72 LIFE-HISTORY OF PARASITES. 



which thus, in conformity with the law of alternation of generations, 

 give rise asexually to a new generation. In these cases a number 

 of generative cells develop, which become collected together in increas- 

 ing numbers, and grow into parasites which are different from the 

 foregoing generations, 1 and are, in fact, small sexless Distomes. 



In many cases this generation finds its way into the body of a host 

 while yet contained within the Sporocyst (or Redia). In Distomum 

 macrostomum, for example, whose life-history has recently been 

 worked out by Zeller, 2 the Sporocyst (the so-called Leucochloridium 

 paradoxuni), having the appearance and colour of a tailed fly-maggot, 

 is swallowed, together with its living contents, by some insectivorous 

 bird, after having bored its way through the tentacle of the Succinia 

 infested by it. About six days after, the young Distome, freed from 

 the Sporocyst, has attained to sexual maturity, having cast off' the 

 earlier thick larval cuticle. 



Such a direct transference into the definitive host is, however, 

 rare ; it is usual for the youny parasite first to enter the body of another 

 animal. 



For this purpose, the young Distomum is generally provided with 

 a tail, and often with a boring tooth at the anterior extremity. In 

 this stage it was formerly regarded as a distinct 

 animal, and named " Cercaria" These Cercariae 

 (Fig. 50) abandon their host and live free for 

 some time in water ; they then seek out a new 

 host, 8 which may belong to the Mollusca, Insecta, 

 or Crustacea, and bore their way through its outer 

 skin. Von Siebold 4 observed these parasites in the 

 act of making their way into the body of their 

 host, and he thus describes the process : " I had 

 obtained a quantity of Cercaria armata from the 

 common pond-snail (Lymnceus stagnalis), and put 

 FIG. 50. A free Cercaria. them into a watch-glass containing a number of 

 larvae of Ephemeridse and Perlidae. I could observe, by the help 

 of the microscope, that the Cercarise, swimming about in the water by 



1 Moreover, we know of cases, especially during the winter time, in which the genera- 

 ting cells of certain Rediae give rise again to Redise. It is more generally, however, the 

 Sporocysts that, by division or budding, give rise to a ramified structure that pierces the 

 tissues of its host in all directions. 



2 Zeller, Zeitschr. f. wiss. Zoo/., Bd. xxiv., p. 564, 1874. 



8 If there be no such change of hosts, the young Distomum has no tail. Some few 

 possess instead a short process which looks like a sucker, and serves to assist them in 

 creeping about. 



* "Ueber Band- und Blasenw tinner," p. 26, " Handworterbuch d. Physiol.," Bd. ii. 

 p. 669, 1843. 



