52 THE LIFE OF CRUSTACEA 



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 antennae 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 Crus- 

 tacea, except the Cirripedes already mentioned, 

 which are normally hermaphrodite. 



The fifth order of the Peracarida, the Amphipoda, 

 is also a very large one. The " Sand-hoppers," 

 which are very common on sandy coasts, belong to 

 this order, as do also a very large number of other 

 forms found in the sea and in fresh water, which 

 have no popular names. A common species is 



