=Geological history.=--The known history of the order is a long one, for remains occur in the Old Red Sandstone (Devonian) of Herefordshire, and in the Coal Measures. (79)[2]. A form which has been named _Archæoniscus brodiei_, and is said to be referable to the recent family Aegidae which is found in some numbers in the Purbeck Beds (Upper Jurassic), of this country (47). Fossil Isopods have also been recorded from the Oolite and from the Oligocene (Isle of Wight). Turning to the Woodlice proper, we find that they first make their appearance in the Miocene (of Oenigen and Baden), and occur also in amber (79); while examples of genera, such as _Oniscus_ and _Porcellio_, have been discovered in late Tertiary deposits (47). =External structure and appendages.=--Woodlice agree in being of a somewhat oval form, and their bodies are arched, the curve varying in different genera and species. A _head_ is to be distinguished; behind this comes the _thorax_ of seven segments which are often considerably broader than the six succeeding ones which form the _abdomen_ (see fig. 1.) [Illustration: FIG. 1.--PARTS OF THE BODY. (_Oniscus asellus._)] The head carries two _large antennae_ (fig. 3) which are very evident, and a careful search with a lens will reveal a second and minute pair (the _smaller antennae_) situated between the base of the others, and really anterior to them. (figs. 2 and 4.) [Illustration: FIG. 2.--THE FIRST ANTENNA. (_Oniscus asellus._)] The larger antennae are customarily bent at certain points, and we can distinguish a terminal part, or _flagellum_, and a basal part, the _peduncle_ (fig. 3). The number of joints in these structures, which varies in different genera and species, forms a useful classificatory character, and the relative length of the component parts is of considerable value in distinguishing species. [Illustration: FIG. 3.--THE SECOND ANTENNA. (_Oniscus asellus._)] There are four pairs of mouth appendages--namely the jaws or _mandibles_ (fig. 5), the _first maxillae_ (fig. 6), the _second maxilla_ (fig. 7), and the _maxillipeds_ (fig. 8). When the head is examined from the underside the last of these organs will be seen first, covering in the others. [Illustration: FIG. 4.--THE UNDERSIDE OF THE HEAD. (_Oniscus asellus._)] A small median plate attached to the front of the head has been called "_the upper lip_" (fig. 9), while inside the mouth appendages is a little bilobed structure "_the lower lip_" (fig. 10). [Illustration: FIG. 5.--THE MANDIBLES. (_Oniscus asellus._)] Before leaving the external features of the head, we must allude to the pair of _eyes_ which are usually present, though never raised on stalks. In the Common Woodlouse (_Oniscus asellus_, from which all our figures to illustrate structure have been made), as in many other species, the eyes are compound (fig. 4), but in some forms these are simple. [Illustration: FIG. 6.--THE FIRST MAXILLAE. (_Oniscus asellus._)]