CRAYFISH 397 



portion, while the portion posterior to the groove is the thorax. The 

 anterior end of the cephalothorax is drawn out to almost a point, and 

 this portion is called the i^ostrum. The mouth is located on the 

 ventral side of the head portion and not at the tip of the rostrum 

 where most people look for it. The lateral portions of the carapace 

 are known as branchial areas or hranchiostegites, and they cover the 

 gills. Their ventral edges are free. On the ventral side of the thorax 

 between the twelfth and thirteenth segments (about the level of the 

 fourth walking leg) of the female is a cuplike pouch called the 

 annulus or seminal receptacle. It serves in reproduction for the 

 receipt and storage of spermatozoa. 



The portion posterior to the thorax, which is frequently called 

 ''tail" by fishermen, is really the abdomen, and the tail proper is at 

 the posterior end of this. The abdomen is divided into six typical 

 segments and the terminal telson, which has no appendages but is 

 often called the seventh abdominal segment. The anus is found on 

 the ventral side of this part. The skeletal part of the abdominal seg- 

 ment consists of: the dorsally arched sternum; a thin, overhanging 

 lateral plate, the pleuron; and the slender ventral sternum in the 

 form of a narrow bar extending from side to side. A thin arthro- 

 podial membrane extends between successive sterna and allows for 

 movement of the segments upon one another. 



The appendages are paired, with one pair attached to each typical 

 segment. There are nineteen such pairs. They are all developed 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 swimmerets 

 or pleopods and all have the fundamental parts consisting of a basal 

 protopodite composed of coxopodite, joining the body and the basi- 

 podite; the exopodite or lateral branch and the endopodite or medial 

 branch each have many joints. The lirst 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 pos- 

 terior pair of swimmerets, attached 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 pos- 

 terior 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 



