xii.] THE FRESH-WATER CRAYFISH. 143 



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.) v 



8. The structure of the cephalothorax. 



a. Note again the carapace, with its frontal spine 

 and cervical suture. 



J3. Turn the animal over and note the very 

 narrow sterna between the points of attach- 

 ment of the thoracic appendages. 



The last thoracic somite is not completely 

 ankylosed with the one in front, on the verti- 

 cal side in the crayfish. In the lobster it is. 



y. Raise with a pair of forceps the free edge of 

 the lateral part of the carapace which lies just 

 over the bases of the thoracic appendages, and 

 is termed the branchiostegite: note that it is 

 formed by the large united pleura of the 

 thoracic segments, and overlaps a chamber in 

 which the gills lie. 



