TREMATODA. 



229 



left behind at hatching. Beneath this there is formed a second 

 membrane (-V), which gives rise to the ectoderm of the just-hatched 

 larva; this is ciliated in some forms (D. hepaticum, Fig. 182), but 

 in others it develops a stout cuticular layer (D. tereticolle, Fig. 

 180, e, EC). 



The peripheral cells of the contained cell-mass now differentiate 

 a third membrane of flattened epithelial cells, while of the remaining 

 cells those at the head end give rise to the rudiment of the alimentary 

 canal the others becoming the germ cells of the larva. The embryo 

 is now hatched, and becomes a small free larva the Miracidium. 

 This, which may be ciliated (Fig. 182) or non-ciliated, or even pro- 

 vided with stiff bristles (D. tereticolle, D. ovocaudatum), has a 

 contractile body and often an x-shaped eye-spot; it is also provided 

 with excretory canals in its walls, and in cases in which the structure 

 has been fully examined, a sucker with a 

 mouth opening, intestine, and a ganglion 

 can be made out (Fig. 181). Further, 

 between the intestine and the body-wall 

 there are some ova-like cells the germ 

 cells. 



This larva leaves the egg and, if ciliated, 

 wanders about independently in search of 

 a host, through the body-wall of which it 

 bores its way ; if non-ciliated it is taken 

 up by the new host in its food. 



Sometimes the unhatched egg is swal- 

 lowed (D. lanceolatwn) and the larva is 

 set free in the alimentary canal. In any 

 its way into the tissues of its host, which is a mollusc, usually 

 a water-snail, and, casting its cuticularized or ciliated skin, becomes 

 a sporocyst or a redia.* The sporocyst (Fig. 182, li) is a hollow sac 

 with excretory canals in its wall, and containing in its cavity a 

 number of germ-cells ; a redia (Fig. 182, c) is like a sporocyst except 

 that it contains a mouth at one end and an intestine, and two lateral 

 processes near its hind end ; it likewise contains a cavity with germ- 

 cells. The germ-cells of these organisms develop into more sporocysts 

 or rediae, or into Cercariae.* The Cercariae are young Trematodes, 

 which eventually reach (often only after two migrations, an active 



* The miracidium generally becomes a sporocyst, rarely a redia (Monostomum 

 flavum and mutabile). The sporocyst may produce other sporocysts (D. cygnoidcs), 

 or Cercariae direct, but generally gives rise to rediae (D. heputicum, Diplodiscus 

 subdavatiis), which produce the Cercariae. 



FIG. 181. Anterior pole of an 

 egg of the liver-fluke with 

 fully developed embryo, in 

 optical section. The probos- 

 cis, with mouth, oesophagus, 

 and intestine, the ganglion 

 and eyes are visible (after 

 Leuckart). 



case the larva makes 



