TREMATODA. 649 



furnished at one end, except in Bilharzia, with a cover or operculum which 

 the mature embryo thrusts open when it escapes. It is usually oval and 

 smooth in the digenetic Trematoda. But in the monogenetic its shape 

 varies, and is determined by that of the ' ootype,' and it is generally pro- 

 vided with a long process at one pole, or with two at opposite poles, 

 and it is often fixed. In the monogenetic Trematoda it is always laid ; 

 in the digenetic it may, or may not commence its development in the 

 oviduct. If development begins, it may be completed before oviposition 

 as in Monostomum mutabile, or only the first stages of fission may be 

 passed through. The germ-cell usually lies near the pole, closed by the 

 operculum, entirely or partially immersed among the vitelline cells. 

 Fission is regular or irregular, but complete, and during its progress 

 the vitelline cells degenerate and become used up more or less completely 

 by the developing germ-cell. It has been found in those cases which 

 have been thoroughly investigated that two layers of cells are successively 

 differentiated from the surface of the embryonic mass (Schauinsland). 

 The outermost layer is formed as follows. A cell at the upper, i. e. 

 opercular pole of the embryonic mass becomes flattened, divides, and the 

 two cells then grow round the mass ; other cells appear by division or by 

 differentiation from the mass itself. Eventually an enveloping membrane 

 is formed which lines the shell. A second superficial layer of flattened 

 cells is next differentiated, which becomes a ciliated coat, or when cilia 

 are not developed, a structureless cuticle as in D. tereticolle. The embryo 

 of the monogenetic Trematoda appears to be non-ciliated as a rule. 

 However, that of Diplozoon (= Diporpa] is ciliated laterally, of Polystomum 

 integerrimum provided with five transverse ciliated bands, three anterior, 

 incomplete dorsally, two posterior, incomplete ventrally. Among the 

 digenetic genera some are, and some are not ciliated, and the cilia may 

 cover the embryo but partially, e. g. they are present only on the anterior 

 region of D. lanceolatum. The ciliated coat or its representative, is 

 subsequently lost, but an underlying layer of flattened cells, afterwards 

 transformed into the cuticle, covers the embryo. A caecal digestive tract 

 is formed anteriorly, and the pharynx is sometimes to be detected. Flame- 

 cells are also present, e.g. as a single pair in the embryo of Fasciola hepatica. 

 One or two black eye-specks with or without a lens-like body may be 

 developed. And the embryo may be furnished at its anterior extremity, 

 either with chitinoid plates or spines as in D. tereticolle, or an extensile 

 boring process as in F. hepatica. When it quits the egg-shell it bursts the 

 enveloping membrane, and then thrusts open the operculum. It may at 



integerrimum. Its existence has been repeatedly denied. Ijima has recently maintained that Zeller's 

 canal also exists in P. ocellatum, in Diplozoon, and an Octobothrium (sp. ?), but that it communicates 

 with the alimentary canal in which he has observed germs in P. integerrimum. See Z. A. vii. 1 883, 

 P- 635- 



