II. The Crustacea comprise the shell-fish commonly called crabs, lobsters, shrimps, and prawns. They have a distinct head, furnished with antennæ, eyes, and mouth; and their bodies are covered with a crust or shell, divided into segments by transverse incisions, the segments being united by a strong membrane. Once a year the larger species of these animals moult, throwing off their old crust or shell, and forming a new one, the animal remaining in a naked and greatly weakened state during the intermediate time. Many of the Crustacea swim with great ease, but on land their motions are generally cramped and awkward; and they are confined to crawling, or leaping by means of the tail. When a limb is injured they possess the extraordinary power of throwing it off, and forming a new one. The Crustacea lay eggs, and the young of some of the species undergo a transformation before they attain their full size. The Crustacea were divided into two sections and seven orders by Latreille, which are as follow:—
Section I. Malacostraca.
Shell solid, legs ten or fourteen, foot-jaws six or ten, mandibles two, maxillæ four; mouth with a labrum.
Sub-section I. Podophthalma, eyes on foot-stalks.
Order 1. Decapoda, legs ten.
Sub-order 1. Brachyura, the crabs.
Sub-order 2. Macroura, the lobsters.
Order 2. Stomapoda, legs more than ten.
Sub-section II. Edriophthalma, eyes not on foot-stalks.
Order 3. Amphipoda, body compressed; mandibles palpigerous.