PLATYELMINTHES. 



207 



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 

 Distommn globiparmn the 

 reverse is true ; but where 

 this does not take place the 

 Rediae or Sporocysts of the 

 second generation give rise 

 to Cercariae. 



The Cercariae are deve- 

 loped from spherical masses FIG. 9:;. VARIOUS STAGES IN THE META- 

 of cells found in the body MORPHOSB OF THE DISTOME* (from Huxley.) 



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. They are probably to be 



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 Butschli, 

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



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 Redia;. D. Redia 

 containing Cercarias. E, Cercaria. F. Full- 

 grown Distomum. 



1 O. Butschli, " Bemerkungen ub. d. excretorischen Gefassapparat d. Trematoden." 

 Zoologiscker Anzeiger, 1879, No. 42. 



