270 TEXTBOOK OF ZOOLOGY 



The appendages are paired, with one pair attached to each typi- 

 cal segment. There are nineteen such pairs. They are all de- 

 veloped on the same plan from the typical biramous (two branched) 

 appendage. The five anterior pairs of abdominal appendages are 

 quite typical of the primitive form except for the modification of 

 the first two in connection with reproduction. This group is known 

 as smmmerets or pleopods and all have the fundamental parts con- 

 sisting of a basal protopodite composed of coxopodiie, joining the 

 body and the hasipodite; the exopodite or lateral branch and the 

 endopod'ite or medial branch each have many joints. The first two 

 are much reduced in the female, but in the male the protopodite 

 and endopodite are fused and extended to serve as an organ for 

 transfer of spermatozoa. The posterior pair of swimmerets, at- 

 tached to the sixth abdominal segment, are broadened into fanlike 

 structures for swimming. They are known as uropods and have 

 oval, platelike exopodite and endopodite. The posterior five thoracic 

 appendages are the walking legs or pereiopods. These are uniramous 

 due to the complete reduction of the exopodite. Each is composed 

 of the two joints of the protopodite and five of the endopodite. Join- 

 ing the coxopodiie (first segment of protopodite) is a sheetlike struc- 

 ture which supports a gill and some chitinous threads. The three 

 anterior walking legs possess pinchers or chela which are formed by 

 the terminal segment being set on the side of the second segment. 

 The walking legs are used in locomotion, ofi'ense, and defense. The 

 three anterior segments of the thorax bear three pairs of biramous 

 maxillipeds. The parts are quite typical in most respects. Each has 

 an epipodite joining the basipodite and all except the first bear gills. 

 These appendages are used in getting food to the mouth. | 



To the segments of the head are attached five pairs of appendages. 

 Just posterior to the mouth and immediately in front of the first 

 maxilliped are two pairs of maxillae, the second of which overlies 

 the first. They are both leaflike and modified. The epipodite and 

 exopodite of the second are fused to form a bladelike hailer or 

 scaphognathite which fits over the gills and by its movement helps 

 circulate the water for respiration. Its endopodite is slender, but 

 the protopodite is broad and foliate. The first maxilla is reduced 

 to a leaflike protopodite and small endopodite. The jawlike man- 

 dible at each side of the mouth is composed of hard protopodite 



