A Brachyuran Crustacean. 105 



A BRACHYURAN CRUSTACEAN 



( Cancer irroratus) . 



External Anatomy. - - Are the ccphalothorax and ab- 

 domen built upon a new plan, or is the plan presented by 

 the lobster again adopted ? Which region of the body has 

 been specially elaborated? The abdomen of the female is 

 much wider than that of the male ; to which sex does the 

 specimen belong? 



Count the abdominal appendages of a female, and see if 

 they agree with the same of the lobster. Count the same 

 of the male. Why are there more appendages on the abdo- 

 men of the female than on that of the male? (Examine 

 a specimen that bears eggs.) Is the number of abdomi- 

 nal somites different in the two sexes ? Does the male show 

 any traces of coalescence of abdominal segments? How 

 does the number compare with the abdominal segments of 

 Homarus ? \ 



Is the number of ambulatory appendages the same as in 

 the lobster? The number of appendages bearing pincers? 

 In what main feature does the carapace differ from the cara- 

 pace of the lobster? Do you find the homologue of the 

 rostrum ? The transverse groove is more clearly shown in 

 many related forms. Can you easily follow the groove to 

 the margin of the carapace? Is this a free margin, as in 

 Homarus ? Note the opening at the base of the first am- 

 bulatory appendage, through which the water enters to bathe 



