2 AMPHIPODA. 



carapace covering the upper portion of the segments of 

 the body, consequently these segments are themselves 

 made dorsally perfect ; second, the gnathopoda, which, 

 in the Macroura, constitute the two outer pairs of mouth- 

 organs, become prehensile instruments, increasing the 

 number of legs to seven pairs; and, third, the tail, or 

 caudal apparatus, consists of four segments instead of 

 two. 



In" the Amphipoda, on the other hand, the various seg- 

 ments are nearly equally developed throughout, whilst, 

 from being generally compressed at the sides, or cylin- 

 drical in form, the animals of this order are easily dis- 

 tinguished from the Isopods, which are, for the most 

 part, broad and flattened in shape.* 



2 



* Hence, to a certain degree, the Amphipoda constitute a group parallel 

 in their own division with the Macroura amongst the Decapoda, whilst the 

 Isopoda form an analogous group to the broad and flattened Brachyura, 

 the Caprettce offering a kind of parallelism with Squilla and its allies. 



