Cod are mostly captured by means of long lines, each about forty fathoms in length, to which a number of smaller lines are fastened at intervals. The hooks are placed on the side lines, and are generally baited with whelks, and then the long lines, or trawls, as the fishermen call them, are anchored in shallow parts of the sea where codfishes, halibut, and the like abound. Each boat carries about eight miles of these lines, with nearly five thousand hooks, so that the work of baiting, lowering, and raising them is very heavy indeed. The fishing takes place in the winter, and the boats are generally out in all weathers for several months at a time. One would think that with so many boats engaged in cod-fishing, each with so many miles of line, nearly all the cod in the sea would soon be caught. But to offset this, a single cod in a single year will often lay eight or nine million eggs, so that notwithstanding the immense number of these fishes which are taken, they still seem as plentiful as ever. FLATFISH The so-called flatfishes, such as the sole, the plaice, the flounder, and the dab, form an interesting group. Although we call them "flat," we ought really to call them "thin," because what we always consider as the back of a sole is really one of its sides, and what seems to be the lower surface is the other side. The explanation is this: when these fishes are quite small, they swim upright in the water, just as other fishes do, and drive themselves along by means of their tails. But when they are about a month old a strong desire comes over them to go and lie down on the mud at the bottom of the sea, and then three remarkable things happen. First their color changes. Up till now, both sides of their bodies have been nearly white. But if a white fish were to lie down on dark-brown mud, of course it would very easily be seen, and most likely would very soon be devoured by one of its many enemies. So as soon as the little fish lies down at the bottom of the water its upper surface begins to grow darker, and before very long it exactly resembles the hue of the surrounding mud. Or if the fish should lie upon sand, as the plaice does, then its upper surface becomes colored like the sand. So as long as it keeps still its enemies may pass quite close to it without noticing it. The next thing that happens is that the little fish changes its way of swimming. Hitherto it has driven itself through the water by means of its tail; now it uses what were formerly its upper and lower fins, but have now been turned into side fins. And by a very graceful waving movement of these fins it winds its way, as it were, through the water.