THE ANATOMY. PRONEPHRIDIA 33 



two or three segments. The latter fact is explained by Vejdovsky as due to the late 

 appearance of the septum, which thus allows the nephridium to grow backwai-ds. As to 

 the first point, the apertures in question, though dorsal in Lumbricus, are ventral in 

 Rhynchdmis. There can be no doubt of the homology of the first pair of embryonal 

 pronephridia in the two forms, and so the different position becomes a matter of sub- 

 ordinate interest. Although the pronephridia of the first segment disappear in Luvi- 

 bricus, this is not universally the case ; I found that in Octochaetus viultiporus the first 

 pair of nephridia persisted and fusing with the next pair became the ' peptonephridia ' 

 opening into the buccal cavity (see p. 46). The persistence of these nephridia in 

 Octochaetus is of course an additional argument in favour of regarding the first pair 

 of these organs in Lwmhricus and Rhynchdmis as equivalent to the pairs which follow 

 and are converted into permanent nephridia. 



The following pairs of pronephridia in both Rhynchdmis and Luvnbricus are 

 short straight rows of cells without a lumen, but ending anteriorly in the case of 

 Rhynchdmis, but not of Luvibricus, in a ' flame-cell ' provided with a long flagellum 

 pointing backwards along the inside of the organ. These nephridia exist in the anterior 

 segments of the body from which in the adult Rhynchelmis they subsequently disappear. 

 In the Lumbricidae the pronephridia arise from a continuous string of cells (not proved 

 in RhyTichelmis) ; this was first discovered by Wilson, it having been previously 

 found by Whitman to hold good for the leech Clepsine. Wilson's results are so 

 far confirmed by Vejdovsky (9). This string originates from a single large cell, the 

 nephridioblast ; the row of cells is called by Vejdovsky the ' nephridiostich ' ; traced 

 forward this row is seen to break up into oblique rows of cells each surmounted by 

 a larger one which is on the ventral side of the body. The large terminal cell of each 

 is the cell from which the funnel will ultimately be formed ; but it never shows the 

 vacuole with the contained flagellum which is to be seen in the corresponding stage of 

 Rhynchelm.is. 



From these pronephridia the permanent nephridia are developed ; but before de- 

 scribing the way in which this development takes place it will be convenient to cast 

 a glance over the general anatomy of the permanent nephridia in the Oligochaeta. 



§ 2. Nephridia of adult. 



Nephridia exist in all Oligochaeta. The only exception which there are good grounds 

 to believe is really an exception in the Naid Uncinais littoralis ; Bouene, who 

 carefully studied this species (5), was unable, after repeated observation, to discover any 

 trace of nephridia. Another peculiar condition of the nephridia has also been described 

 by Benham, and noticed by others, in certain Naids : in a few forms the nephridia are 



F 



