardly adduce special cases to prove. The parasites 

 are in this way handed on from one animal to another, from an 

 aquatic to a terrestrial animal, from a cold-blooded to a warm-blooded 

 creature. x The bearer of the encysted parasite falls a prey to some 

 more powerful foe, and is devoured by it : neither herbivorous nor 

 carnivorous animals are secure from the invasion of parasites. The 

 possibility of the transference of parasites increases of course with 

 the number of animals that are devoured, and especially since the 

 bearers of encysted parasites are usually small invertebrates. The 

 larger animals, which need more nourishment, thus take in a gradu- 

 ally increasing number of parasites ; and it is easy, therefore, to 

 understand how it is that of all animals the Vertebrata are most 

 affected by these creatures (see p. 11). 



Only when the parasite has been transferred to the body of its 

 risjht host, and other circumstances are favourable, does it arrive at 



O ' ' 



sexual maturity ; otherwise it rapidly dies. In the same way, the 

 eggs, unless they reach the body of their proper host, die and decay. 



The first change that takes place is the dissolution of the 

 cyst, which, as in the case of the egg-shell, is accomplished 

 by the action of the digestive 

 juices of the stomach ; the 

 parasite then usually makes its 

 way into the intestine. It re- 

 mains for some time exposed 

 to this action of the digestive 

 juices, longer, perhaps, than the 

 embryos hatched from the eggs, 

 which, on account of their small 

 size, can move about more freely, 

 and also, possibly, bore into the 

 walls of the stomach. A longer 

 contact with the digestive juices 

 is but rarely dangerous, since 



they are protected by their large size, and relatively small super- 

 ficies, as well as by the thickness of the cuticle. Sometimes, how- 







The same thing holds good for the so-called Leucochloridiuiii, and its brood of Distonies 

 (see p. 71). 



1 That temperature has an effect upon Entozoa is shown by the fact that the Dis- 

 tomum of the bat undergoes no further development during the winter sleep of its host, 

 (van Beneden, " Les Parasites des chauves souris," Mtm. Acad. Eelgique, t. xl., p. 23, 

 1873). [In the same way, the Entozoa of cold-blooded animals, when they have not 

 arrived at maturity, stop their metamorphosis during the winter, and produce no eggs, or 

 only very few ; and also, under similar circumstances, Redise, instead of producing Cer- 

 cariae, give rise to new Redise. R. L.] 



FIG. 53. -Bladder- 

 worm with extruded 

 head. 



FIG. 54. Bladder- 

 worm head after di- 

 gestion of the caudal 

 bladder. 



