32 OLIGOCHAETA 



periods in the life-history of these worms there are four sets of excretory organs ; the 

 very young embryo is furnished with certain epiblastic cells probably of an excretory 

 function ; the older embryo has a pair of larval pronephridia ; the older embryo a set 

 of embryonal pronephridia, and finally there are the definitive nephridia of the adult ; 

 the terms used are those introduced by Vejdovskt in his work already quoted (9). 

 These various excretory organs will now be considered seriatim. 



(a) Excretory cdls. In the gastrula stage of various species of Lumbricus and 

 Allolohophora there are a few large cells which are thus early set apart to perform an 

 excretory function ; so at least it is believed from the fact that they contain canals in 

 their interior which are often coiled in quite a complicated fashion. They are epiblast 

 cells distinguished by their large siZQ and more granular appearance ; they always mark 

 the anterior end of the embryo and only persist during the younger stages ; they do not 

 exist in Allolohophora foetida, nor were they found in Rhynchelmis. There are three of 

 these cells. But they get to be completely fused so that no cell-outlines, but only the 

 three nuclei, are distinguishable. The canal appears to become lost in the primitive 

 body-cavity, lying between the epiblast and hypoblast. When the cells are kept under 

 observation the liquid contained in the canals is seen to be evacuated ; after this has 

 taken place the canals are no longer visible, but the cell-boundaries come into view; 

 for further details the reader must refer to Vejdovsky's work (p. 208 et seq.). 



(6) Larval pronephridia. These were first seen by Vejdovsky in Allolohophora at 

 a stage when the blastopore was still large : it is a fine canal running in the primary 

 body-cavity. There are ultimately a pair of these tubes ; these tubes do not exist in 

 Rhynchelmis nor in Allolohophora foetida, but they are as described by Beegh (4) 

 enormous in Criodrilus. The tubes are ciliated and open on to the exterior anteriorly ; 

 in Lumbricus ruhellus alone did Vejdovskt find an iuternal ' flame-cell.' The organ 

 opens on to the exterior through the lumen of the excretory cells ; under the description 

 of these the extension of that lumen into the primary body-cavity was mentioned. 

 These tubes persist during the first formation of the embryonal pronephridia. 



(c) The embryonal pronephridia have no relation at all to the last, but they give rise 

 to the permanent nephridia. These occur in every segment of the body, and the first 

 pair open on to the exterior by the headpore of the embryo Lumbricus ; this first pair 

 commonly occupy two segments and their lumen is not always ciliated. They are 

 developed before the others, and disappear early, in a number of forms such as 

 Lumbricus and the aquatic Oligochaeta. The name 'headkidney' has often been 

 applied to this first pair, and they have been supposed to be different from the pairs 

 which follow them. The principal difficulty in comparing them is the fact that the 

 external pore is differently placed, being dorsal instead of ventral, and that they occupy 



