74 THE TREMATODA 



parasitism ; it loses its cilia and the cells that bear them. The 

 enteron, meanwhile, undergoes obliteration and degeneration. 1 



The organism is now known as a " sporocyst " (Filippi), and 

 the " germ cells " which occupy the cavity begin to divide up to 

 form " egg-balls " (Fig. XVI. 2). According to some authorities, 

 the "germ cells" are directly derived from undifferentiated 

 blastomeres (Leuckart, Schauinsland), whilst others have described 

 them as arising by division from the cells of the body wall 

 (Thomas, Biehringer, 7 ; Heckert, 20). 



Anyhow, the germ balls consist of cells of different sizes, and 

 soon a flat epithelium is differentiated around a central mass, the 

 epithelial cells are said to lose their nuclei and become cuticularised 

 (Leuckart), while from the central mass a new outer layer becomes 

 differentiated, and thereafter the history is very similar to that by 

 which the miracidium was produced. By a series of changes there is 

 developed from each germ ball another larval form, which is known 

 as "Redia" (Filippi), or "king's yellow worm"(Bojanus and Swam- 

 merdam were the first to observe this stage, 1737); within this a new 

 generation of " germ balls " is already formed (Fig. XVI. 2, 3). 



The redia differs from the miracidium, in the absence of cilia 

 and of eyes, in the possession of a pharynx, and in the general 

 shape. The rediae escape from the sporocyst, the aperture closes, 

 and the wound heals. The rediae in their turn produce a new 

 generation, the " Cercaria " (0. F. Miiller), in the same way, no 

 doubt, as they themselves were produced. But one or more new 

 generations of rediae may be produced by rediae ere the cercariae 

 are formed. 



The cercariae, several of which are produced in a redia, escape 

 one by one through a definite birth-pore (as was first noted by 

 Bojanus). The cercaria possesses all the organs of the young fluke 

 in a rudimentary condition, even the foundations of the genital 

 organs are present; in addition, there are the tail, cystogenous 

 glands, and, in some cases, eyes, stylets, and rod cells, organs only 

 used during the brief larval life (Fig. XVI. 5). 



This third generation now leaves the snail, swims freely in the 

 water by the movement of its tail, and having attached itself to 

 a blade of grass by means of its ventral sucker, secretes a "mucous" 

 substance around itself which soon hardens to form a cyst (this fact 

 was known to Nitzsch, 1807). This cyst is devoured with the 

 grass by a sheep, the final host of D. hepaticum ; the cyst is dis- 

 solved in the host's stomach, and the tail having in the meantime 

 dropped off or undergone degeneration, a young fluke emerges, and 

 makes its way up the bile duct and into its finer branches, where 

 it grows into an adult fluke. 



1 It appears that in Victoria the intermediate host of D. hepaticum is Bit-limits 

 tenuistriatus, according to T. Cherry, Proc. Roy. Soc. Viet. viii. (n. s.), 1896, p. 183. 



