Fig. 20—A Woodlouse (Porcellio scaber), One of the Isopoda. Enlarged. (From Lankester's "Treatise on Zoology," after Sars.)

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The third Order, that of the Tanaidacea ([Fig. 19]), is of special interest, since in many respects it forms a transition to the next. It comprises a number of minute Crustacea, generally found burrowing in mud in the sea. They have a small carapace, which only involves the first two thoracic somites, the rest of the somites being distinct. The side-folds of the carapace enclose a pair of small cavities, within which lie, as in the case of the last two orders, the epipodites of the maxillipeds. The eyes are not movable, although they are set on little side-lobes of the head, representing the vestiges of eye-stalks. The first pair of thoracic limbs are maxillipeds, and the second pair are very large, and form pincer-claws (chelæ). Minute vestiges of exopodites are sometimes found on the second and third pairs, but they are not used for swimming, and only help to keep a current of water flowing through the gill cavities. The abdomen is very short, with small swimmerets, and the telson is not separated from the last somite. The uropods are generally very small, and do not form a tail-fan.

Unlike the Tanaidacea, the Isopoda, which form the fourth order of the Peracarida, are very numerous in species, and very varied in structure and habits. The most familiar are the Woodlice, or Slaters, which are commonly found in damp places, under stones and the like. Besides these, however, the order includes a vast number of forms living in the sea and a few that live in fresh water. The examination of a common Woodlouse, such as Oniscus or Porcellio ([Fig. 20]), will give a general idea of the form and structure of a typical Isopod, although many curious modifications are found, some of which will be mentioned in later chapters.

There is no distinct carapace, but the last vestige of one may be indicated by the fact that the first thoracic somite is completely fused with the head. All the other somites of the body are distinct (in some Isopods, however, the abdominal somites are coalesced), but the telson is not separate from the last somite. The eyes are not stalked, but are sessile on the sides of the head. The antennules have only a single branch, and in the Woodlice are very small. The antennæ have no exopodite, although in a few other Isopods a minute vestige is present. The thoracic limbs never have any trace of exopodites. The first pair are maxillipeds, and if they carry an epipodite it is never enclosed in a gill cavity, as in Tanaidacea. The swimmerets form one of the most characteristic features of the Isopoda, for they are always flattened into thin plates, which act as gills. In the Woodlice, which breathe air, certain curious modifications of the swimmerets are found, which will be described in a later chapter. In some Isopods that live as parasites on fish or on other Crustacea, each individual is at first a male, and later becomes a female. They are almost the only Crustacea, except the Cirripedes already mentioned, which are normally hermaphrodite.

Fig. 21—An Amphipod (Gammarus locusta). Enlarged. (From Lankester's "Treatise on Zoology," after Sars.)

a′, Antennule; a″, antenna; acc, accessory (inner) flagellum of antennule; br, gill plate; cx, coxal plate (the expanded first segment of the leg); gn, the two pairs of gnathopods (prehensile legs); plp′′′, abdominal appendage of third pair; prp′, prp″, first and second peræopods, or walking-legs; t, telson; ur, uropod; II, VIII, second and eighth thoracic somites; 1, 6, first and sixth abdominal somites

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