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It is quite impossible, in an article of this nature, to treat the invertebrate fauna of the sea fringing the coast of Lancashire in anything like detail. An immense amount of investigation has been carried out during the last twenty years, and the fauna and flora of the Irish Sea have now been investigated more completely than most other similar areas of the British seas, the Firth of Forth and St. Andrew's Bay in Scotland and the English Channel being excepted. There are now two biological stations in the northern part of the Irish Sea—one at Piel in the Barrow Channel, and the other at Port Erin in the Isle of Man. Four distinct organizations—the Lancashire Sea Fisheries Committee, the Liverpool Biological Society, the Liverpool Marine Biology Committee, and the Southport Society of Natural Science—are now in existence and are still investigating Lancashire waters. The marine zoology of this area has therefore received and is still receiving very considerable attention.

Physically the Irish Sea is for the most part a shallow water basin. The North Channel which connects it with the Atlantic and the Firth of Clyde is, in places, of considerable depth (over 140 fathoms), and on the south St. George's Channel varies from 40 to about 90 fathoms. To the westward of the Isle of Man there is a deep depression in which depths of 50 to 80 fathoms may be found. With these exceptions the greater extent of the Irish Sea area is comparatively shallow. The southern entrance is wide, but the northern inlet is very restricted, and to this cause is due the peculiar conditions of the tides. The tidal wave coming in from the Atlantic impinges obliquely on the south-west coast of Ireland, and there splits up into three main streams. One of these passes up the English Channel and enters the North Sea through the Straits of Dover, but, becoming reflected from this narrow outlet, sets up very peculiar tidal phenomena. Another main stream passes up the Bristol Channel, producing the high tides in the Severn. The remaining stream passes up through St. George's Channel into the Irish Sea. Continuing on, the Atlantic tidal crest passes round the north of Scotland, entering the North Sea, but a part of it also runs down the North Channel, and so enters the Irish Sea from the north. Thus there are two main tidal streams entering the latter basin from different directions, but in consequence of the much wider southern channel, more water enters the Irish Sea from the south than from the north. There is therefore a very evident surface drift of the water from south to north, helped no doubt by the prevailing west to south winds.

These two tidal streams meet in a straight line drawn from the north of the Isle of Man across to Morecambe Bay, and from the Isle of Man to the Irish coast. Between the Irish and Manx coasts there is a large area where tidal streams practically do not exist, and where the water simply rises and falls. All along the east Irish coast the velocity of the stream is small, but 87