( 842 ) 



t'oelom luis almost eiitirelv disappeared, while the üenital apparatus 

 luis obtained a inaxiinuni degree of coniplieatioii. 



At the outset it seems to me to be less probable that at the base 

 of the pedigree of the Annelids such animals should stand like the 

 hermaj)hi'odite Plathelminths with their ovaries, testes, vitellaria, so 

 greatly varying in size and shape ; with their shell-glands, ootype, 

 cirrus, penes, uterus, spermatheca, etc., not even to mention the 

 vitello-intestinal, the Laurer- and other canals. Does not this very 

 complication force us to place such animal forms rather in the 

 peripheral branches than near the root of any pedigree ? 



On the other hand we can state that in those Polychaeta which 

 liave retained archaic characters, such as Polygordius, Protodrilus and 

 Saccocirrus, various peculiarities draw our attention which in Plathel- 

 minths are further developed. So the phylogenesis of the Plathel- 

 minths wonld not necessarily ha\e to be so long, via Polychaeta, 

 Oligocliaeta, Hirudinea, but the type of Plathelminths might already 

 at an early period \\a\e been a deviation of the origiiuil coelomatous 

 ancestral forms, while in the conrse of this evolutionary process also 

 the present Oligochaeta and Hirudinea might have sprung off laterally. 



Meanwhile the stronges argument for the degeneration of the 

 Plathelminths seems to nie to be fonnd in their early ontogenesis. 



When we consider this in the light wliich not long ago especially 

 Americaii workers have procured to us, we ought to pay attention 

 to the phenomena of ceU-Iiueage : the descent of special groups of 

 tissue from certain mothercells. Wilson, Conklin, ^Ikad and others 

 have shown iis the way here. 



Of paramount importance is the fact that Annelids (Polychaeta 

 Oligochaeta, Hirudinea) and Molluscs in those earliest phases of 

 development show a striking uniformity and that e.g. in all of them 

 the couple of mothercells of the so-called mesoblast-bands, within which 

 the coelom and metamerism appear tirst, originate in a similar 

 manner from one cell, the oldest, unpaired, mesodermic mother-cell, 

 belonging to the 64-cellular cleavage phase. 



This cell lies in the second quartet of cells reckoned from the vege- 

 tative pole and is produced by a plane of division slanting to the left. 

 The next cleavage always divides this cell into a right and left 

 mesodermic cell ; these two develop into the paired mesodermic bands. 



All this is always observed in the animal phyla above-mentioned. Con- 

 cerning the Plathelminths Lang provided us already t\venty years ago 

 Avith extensive data, which however do not constitute an unbroken 

 series such as is necessary for establishing the cell-lineage. Such 

 a series was given us a few years ag(» ( l-Sy.S) tor J^e|»t<)phiiui 



