TREMATODA. 



319 



bryonic development, the contractile, usually ciliated embryos* (fig. 

 255, ), which already possess the first rudiments of an excretory 

 system and more rarely a sucker with a mouth and alimentary canal, 

 leave the egg and wander about independently in search of a new host. 

 The latter is, as a rule, a snail, into the interior of which they pene- 

 trate and there become transformed into simple or branched Sporocysts 

 (without mouth and alimentary canal, fig. 255, c), or into JRedice 

 (with mouth and alimentary canal, fig. 255, d). These give rise, by 

 means of the so-called germs [cells lying in the body cavity of the 



D 



fh 



FIG. 255. Developmental history of Distomitm (partly after R. Leuckart). u, free swimming 

 ciliated embryo of the liver fluke. 4, the same in a state of contraction with rudimentary 

 alimentary canal (D) and nell mass (Or) (rudiments of the genital glands). Ex, ciliated 

 apparatus of the rudimentary excretory system, c, sporocyst developed from a Distomum 

 embryo, filled with Cercarire (C) ; B. Boring spine of a Cercaria. d Redia with pharynx, 

 (Ph), and alimentary canal (D) ; O, mouth ; Ex, Excretory organ ; C, Cercaria inside 

 Redia. e, Free Cercaria ; S, sucker ; D, alimentary canal. 



sporocyst or redia], which probably correspond to the germinal cells 

 (primitive ova) of the rudimentary ovary, to the generation of the 



* As R. Leuckart has rightly observed, the Dicyeniitlre. which were regarded 

 as Jli'Mziiti by Ed. v. Beneden, as well as the Orthonectidce, which ha\v 

 recently been especially investigated by Giard and E. Metsctmikofl:. and which 

 in the reproductive stage do not rise above a form corresponding to the embryos 

 of Trematodes, recall these Distomum larvae. 



