The chief oyster fisheries in Britain are at Whitstable, Colchester and Brightlingsea. Nearly 40,000,000 oysters were gathered on the coasts of England and Wales in 1920, and were sold for about £250,000. Perhaps the next most important edible bi-valve is the mussel. Frequently, mussel beds are situated near the mouth of rivers, and consequently tend to be contaminated by sewage. It has been established by various investigators—notably Dr. Klein and Professor James Johnstone—that mussels are able to cleanse themselves of sewage pollution in a comparatively short time if they are re-laid in sterilized water. Experiments on a large scale have been carried out with the mussel beds at the mouth of the Conway river since September, 1916. The mussels are gathered from the beds and placed about two deep on wooden grids in a large concrete cleansing tank of 40,000 gallons capacity. The mussels are first thoroughly hosed with water at high pressure to remove all adherent mud, etc. The tank is then filled with sterilized sea-water and the mussels are allowed to remain in it for 24 hours. During this period the mussels effectually free themselves from bacteria. The tank is then emptied, the mussels are hosed again, the tank is again filled with sterile water and after a further 24 hours is emptied. The mussels are once more flushed with the hose. After this treatment the mussels reach a high standard of purity. The scheme has proved to be a complete success, not only from a scientific point of view, but also as a commercial proposition. The sum of 1s. per bag of mussels (140 lbs.) is charged to the fishermen for this treatment. CHAPTER VIII FISHERIES FOR WHALES Whales are the most important members of a large family of land animals including also the seals, walrus, and porpoise, that have gradually become adapted to live in the sea. They have acquired an externally fish-like form, but in every other respect they retain the characteristic features of mammalian structure. They are warm-blooded, air-breathing quadrupeds, that suckle their young. In the whale, the fore-limbs have become simple five-fingered flippers, while only isolated, vestigial bones of the hind-limbs remain buried uselessly in the body. Unlike fishes, the tail is set horizontally, thus enabling the creature to rise easily to the surface to breathe. The warm-blooded body is kept warm by a layer of fat placed immediately beneath the skin, and varying in thickness from 8 to 20 ins., and known as the blubber. The nostrils, instead of being situated at the end of the snout, are placed far back at the apex of the head to form the blowhole. Whales are divided into two well-marked groups, known as the whaleboned and the toothed whales respectively, according to the particular form of their dentition.