THE CLASSIFICATION OF CRUSTACEA 51 



times 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 men- 

 tioned in later chapters. 



There is no distinct carapace, but the last vestige 

 of one may be indicated by the fact that the first 



FIG. 20 A WOOD- 

 LOUSE (Porcellio 

 scaber), ONE OF 

 THE ISOPODA. EN- 

 LARGED. (From Lan- 

 kester's " Treatise on 

 Zoology," after Sars.) 



