320 



PLA.TYIIELMINTHES. 



tailed Cercarice, or to another generation of Sporocysts or Redice* 

 which then produce the Cercarise. 



The Cercarice are nothing else than Distomum larvse, which 

 eventually reach (often only after two migrations, an active and a 

 passive one) the final host, where they become sexually mature. 

 They are furnished with an exceedingly motile caudal appendage, 

 frequently with a buccal spine, and occasionally with eyes, and they 

 present in the rest of their organization great resemblances to the 

 adult Distomum, excepting that the generative organs are not 

 developed. In this form they leave independently the body of the 

 Redia or Sporocyst and of the host of the latter, and move about 

 in the water, partly creeping and partly swimming. Here they 

 soon find a new host (Snail, Worm, Insect larva, Crustacean, Fish, 

 Batrachian), into which they penetrate, aided by the powerful 



vibrations of their tail ; they then 

 lose the latter and encyst. 



The Cercarice from the interior 

 of the snail thus become distributed 

 amongst a number of hosts, and each 

 of them gives rise to an encysted 

 young Distomum without generative 

 organs. This young Distomum mi- 

 grates passively with the flesh of its 

 host into the stomach of another 

 animal, and thence, freed from its 

 cyst, into the organ (intestine, bladder 

 ^-)> in wnich ^ becomes sexually 

 mature. 



There are, then, as a rule, three 

 different hosts in the organs of which 

 the different developmental stages 

 (Redia or Sporocyst, encysted form, 



sexually mature animal) of the Distomum bury themselves. The 

 transitions from one host to another are effected partly by inde- 

 pendent migration (embryos, Cercarise), partly by passive migration 

 (encysted young Distomuni). 



Modifications of the ordinary course of development may, however, 

 take place ; these may be either complications or simplifications. . The 

 embryo at hatching may contain a single Redia (as in Monostomum 



* In Cercaria cystoplwra from Planorbis marr/inatus ; according to G. 

 Wagoner, the primaiy asexual individual is a Spc.rocyst, the secondary a Redia. 



FIG. 256. o, Embryo of 



subclavatus (after G. Wagener). 

 D, Alimentary canal ; Ex, excre- 

 tory system. I, Embryo of Mo- 

 nostomum mutabile (after v. Sie- 

 bold). P, Pigment spots ; J2, 

 redia in the interior of the embryo. 



