438 



ARTHROPODA. 



Legion III. Arthrostraca (EdriopUthalmata). 



Although the head of the Arthrostracan consists of six seg- 

 ments, it is remarkably short. It bears six pairs of appendages, 

 one of the normal thoracic pair being added to it as maxillipeds. 

 Eyes, when present, are aggregates of ocelli situated on the sides 

 of the head. There are seven thoracic segments, the appendages 

 of which are walking feet which lack exopodites. The abdominal 

 appendages, when present, are always biramous, the telson never 

 bearing appendages, and in the Amphipods is greatly reduced, 

 sometimes being split nearly its whole length. 



The nervous system (figs. 75, 448) is of the ladder type. The 

 alimentary canal is straight and has an anterior enlargement, the 



FIG. 448. Male Orchestia cavimana. (After Nebeski.) a', a 2 , antennae ; ao, aop, 

 anterior and posterior aortse ; c, heart ; d, digestive tract ; 0, brain and eye ; ft, 

 testes ; /c, gills; 7c/, maxilliped ; Z, liver; ra, excretory organ ; ?i, ventral nerve cord; 

 o, rudimentary ovary; vd, vas deferens ; I- VII, thoracic feet; 1-3. anterior, A-6', 

 posterior abdominal feet. 



chewing stomach, behind which empty one or more pairs of long 

 liver tubes, while in a few Amphipods a pair of excretory tubes, 

 the so-called Malpighian tubules, empty into the intestine near its 

 end. Respiratory and circulatory systems vary so that they are 

 best described in connexion with the two orders. 



Order I. Amphipoda. 



The Amphipods are almost exclusively aquatic, a few species 

 living on the shore near high-tide mark. A few live in fresh 

 water (Gammarus, Allorchestes), the majority being marine. On 

 land they move by a leaping motion, whence the common name, 



