MARINE ANIMALS ON THE ESSEX COAST. 21 



of probably one or two square miles, so that the number may 

 have been something like a million. They were apparently all 

 males probably of the species Nereis dumerilii. Not one was seen 

 the day before or the day after. Some years earlier at Queen- 

 borough, I saw on one evening a similar large number of a some- 

 what different kind of Annelid, larger and coloured much more 

 red by haemoglobin, which were both males and females, and 

 though I most carefully looked for them that year and subse- 

 quently I never saw another. Once before I witnessed a similar 

 display off Sheerness. It thus seems probable that these worms 

 usually live in tubes which they build amongst algae, and just 

 occasionally, for a few hours, swim about on the surface in the 

 Heteronereis condition, probably when scattering the ova or 

 spermatozoa ; the remarkable thing being that apparently all do 

 so at the same time. It is impossible to sa)- what occurs lower 

 down out of sight, and the swimming at the surface probably to 

 some extent depends on the weather being calm. 



I must now describe a series of changes which seem more 

 easy to understand than those already noticed. Until a few 

 years ago the bottom of the Orwell at Pin Mill was almost 

 entirely free from mud, and was to a large extent covered with 

 Sponges, Alcyoniditim, and compound Ascidians. The number 

 of Caprella linearis, a small Terehella {Nicolea zostericola), and the 

 curious worm, Siphonostoma diplochaiios, was astonishingly great ; 

 and a larger number of different species of animals could 

 easily be obtained by dredging than in any other place along the 

 coast. In 1898 and 1899 I however found the bottom covered 

 with a fine tenacious mud, built into short, stout, soft tubes by 

 enormous numbers of the small Amphipod, Jassa pulchella, which 

 had increased so much as to have smothered and almost exter- 

 minated most of the animals previously living at the bottom. 

 There had been little, if any, change in the mudbanks left dry at 

 low water, nor any niarked change in the animals living there. 

 To my surprise I found the bottom clean in 1900, near Tin Mill, 

 though lower down the bottom was more muddy than it used to 

 be. In 1901 the bottom was clean and there was a most extra- 

 ordinary number of small specimens of simple Ascidians, to the 

 almost complete exclusion of other animals. Probably some of 

 these changes have been due to the extensive dredging operations 

 carried on to improve the navigation to Ipswich, and perhaps 

 in a few years the original conditions may be restored. 



