IIS" THE DISTRICT OF THE THAMES ESTUAET. 437 



about and voided eggs, wliieli I mounted on a lantern-slide, and 

 carefully estimated their numbers at about 20,000. It bad not 

 passed into tbe Heteronereis condition ; but I bave mounted 

 specimens of smaller size obtained elsewbere wbicb are in that 

 state and yet are full of eggs. Tbe female, 3 inches long, wbicb 

 laid the eggs was mounted in balsam, and in general appearance 

 differs remarkably from those full of partially developed eggs. 



Another peculiarity of iV. Dumerilii is that the large specimens 

 from the Orwell, when mounted in balsam, turn from a pale red 

 to a dark brown, even when the blood-vessels are well preserved, 

 so that it is not due to decomposition, but as though some 

 unusual pale substance changed gradually into a dark brown one. 

 In no other species of Chaetopod bave I seen such a change. 



The only other Ch^etopod that I have seen swimming in con- 

 siderable numbers near the surface is Siphonostoma {FlahelUgera) 

 affine (M. Sars). In several years, before and after 1890, this 

 was so abundant in tbe Orwell, that many objects dredged up 

 were covered by scores of individuals. I have never seen 

 any elsewhere or in the Orwell since. Many left the bottom 

 and came near the surface, where they wriggled about and were 

 carried along by the tidal current, and this continued day after 

 day. Many were full of eggs, which they voided when kept alive, 

 and my impression was that when swimming they were dispersing 

 their ova. This case is interesting because it difiers so completely 

 from what I have observed in tbe case of Heteronereis, and so 

 easy to understand. 



N. Gultrifera. — Baron de Saint-Joseph, in his work on the 

 PolycJicsta of the coast of Erance, describes fully tbe characters 

 of the male and female Heteronereis oi this species, but does not 

 allude to the Nereis form so common in South Britain. This is 

 remarkable because, though during the whole of the summer 

 months I have collected large numbers on the coast of Essex, I 

 never saw one in the Heteronereis condition. I may also say 

 that none of my numerous mounted specimens contain ova. 

 It is of course possible that their development and the change to 

 Heteronereis may occur at a time of the year when I was not on the 

 yacht, or occurs very rarely. Dr. Allen informs me tbat it does 

 occur at Plymouth, but in much smaller numbers than the 

 Nereis. * 



N. pelagica. — I have never obtained a single specimen of the 

 Heteronereis, which occurs in some localities abundantly between 

 tide-marks. 



