HABITS AND STRUOTURbl OF AKENICOLA MARINA. 27 



of the otocyst where the movement has been noticed, but we 

 are of the opinion that the quivering motion of the otoliths is 

 not a normal phenomenon, but is due to diffusion currents. 



9. Nephridia. 



There are six pairs of nephridia, belonging to somites 4 to 9. 

 Of these the first pair seems to be unrepresented in any other 

 species of Arenicola, and its variation in A. marina points 

 clearly to a gradual degeneration which it appears to be under- 

 going at the present time. It is not only the smallest of the 

 series, but is sometimes represented merely by a funnel or by 

 the secretory and terminal portions. Very rarely both the first 

 nephridia are mere funnels, and again one may be fully 

 developed and the other rudimentary, but they are never abso- 

 lutely wanting. Their small funnels, which are of a bright 

 pink colour, are placed on the anterior face of the third 

 diaphragm with the long axes vertical (PL 2, figs. 13 and 14). 

 One lip (the outer) is produced into processes corresponding to 

 the dorsal lip of the other nephridia. The secretory portion is 

 elongated, narrow, and usually brownish in colour, and the 

 terminal portion opens just above the fourth neuropodium 

 (PI. 1, fig. 1) at a decidedly lower level than is the case in the 

 succeeding nephridiopores. 



The remaining five pairs are always in adults fully de- 

 veloped. They are attached to the body- wall partly by connec- 

 tive tissue, partly by the broad bands of oblique muscle 

 which obscure them at first sight (PI. 2, fig. 5). The nephro- 

 stomes are very long, and bent upon the rest of the organ. 

 The narrow slit-like aperture has a dorsal vascular lip bearing 

 finger-shaped or spatulate ciliated processes, and an entire 

 ventral one. The cilia just within the mouth of the funnel 

 are exceedingly long, and produce a current tending to carry 

 coelomic fluid and corpuscles into the cavity of the organ. 

 The middle or secreting portion is brownish (in old worms 

 almost black), owing to the excretory granules which are 

 formed in its cells. The terminal rosette-shaped bladder, which 

 is slightly lighter in colour, opens by a minute slit-like aper- 



