V PLATYHELMINTHES 117 



ment of Polyclada, we desire to know the origin of the extraordinary 

 excretory system which is so characteristic of Platyhelminthes and, 

 so far as we know, entirely unrepresented in any Ctenophore ; also a 

 knowledge of the development of the genital cells and of the genital 

 ducts would be of extreme interest. 



On the subject of the development of the excretory system Lang 

 has a few observations. He has observed two ectodermic ingrowths 

 at the sides of the embryo anteriorly, and these he looks upon as the 

 rudiments of the main excretory canals ; but we ought to have a 

 detailed knowledge of their development. 



When the development of Polyclada is thoroughly known the 

 embryonic development of the other groups of Platyhelminthes will 

 become of greatly increased interest, for it will show what alterations 

 in a known type are produced by access to stores of food, and in this 

 way throw light on the laws of variation, the ascertaining of which 

 constitutes one of the root problems of the Science of Biology. 



LITERATURE REFERRED TO 



Eeeble and Gamble. The Origin and Nature of the Green Cells of Convolida 

 roscoffensis. Quart. Journ. Micr. Soi., vol. 57, 1907. 



Lang. Die Polycladen des Golfes von Neapel. Fauna and Flora des Golfes von 

 Neapel, vol. 11, 1889. 



Surface. The Early Development of a Polyclad Planocera inquilina. Proo. Acad. 

 Nat. Sei. Philadelphia, 1907. 



