Malacostraca

The orders composing the Malacostraca may be grouped in the following five Divisions:— I. Peracarida. The Mysidacea comprise shrimp-like swim ming forms, nearly all marine. Thermosbaenacea includes only the minute, blind, creeping Thermosbaena, found in a hot spring in Tunis. Cumacea are marine mud-burrowers but the males are, to some extent, free-swimming. Tanaidacea are also part of the micro-fauna of the sea-bottom, less natatory than the Cumacea. Isopoda are a very varied and successful group, creeping, mud burrowing and sometimes actively swimming in the sea and a few in fresh-water. Parasitism appears in many different families and leads to extremes of specialization and degeneration. One sub-order, Oniscoidea, consists of the terrestrial, air-breathing, Woodlice. Amphipoda, another varied and successful group, con trasting with the Isopoda in that its multitude of species have only a limited range of morphological differentiation. Creeping and swimming forms abound in fresh-water and in the sea, a few sandhoppers become completely terrestrial, one sub-order (Hy periidea) is planktonic and another (Cyamidae or Whale Lice) exclusively parasitic.

2. Phyllocarida. The single order, Nebaliacea, includes only three or four living genera, all marine mud-burrowers. Their doubtful affinities are mentioned above.

3. Syncarida. The Mountain Shrimp (Anaspides) of Tasmania and a few allies in the Australian region are fresh-water animals and the Bathynellidae of Cen tral Europe are minute, blind, degenerate, subterranean forms. They appear to be survivors of a group widely distributed in Carboniferous and Permian times.

4. Eucarida.

The Euphausia cea are a little group of planktonic, phosphorescent shrimps which, perhaps, do not deserve to be separated as a distinct order. The Decapoda, on the other hand, form the most extensive and diversified of all the orders of Crustacea. It includes the largest representatives of the class and, indeed, of living Arthropoda, and, since many of them are used for food, they are more generally familiar and more thoroughly studied than any other Crustacea. The most usually accepted classification recognizes two sub-orders :—Natantia comprising the Prawns and Shrimps, and Reptantia the Lobsters and Crayfishes, the Hermit-crabs and their allies, and the true Crabs (Brachyura). The headquarters of the group is in the sea but many of its members have invaded fresh waters (river prawns and river crabs in the tropics and crayfishes in temperate regions) and although a few are so far terrestrial as to deserve the name of land crabs, anct may even climb lofty trees like the Coconut Crab, they pass their young stages in salt or fresh water.

5. Hoplocarida. The members of the single order Stomatopoda are sometimes known as "Mantis Shrimps" from the resemblance of their large prehensile claws to those of the Mantis insect. They are exclusively marine, burrowing in sand or lurking in crevices in the shallow waters of all the warmer seas.

See also CRAB; CRAYFISH ; LOBSTER; PRAWN ; SHRIMP; WOOD LOUSE; for definitions of terminology, see ZOOLOGY.

(W.

T. C.)