MARINE ZOOLOGY
both the Ribble and Wyre, and the fresh water carried down by these rivers was, even a week later, floating at the surface of the sea in admixture with normal sea water.
The characteristic marine fauna of the Lancashire inshore waters is the result of these physical conditions—the shallow depths, the extensive sand and mud deposits, the rapid tidal streams and the great rise and fall of the tides, and the somewhat low salinity due to river water. While these conditions produce a fauna which to the marine zoologist is somewhat lacking in variety, and may be described as commonplace, they have at the same time made the Lancashire inshore waters and the foreshore between tide marks one of the most valuable inshore fishing grounds round the British Islands, and one which presents many features of interest.
Shellfish beds are thickly distributed over the whole of the Lancashire coast, and the cockle fishery of Morecambe Bay is without exception the most valuable round the British Islands, while some parts of the coast yield mussel fisheries not much less important. Practically the whole of the northern part of Morecambe Bay consists of cockle-bearing sands. Here and there over this extensive area, and also at the mouth of the Ribble estuary and out from the Mersey along the Lancashire coast from Liverpool to Formby Point, cockle beds are abundantly distributed. The exact positions of these beds are always changing, for the formation of such a shell-fish bed depends on the deposit of the cockle 'spat' or 'seed'—that is the minute free swimming larvæ of the mollusc. During the spring of the year the cockle spawns, and after a week or two the eggs so produced develop into larvæ provided with ciliated swimming organs. These larvæ are borne in the water by the tides and currents, and the place where they settle down depends on the winds, tides, and other conditions. When they do settle down in the sand a cockle bed is produced and sometimes an incredible abundance of these shellfish results, so that the molluscs may actually smother each other. In a few months these shellfish may grow from half an inch in diameter to nearly twice that size and become big enough to be taken by the fishermen. Walking over a cockle bed one does not at first see many signs of the presence of these bivalves, for they are buried in the top layer of the sand with only the tops of their siphons projecting, presenting the appearance of a pair of small dark holes (the 'eyes' of the cockles). Sometimes a tuft of algæ attached to the posterior end of the shell betrays the presence of the mollusc, and the appearance of the 'groats' (a north Lancashire term) also indicates where a cockle lies hidden. The 'groats' are the extruded strings of fæcal matter lying on the surface of the sand. Few people have any idea of the value of this humble mollusc to the Lancashire fishermen, and it will surprise most to learn that from five to ten thousand tons of cockles may be taken annually from the Lancashire foreshores.
The habitat of the mussel is somewhat different from that of the cockle. While the latter mollusc lies buried in the sand and unattached to any substance, the mussel lives above the surface and is attached to stones, etc., by means of its byssus. In almost every case a deep deposit of mud, sometimes several feet thick, may be formed between the layer of mussels and the solid substratum of stones or gravel, etc. (the mussel 'skear'), to which the molluscs are attached, this process being accompanied by the gradual lengthening of the
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