I 



PLATHELMINTHES 173 



Hallez proves to be true, that portions of the migratory cells are 

 employed even after this in the formation of the ectodermal layer, 

 then such a mode of formation of the nervous system could be more 

 readily referred to the ectodermal method of origin, which was met with 

 in the Polycladida. However, it is not to be denied that the earliest 

 appearance of the nervous system in the Tricladida points to a meso- 

 dermal mode of origin, such as was attributed to it by the brothers 

 Hertwig {Ccelomtheorie) even at the time they wrote. Recently, too, in 

 the related Nemerteans, the nervous system has been derived from the 

 mesoderm (Hubkecht). 



When the embryonal pharynx has fulfilled its function, the 

 provisional mouth-opening closes, the pharynx degenerates, 

 and an irregular heap of cells lies in its place. In this a 

 cavity then arises, the cellular lining of which represents the 

 internal epithelium of the pharyngeal pocket, for the per- 

 manent pharynx is formed at the same spot. This therefore 

 arises, as it seems, from the entoderm (or mesoderm), whereas 

 in the Poly dads an invagination of the ectoderm gives rise to 

 its formation. The cylindrical form of the pharynx is due 

 to the fact that the surrounding cells take part in its forma- 

 tion. Before the pharynx attains its final shape, the union 

 with the lumen of the intestine takes place, and later the 

 mouth-opening also breaks through to the exterior. 



During the developmental processes described, the embryo 

 has frequently changed its form. At first ovate, it becomes 

 spherical after the introduction of the yolk-cells ; then at the 

 time of the formation of the permanent pharynx it again 

 elongates and becomes flattened on the ventral side (Fig. 85 C). 

 The pointed portion corresponds to the anterior part of the 

 body. 



III. Rhabdoccelid^. 



The development of the rhabdocoelous Turbellaria is still 

 the least known. Various forms belonging to the genera 

 Frorhy7ichus, Frostomum, Mesostomum, Schizostomum, and 

 Macrostomum were studied by Hallez (No. 6) in some stages 

 of development, but he studied only the winter eggs. These 

 eggs, which are surrounded by a firm capsule, are attached 

 to aquatic plants by means of a mucilaginous secretion. In 

 many forms (Prostomum lineare and P. Steenstrupii) the cap- 



