Within the last thirty years the Crayfish fisheries of Western Europe have suffered heavily from outbreaks of an epidemic disease which has all but exterminated these animals in certain districts. In this country it is said to be responsible for the almost complete disappearance of Crayfish from localities where they were formerly plentiful, as, for instance, in the neighbourhood of Oxford. The cause of the disease is believed to be a protozoan parasite belonging to the group Myxosporidia.

In other parts of the world it does not seem that the fresh-water Crayfishes are of much importance as an article of food. Some species of Cambarus are so used to a limited extent in the United States, and the gigantic Astacopsis serratus ([Plate XX].) is known as the "Murray River Lobster" in the markets of Sydney and Melbourne.

Fig. 78—The Common Shrimp (Crangon vulgaris). Natural Size

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The Decapods of the suborder Natantia comprise a large number of edible species, generally known as Shrimps and Prawns. The Common Shrimp, Crangon vulgaris ([Fig. 78]), which is plentiful on the British coasts wherever the bottom is sandy, is about two or three inches long, and when alive is of a translucent greyish colour speckled with brown. It differs from most of the Natantia in having the body somewhat flattened from above downwards, and the rostrum very short. When boiled, it is of a reddish-brown colour, and from this it is sometimes known as the "Brown Shrimp." On many parts of the coast the Shrimp fishery is of considerable importance. Most often the Shrimps are caught by means of a large bag-net attached to a semicircular hoop with a long handle, and pushed over the surface of the sand by a fisherman wading in the water at ebb-tide.

A variety of species are sold in England under the name of Prawns. The largest of the native species, to which the name of Common Prawn is perhaps most properly restricted, is Leander serratus. It grows to a length of over 4 inches, and has a long serrated rostrum extending beyond the antennal scales and curving upwards at the point. The first and second pairs of legs end in small pincer-claws. When alive the animal is very transparent, and beautifully marked with bands of brown and red on the body and limbs. A smaller species of the same genus (L. squilla), distinguished by the much shorter and straighter rostrum, and another very similar species of which the proper name appears to be L. adspersus (often known as L. fabricii), are said to be sold on some parts of the English coast as "Cup Shrimps."

Much commoner, at least in the London market, than the species of Leander is Pandalus montagui, often sold under the general name of Prawn, but sometimes called the "Pink Shrimp." This resembles Leander serratus in having a long, serrated, up-curved rostrum, but differs from it strikingly in the form of the anterior pairs of feet. The first pair appear to the naked eye to have no pincer-claws, but to end in a sharp point, resembling the third maxillipeds, which are just in front of them. As a matter of fact, they do have pincers, but so minute that they can only be detected by microscopic examination. The feet of the second pair are unequal in length on the two sides, that on the left side being the longer, and are very slender. They end in small pincers, and examination with a pocket-lens will show that the carpus, or "wrist," and the segment below it (merus) are broken up into a large number of short segments, so that the limb is extremely flexible. When alive, the animal is even more handsomely marked than the Common Prawn.