PLATYELMINTHES. 



2O7 



further development one of two things may happen. They may 

 either (i) develop fresh Rediae or Sporocysts by a process of 

 internal budding (fig. 95 C) ; or else (2) there may be formed in 

 them, by an analogous pro- 

 cess, larvae with long tails 

 known as Cercariae (fig. 95 

 D.) The direct develop- 

 ment of Cercariae is the 

 usual course, though in 

 Distomum globiparum the 

 reverse is true ; but where 

 this does not take place the 

 Rediae or Sporocysts of the 

 second generation give rise 

 to Cercariae. 



The Cercarias are deve- 

 loped from spherical masses 

 of cells found in the body 

 cavity of the Sporocyst or 

 Redia. The exact origin of 

 these masses is still some- 

 what obscure, but they are 

 stated by Wagener (No. 212) 

 to be derived from the body wall, 

 regarded as internal buds. 



The spherical bodies grow rapidly in size, their posterior 

 extremity is prolonged into a process which forms the tail, while 

 the anterior part forms the trunk. When fully formed (fig. 95 E), 

 the trunk has very much the organization of an adult Distomum. 

 There is an anterior and a ventral sucker, the former of which 

 contains the opening of the mouth, and is often provided with a 

 special chitinous armature. The mouth leads into a muscular 

 pharynx, and this into a bilobed caecal alimentary tract. An 

 excretory system of the ordinary type is present, consisting of 

 longitudinal contractile trunks continuous anteriorly with branch- 

 ed ciliated canals, which, as has recently been shewn by Biitschli, 

 may be provided with funnel-shaped ciliated internal openings 1 . 



FIG. 95. VARIOUS STAGES IN THE META- 

 MORPHOSIS OF THE DISTOME^; (from Huxley.) 



A. Ciliated larva of Monostomum muta- 

 bile. a. larval skin. b. Redia developed 

 within it. B. Redia of Monostomum muta- 

 bile. C. Redia of Distomum pacificum, with 

 germs of a second brood of Rediae. D. Redia 

 containing Cercariae. E. Cercaria. F. Full- 

 grown Distomum. 



They are probably to be 



1 O. Biitschli, "Bemerkungen iib. d. excretorischen Gefassapparat d. Trematoden." 

 Zoologischer Anzeiger, 1879, No. 42. 



