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. ORDER 4. _Læmodipoda_, abdomen rudimental, with only the rudiments of one or two pairs of appendages. ORDER 5. _Isopoda_, body depressed; abdominal appendages flat; mandibles not palpigerous. SECTION II. _Entomostraca._ Shell not solid; legs variable in number; mouth variable. ORDER 6. _Branchiopoda._ Integuments horny, branchiæ feathery, forming part of the feet. It is to this division of the Crustacea that the Cirrhopoda are now referred. ORDER 7. _Pæcilopoda_, mouth suctorial. Sub-order 1. _Xiphosura_, or king-crabs. Sub-order 2. _Siphonostoma_, or fish parasites. III. _The Arachnida_ are defined by Lamarck to be oviparous animals, provided with six or more articulated legs, not subject to metamorphosis, and never acquiring any new kinds of organs. It is now known, however, that some mites undergo a sort of metamorphosis, having only six legs when first hatched, and passing through a quiet pupa stage before acquiring their perfect form. Their respiration is either by means of air-sacks, which serve for lungs, or of a kind of tube with circular openings for the admission of air. There is a rudimentary heart and circulation in most of the species. There are two orders; those with lungs, and those without.