ANATOMY AND CLASSIFICATION OF THE ARENIC0LIDJ5. 609 



means of the characters of the nephridia, the first section con- 

 taining A. marina, A. Claparedii, and A. cristata, the 

 second A. ecaudata and A. Grnbii. 



I. The nephridia of the "marina" section agree thi'ough- 

 out. Each consists of a funnel or nephrostome, a secreting 

 portion, and a posterior terminal vesicle or bladder (PI. 24, 

 fig. 29). The funnel is always bright red in colour, owing to 

 its rich vascular supply. Its narrow slit-like aperture, which 

 is generally directed forwards and inwards, is bordered by 

 an entire plain ventral lip, and a somewhat large dorsal 

 lip fringed by ciliated vascular processes which are placed 

 moderately close together on the edge of the lip. The pro- 

 cesses are flattened, spatulate, or triangular structures fixed by 

 their narrower end to the lip of the nephridium; their flat 

 faces are turned towards each other. Each process, which 

 may be branched distally into two, three, or four, contains a 

 large blind diverticulum of the blood-vessel which traverses 

 the edge of the dorsal lip ; hence the processes are always 

 bright red. The free edge of the ventral lip is almost serai- 

 circular. The edges of both dorsal and ventral lips are 

 richly ciliated, and there are also very numerous long cilia 

 within the mouth of the funnel, the motion of which is very 

 obvious in fresh nephridia examined on a slide in sea water. 

 The secreting portion of the nephridium is a moderately thin- 

 walled sac, usually dark brown, sometimes almost black in 

 colour, owing to the presence of enormous numbers of brown 

 excretory granules in the cells lining the sac. The secreting 

 portion of the nephridium tapers posteriorly and opens into 

 the bladder, which is also usually brownish in colour. When 

 expanded the bladder is more or less spherical, but in the 

 contracted condition it is rosette-like. Each bladder opens 

 to the exterior by a small oval aperture just above and 

 behind the dorsal portion of a neuropodium. 



Immediately behind the posterior portion of the nephro- 

 stome there is a small whitish or pinkish, ovoid, club- 

 shaped, or cylindrical mass of cells. These masses of cells 

 are the reproductive organs of the worm, ovaries or testes, as 



