In this order the head is always distinct from the segment bearing the first pair of feet. The respiratory organs are not thoracic, as in the two preceding orders, but are attached to the inferior surface of the abdomen, and consist of branchiae, which in the terrestrial species are protected by plates which fold over them. The thorax is composed of seven segments, bearing seven pairs of limbs, which, in the females, have marginal plates, attached to their bases, and serving to protect the ova. The number of segments in the abdomen varies, but is never more than seven. The abdominal segments are coalescent, and form a broad caudal shield, beneath which the branchiae are carried. The eyes are two in number, formed of a collection of simple eyes, or sometimes truly compound. The heart is sometimes an elongated tube, with three pairs of fissures (as in the Amphipoda), sometimes short or spherical, removed towards the abdomen, and with more or fewer fissures than the above. The young Isopod is developed within a larval membrane, destitute of appendages. After a time this membrane bursts, and liberates the young, which resembles the adult in most respects, but possesses only six instead of seven pairs of limbs. Of the members of this order, many are aquatic in their habits, and are often parasitic, but others are terrestrial.
By Milne-Edwards the Isopoda are divided into three sections, termed respectively, from their habits, the Natatorial, Sedentary, and Cursorial Isopods. In the Natatorial Isopoda the extremity of the abdomen and the last pair of abdominal legs are expanded so as to form a swimming-tail. Some of this section are parasitic upon various fishes (Cymothoa), whilst others are found in the sea (Sphaeroma). In the Sedentary Isopoda the animals are all parasitic, with short, incurved, hooked feet. This section includes the single family of the Bopyridae, all the species of which live parasitically either in the gill-chambers, or attached to the ventral surface, of certain of the Decapod Crustacea, such as the Shrimps, (Crangones) and the Palaemones.
The Cursorial, or running Isopods mostly live upon the land, and are therefore destitute of swimming-feet. The most familiar examples of this section are the common Wood-lice (Oniscus). Here, also, belongs the little Limnoria terebrans, so well known for the destruction which it produces by boring into the wood-work of piers and other structures placed in the sea. Other well-known Isopods are the Water-slaters (Asellus) of fresh waters, the Rock-slaters (Ligia) of almost all coasts, the Box-slaters (Idothea), the Shield-slaters (Cassidina), and the Cheliferous Slaters (Tanais). These last are remarkable as being the only Isopods in which there is a carapace. The lateral parts of the carapace, also, are highly vascular, and respiration is effected by these, and not by the abdominal feet.
Many Isopods undergo an extensive metamorphosis. "In some Fish-lice (Cymothoa) the young are lively swimmers, and the adults are stiff, heavy, stupid fellows, whose short clinging feet are capable of little movement." In the Bopyridae the adult females are usually blind, the antennae are rudimentary, and the abdominal appendages from natatory become respiratory organs. The males, on the other hand, are dwarfed, and sometimes lose all the abdominal appendages and all traces of segmentation; until we get forms which, like Cryptoniscus planarioides, "would be regarded as a Flat-worm rather than an Isopod, if its eggs and young did not betray its Crustacean nature " (Fritz Muller).
The members of this division have compound eyes supported upon movable stalks or peduncles, and the body is always protected by a cephalo-thoracic carapace. Most of the Podophthalma pass through Zoea-stages in their development. It comprises the two orders Stomapoda and Decapoda, of which the latter includes all the highest and most familiar examples of the class Crustacea.
Fig. 155. - Isopoda. Woodlice (Oniscus), twice the natural size.