1902.] NATURAL SCIENCES OF PHILADELPHIA. 223 



imjDortauce, as far as I cau recall ; but without wishing to preju- 

 dice the question it may be asked, is this dorsal position to be 

 explained simply as due to peculiarities in the external skeleton ? 

 If it cannot be so explained, then it is a characteristic which 

 should receive more attention than it has heretofore. The pro- 

 boscis of au Echinorhynchus is morphologically a part of the body 

 wall, that of a Turbellariau a fold of the pharynx alone; this 

 difference of position is sufficient to disassociate these structures, even 

 though they should agree closely in anatomical details, and here, 

 then, relative position would be of more value than structural 

 detail of the organ itself. The relative position of internal organs, 

 which are not immediately exposed to change in external environ- 

 ment, is a characteristic which must receive careful consideration. 

 The other characteristics of an organ, form and differentiation 

 of its parts, are especially important. The general form is of more 

 importance for phylogeny than its minute histological structure, and 

 its grosser than its finer structure. This point has been well ex- 

 pressed by E. B. Wilson:^* " The nephrostome Ls a nephrostome 

 all the same whether it consist of one cell, two cells, or many 

 cells. Its form and function are both independent of the number 

 of component cells. Cells midtiply, but the organ remains the 

 same throughout. So far as homologies are concerned, the existence 

 of cells may be ignored." And to consider the nephridia of 

 TurbeUaria, Nemertini and Annelida. The nephridia of the first 

 two mentioned groups are rightly considered to be of essentially 

 the same type and different from the nephridia of Annelida, since 

 they cou.sist of continuous longitudinal ducts with numerous lateral 

 branches, while a nephridium of an Annelid is essentially a single, 

 unbrauched tube; we cannot say there is a sharp distinction 

 between the nemerteau and the turbellariau type, because the 

 terminal end of a branch in the former is multicellular, in the 

 latter unicellular. Similarly, in certain Nematoda the intestinal 

 lumen is intercellular, in others intracellular; but this histological 

 difference should not lead us to doubt the correspondence of these 

 intestines. In regard to the nephridia of Xematoda, there is still 

 reasonable doubt whether they should be considered comparable to 

 the protoiiephridia of TurbeUaria; but this doubt is based upoii 



'* The Emhryological Criterion of Ilomoloiju, Woods IIoll Lectures, 

 1806. 



