PHYLUM ARTHROPOD A 221 



with even the thoracic appendages biramous. In later develop- 

 ment, the exopodites of these biramous thoracic appendages 

 disappear (Fig. 95 C) leaving the unbranched walking legs of 

 the adult. In the crabs, the thoracic appendages of the zoea, 

 which are later destined to become the walking legs, develop in 

 the free-swimming larva as rudimentary budlike appendages only 

 (Fig. 103) and never acquire the biramous condition characteristic 

 of the mysis. Thus in the crabs the mysis stage has been elimi- 

 nated and the zoea in passing to the adult form involves a more 

 advanced type of larva which has been called the megalops (C). 

 Some of the prawns have the zoea preceded by a nauplius and 

 metanauplius and thus show unique combinations of larvae of 

 both the lower and the higher crustaceans. 



V. Order Amphipoda 



Almost all of the Amphipoda are aquatic. Characteristically, 

 the body is compressed laterally. The common name of beach 

 fleas or sand fleas is due to the leaping movements with which 



Fig. 104. — An amphipod, Orchcstia palustris. (After Kunkel). 



they spring into the air when out of the water. The head bears 

 six pairs of appendages. There are seven segments in the thorax. 

 The first of these bears a pair of small appendages which function 

 as maxillipeds. 



In the anterior region of the thorax, the sides of the body are 

 frequently prolonged ventrally by epimeral plates which are 

 borne upon the legs and serve to enclose the gills or gill sacs. 

 In the same region, scales from the two sides of the body fre- 

 quently enclose a brood pouch ventral to the ventral body wall. 

 Within this pouch, eggs and the young are carried. 



The abdominal appendages are of two types. On the anterior 

 three somites of the abdomen are borne biramous feet with 

 many joints and numerous hairs. Posterior to these the append- 

 ages are biramous but the branches are not segmented and form 



