558 



CRUSTACEA. 



of parasitic Amphipods included under the name of LcEmodipoda 

 have been as yet discovered. 



Order II. Isopoda. — 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 preceding order, but are attached to the 

 inferior surface of the abdomen, and consist of leaf-like branchice, 

 which in the terrestrial species are protected by plates which fold over 

 them. The thorax is composed of seven segments, bearing typically 

 seven pairs of limbs, which, in the females, have marginal plates, 

 attached to their bases, and serving to protect the ova. The num- 



Fig. 419. — Recent Isopoda. A, Idotea entojnon, enlarged; b, Arcturus longicorm's,&n\zrgQd- y 

 c, Serolis Scythei : an, Antennae ; a, Antennules. (After Gerstaecker, Spence Bate and West- 

 wood, and Liitken.) 



ber of segments in the abdomen varies, but is never more than 

 seven. The abdominal segments are in many Isopods coalescent, 

 and form a broad caudal shield, beneath which the branchiae are 

 carried (fig. 419, a). 



The recent Isopods are for the most part of small size, and are 

 of the most varied habit. Many forms are strictly marine, some 

 forms occurring even at great depths (over 2000 fathoms), but very 

 many types are either inhabitants of shallow water or live between 

 tide-marks. Some forms, such as the Bopyridce, are parasitic in the 

 adult condition. Many Isopods live in fresh waters, some of these 

 belonging to genera which are also represented by marine forms. 

 Other Isopods, like the Wood-lice {Oniscidce) are terrestrial in 

 habit. 



