Sub-Class 2. Malacostraca . Order 4. Isopoda. 217 



to form the clielse, are often provided with rudimentary, but distinct, 



e X o p o d a . The abdominal appendages do not sen'e as gills. 

 They occur on aU European coasts. 



Order 5. Amphipoda. 



The Amphipoda are like the Isopoda in many respects ; there is 

 no carapace, the eyes are sessile, there are seven free thoracic 

 segments ; the first thoracic appendages are maxillipeds, the others 

 are ambulatory, consisting merely of endopod, etc. An important 

 difference is that the abdominal appendages do not act as gills, 

 but the organs of respiration are peculiar, lamellate, membranous 

 processes of the inner side of the basal joint of some of the thoracic 

 legs.* The exoskeleton is usually not so hard as in the Isopoda. 



■"^^■^ss^ 



Fig. 179. An Amphipod nearly related to Qammarus (somewh&t enlarged). A^ — A^ 

 first and second antennae, 1 maxilliped, 2, 3 second and third thoracic feet, ce brood-ponch, 

 g gill, if 3, H4, Hg third, fourth, and sixth abdominal feet. — After Sars. 



The abdomen is seven-jointed (the terminal segment small). The 

 maxillipeds are fused at their bases. Some of the walking legs, 

 chiefly the anterior ones, are also prehensile organs, since the terminal 

 joint can move upon the penultimate. The basal joint of these 

 appendages (especially of the first four pairs) is laminate and 

 directed downwards, which gives the body a compressed appearance 

 (the thorax itself is not compressed. Pig. 180). The first three pairs 

 of abdominal limbs are powerful swimming legs, the 

 three hindmost, on the other hand are smaller, somewhat stiff and 

 turned back. The Amphipoda have just such a marsupium as have 

 the Isopoda, but the larvae do not leave the eggs until all the limbs 

 are developed. 



* This process cannot be taken as representing the epipod, which arises from the 

 outer side of the basal joint (see Fig. 180). 



