xii.] THE FRESH-WATER CRAYFISPL 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.) 



8. The structure of the cephalothorax. 



a. Note again the carapace, with its frontal spine 

 and cervical suture. 



/3. 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. 



