PLATYELMINTHES. 205 



TREMATODA. 



The eggs of the Trematoda consist of a germ or true ovum 

 enclosed in a mass of yolk cells, which undergo disintegration 

 and subsequent absorption at varying periods of the develop- 

 ment From the observations of E. van Beneden (No. 218), 

 Zeller (No. 217), etc. it is known that the segmentation is 

 usually complete, but generally somewhat irregular. 



Unfortunately we are still completely in the dark as to the 

 mode of formation of the germinal layers. The embryos of the 

 entoparasitic forms or Distomeae become free in a very imperfect 

 condition, and the ova are small ; while in the Polystomeae the 

 development is as a rule nearly completed before hatching, and 

 the ova are large. It will be convenient to treat separately the 

 development of the two groups. 



Distomeae. The embryos of the Distomeae are hatched 

 either in some moist place or more usually in water. In the 

 majority of genera the larvae pass through a complicated meta- 

 morphosis, accompanied by alternations of generations. But for 

 some genera, e.g. Holostomum, etc., the life history has not yet 

 been made out. The whole life history of comparatively few 

 forms has been followed, but sufficient fragments are known 

 to justify us in making certain general statements, which no 

 doubt hold true for a large proportion of the Distomeae; 



The larvae are usually ciliated (fig. 95 A), but sometimes 

 naked. 



The ciliated forms are generally completely covered with cilia, but in 

 Distomum lanceolatum the cilia are confined to an area at the front end of 

 the body, in the centre of which a median spine is placed. An x shaped 

 pigment spot, sometimes provided with a rudimentary lens (Monostomum 

 iutabile\ is also generally situated on the dorsal surface. 



In some intances a more or less completely developed alimentary tract is 

 present {Monostomum capitelliun, Amphistonntm subclavatum\ but usually 

 there can only be distinguished in the interior of the larva a transparent 

 mass of cells bounded by a more or less distinctly marked body wall with 

 ciliated excretory channels. 



Ed. van Beneden has shewn that the ciliated covering is developed 

 while the embryo is still in the egg, and long before the yolk cells are com- 

 pletely absorbed. It would seem that even before hatching this ciliated 

 covering is to a great extent independent of the mass within. In the 



