NECESSITY OP A SECOND MIGRATION. 



71 



CHANGE OF HOST — MIGRATION. 



Neglecting for the present parasites that develop directly (Tricho- 

 cephal'iMs, Oxyuris, Dochmius, &c.), and the other two instances just 



'k 



Jj 



Fig. 49. — Sporocyst and Eedia, with Cercarice in the interior. 



quoted, the second stage of development leads onl}^ to a certain point, 

 which is more or less distant from the final stage of sexual maturity. 

 But this stage lasts for a considerable time, even several years in 

 many parasites ; indeed, until a favourable opportunity affords the 

 conditions suitable for further development. If this opportunity do 

 not occur, they remain in the asexual state, and finally perish. 



The progress of recent research has made us acquainted with the 

 fact that these intermediate forms sometimes, of their own accord, seek 

 out a new host, in the body of which they arrive at sexual maturity. 

 This has been proved in the case of some marine tape-worms 

 {Tetrarhynchus), and will perhaps be ultimately shown to be of more 

 frequent occurrence. This migration is often accomplished by a brood 

 produced asexually from the quiescent form of the second develop- 

 mental stage, — it being, of course, supposed that this is active, and not, 

 like the heads of bladder-worms, attached to the mother. This is what 

 takes place in the Distomidae and allied forms of Trematodes. The 

 embryos (Fig. 49) are formed in the interior of saccular parasites, pro- 

 vided or unprovide(^w^hg^i^ali^^^|r^^^nal (Eedife or Sporocysts), 



