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.