Wood louse; pill bug

Isopods are typically flattened dorso-ventrally, although many species deviate from this plan, particularly those from the deep sea or from ground water. Isopods lack an obvious carapace, which is reduced to a "cephalic shield" covering only the head. Gas exchange is carried out by specialised gill-like pleopods towards the rear of the animal's body. In terrestrial isopods, these are often adapted into structures which resemble lungs, and these "lungs" are readily visible on the underside of a woodlouse. Eyes, when present, are always sessile, never on stalks. They share with the Tanaidacea the fusion of the last abdominal body segment with the telson, forming a "pleotelson", and the first body segment of the thorax is fused to the head. The pereiopods are uniramous, but the pleopods are biramous.