PL A TYELMINTUES. 



171 



velope fresh Redise or Sporocysts by a process of internal budding 

 (fig. 95 C; ; or else (2) there may be formed in them, by an analogous 

 process, larvaB with long tails 

 known as Cercarise (fig. 95 

 D). The direct development of 

 Cercariae is the usual course, 

 though in Distomum globiparum 

 the reverse is true ; but where 

 this does not take place the Re- 

 dise or Sporocysts of the second 

 generation give rise to Cercariae. 

 The Cercarise are developed 

 from spherical masses of cells 

 found in the body cavity of the 

 Sporocyst or Redia. The exact 

 origin of these masses is still 

 somewhat obscure, but they are 

 stated by Wagener (No. 212) to FIG. 95. VARIOUS STAGES IN THE META- 

 be derived from the body wall. M RPH sis OF THE DISTOME^: (from Huxley). 



They are probably to be re- , , A ' Cili f ted , lar y a of Jfojwstommn muta- 



J , . * ill ljlle - a - larval skin. b. Kedia developed 



garded as internal buds. within it. B. Eedia of Monostomum muta- 



The spherical bodies grow bile. C. Kedia of Distomum pacificum, with 



rinidlv in size their nostpri or germs of a second brood ofKediae. D. Kedia 

 rapidly in size, tnen posterior contaiuing CercarijBt E . cercaria. F. Full- 



extremity is prolonged into a grown Distomum. 

 process which forms the tail, 



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

 the trunk has very much the organisation 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 ca?cal alimentary tract. An excretory system of the ordinary 

 type is present, consisting of longitudinal contractile trunks con- 

 tinuous anteriorly with branched ciliated canals, which, as has re- 

 cently been shewn by Biitschli, may be provided with funnel-shaped 

 ciliated internal openings 1 . The contractile trunks unite posteriorly, 

 but instead of opening directly to the exterior are prolonged into 

 a vessel which traverses the substance of the tail, and after a longer 

 or shorter course bifurcates into two branches which open laterally. 



The tail is provided with an axial rod of hyaline connective 

 tissue, like the notochord of the tail of a larval Ascidiau, and is fre- 

 quently provided with membranous expansions. It is used as a 

 swimming organ. Beneath the epidermis are layers of circular and 

 longitudinal muscular fibres, the latter arranged in the tail as two 

 bands. 



The Cercariae when fully developed leave the Sporocyst or Redia, 

 and then their host, and become free. In most Redia? there is a 



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

 Zuoloyischcr Anz<?i(/er, 1879, No. 42. 



