XII.] THE FRESH- WATER CRAYFISH. 137 



a. The protopodite : represented by a single short 

 strong joint. (In the lobster there is an in- 

 complete basal joint.) 



fi. The exopodite and endopodite: wide plates 

 fringed with setae: the exopodite divided into 

 two portions by a transverse joint. 



5. The telson. 



A flattened plate bearing no appendages : sub- 

 divided by a transverse joint (it is undivided in 

 the lobster) : the membranous character of the greater 

 part of the ventral surface of its anterior division. 



The tail-fin ; formed by the telson and the append- 

 ages of the sixth abdominal segment. 



6. The second abdominal segment. 



Closely resembling the third in the female : in the 

 male its appendages are modified : the protopodite 

 and basal joint of endopodite much elongated, and the 

 latter produced into a plate rolled upon itself so as 

 to form a demicanal, concave inwards. (In the lobster 

 the endopodite is produced inwardly, into an oval 

 process.) 



7. The first abdominal segment: its appendages; rudi- 

 mentary in the female (it has only one instead of 

 two terminal divisions in the lobster) : in the male 

 consisting of a single plate rolled in upon itself. (In 

 the lobster the single terminal division has the form 

 of a flat scoop or a narrow spoon with its concave side 

 turned inwards.) 



8. The structure of the cephalothorax. 



a. Note again the carapace, with its frontal spine 

 and cervical suture. 



