218 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



cells. The appreciation of this difference gave rise, not un- 

 naturally, to a suspicion — which would now appear to be 

 unfounded — that under Lankester's name nephridium were included 

 excretory tubes of two morphologically distinct types and the use of 

 the word nephridium was often restricted to the one of these types 

 in which the coelomic funnel was present. 



Later researches brought out the fact that in some cases — certain 

 Polychaete worms — the excretory tube may possess both flame-cells 

 and coelomic funnel. And finally the hypothesis was developed — by 

 Meyer and especially Goodrich (1895) — that the nephridial tube 

 and the coelomic funnel were originally quite distinct organs with 

 separate openings to the exterior. On this view the primitive 

 excretory tube or protonephridium (Goodrich) was provided with 

 flame-cells at its inner end, while apart altogether from it and opening 

 independently to the exterior was the coelomic funnel which formed 

 the primitive exit for the reproductive cells. In the course of 

 evolution there came about a fusion of the two structures, the 

 coelomic funnel becoming as it were grafted on to the nephridium 

 and in many cases shifted up the wall of the tubule right to its 

 inner end. Such a compound organ (Nephromixium — Goodrich) 

 might retain for a time both flame-cells and coelomic funnel — as in 

 the Polychaetes alluded to above — or the flame -cells might, as is 

 more usual, disappear leaving an excretory tube possessing at its 

 inner end a coelomic funnel which shows no trace of its morpho- 

 logically independent origin. To support the hypothesis which has 

 just been outlined there is brought in the evidence of embryology 

 which testifies (see Vol. I. p. 158) that the main' part of the excretory 

 tube is developed as an ingrowth of the ectoderm, while the coelomic 

 funnel arises as an outgrowth of the mesoderm. 



This hypothesis has met with very general acceptance not 

 merely with regard to the excretory organs of Annelids alone but 

 also as a theory of the morphology of excretory tubes in general. 

 As, however, the writer of this volume takes up a somewhat different 

 standpoint it will now be necessary to state shortly what that 

 standpoint is. 



The word nephridium will be used in the original sense as 

 meaning an excretory tube whether possessing flame -cells or a 

 coelomic funnel at its inner end. 



Physiologically the open funnel and the flame-cell appear to be 

 associated primarily with two different sets of spaces. The funnel 

 is associated with coelomic spaces and it serves to transmit to the 

 exterior the products of the lining of such spaces — fluid, excretory, 

 or reproductive. The flame-cell is associated rather with the meshes 

 of the mesenchymatous spongework: it serves to filter off from 

 these spaces watery fluid containing excretory salts in solution. 

 The activity of the " flame " is in direct relation to the pressure of 

 fluid within these spaces : if the pressure is lowered by making a 

 minute puncture in the body-wall the movement at once ceases — to 



