MALACOSTRACA, the largest sub-class of the Crustacea (q.v.), including the lobsters, crawfish, crabs, shrimps, prawns, beach-fleas, sow-bugs and various other crustaceans. They may be briefly defined as Crustacea having the body composed of nine teen somites, all, typically, bearing appendages, the trunk-limbs differentiated into two series, a thoracic of eight and an abdominal of six pairs ; and the genital openings of the female on the sixth, those of the male on the eighth thoracic somite.
A study of the comparative morphology of the Malacostraca permits us to draw up a scheme for the probable course of evolu tion of the group which is, at least, not contradicted by our scanty knowledge of its fossil representatives. According to this scheme, the earliest Malacostraca exhibited what has been called the "caridoid facies"; that is to say, they were shrimp-like in general form, with a carapace enveloping, but not coalesced with, the thoracic somites, with stalked eyes, biramous antennules, and a scale-like exopodite on the antenna, with the thoracic limbs forming walking-legs with swimming exopodites and branchial epi podites and with a tendency for one or more of the anterior pairs to be assimilated to the mouth-parts as maxillipeds ; with the abdominal appendages forming biramous swimmerets, except the last pair which are large, lamellar, and form with the telson a "tail-fan." The earliest fossils that can be definitely referred to the Mala costraca occur in the Carboniferous rocks and present, with little modification, the caridoid facies described above. Some of them (Pygocephalus) have a brood-pouch formed by overlapping plates (oostegites) from the bases of the thoracic legs and appear, there fore, to be referable, to the Mysidacea. From the caridoid Mysi dacea a series can be traced in which the carapace is progressively reduced, the thoracic exopodites are lost and the eyes become sessile. Although palaeontology gives no help, the steps of this series are indicated by the specialized offshoots which have given the Cumacea, Thermosbaenacea, Tanaidacea and Isopoda. The
Amphipoda belong to the same series but their precise place in it is less clear. The other orders of Malacostraca have no brood pouch and appear to have diverged very early from the primitive stock. Already in Carboniferous times the Syncarida had lost the carapace and had much the same general structure as the recent Anaspides and its allies. Another series in which the cara pace coalesced with the thoracic somites gave rise to the great group of the Decapoda, from which the Euphausiacea are perhaps an offshoot. The Decapoda, beginning with caridoid forms, have, in several independent lines, assumed the crab-like or "carcinoid facies" by reduction of the abdominal region (Brachyura or true crabs, and crab-like Paguridea, Hippidea and Galatheidea). _ The Stomatopoda had assumed nearly their typical structure in Juras sic times but their earlier history is unknown.
In the scheme of phylogeny thus outlined no mention has been made of the Phyllocarida, which, in most systems of classification, are ranked as the most primitive Malacostraca, forming a link with the Branchiopoda. They differ from the other Malacostraca in having an additional somite in the abdomen, the telson termi nating in a "caudal fork," and a bivalve carapace with an adductor muscle. The thoracic limbs are more or less flattened and leaf-like and have a general resemblance to the trunk-limbs of the Branchiopoda though a close comparison is difficult. The existing genera, Nebalia and its allies, are believed to show affinity with the fossil Ceratiocaridae, the earliest of which appear in Cambrian rocks and are thus vastly more ancient than any other Malacostraca. Recent work, however, tends to diminish the signifi cance of the differences between Phyllocarida and other Mala costraca, and, in particular, the recognition of a vestigial seventh abdominal somite in certain primitive Mysidacea suggests that the Phyllocarida may, after all, be more closely related to the Peracaridan series than has been supposed.