36 INVERTP2BRATE ZOOLOGY 



CRUSTACEA 



A BRACHYURAN DECAPOD. A CRAB 



The crab is a representative of the more highly specialized 

 of the two divisions of the Decapoda, the Brachyura, which 

 include those decapods with short weak abdomens. The lobster 

 and the crayfish represent the other and less highly specialized 

 of the two divisions, the Macrura, which comprise those decapods 

 with long abdomens. 



Compare the crab with the lobster or the crayfish. Note the 

 broad shield-shaped cephalothorax and the abdomen bent under it. 

 The abdomen of the male crab is narrow while that of the 

 female is broad. Which sex is your animal ? In what ways is 

 the higher specialization of the cephalothorax and the abdomen 

 of the crab shown ? 



The body of the crab is composed of twenty somites, like that 

 of the crayfish and the lobster, thirteen of which belong to the 

 cephalothorax and seven to the abdomen. The cephalothorax 

 is covered by a carapace. Notice the short transverse suture 

 which separates the cephalic from the thoracic portion. At 

 the ends of this suture notice the longitudinal depressions 

 which mark off the lateral branchial areas and separate the 

 branchiostegites from the median portion of the carapace. The 

 branchiostegites are not applied closely to the body as they are 

 in the lobster and the crayfish, but stand out from it, very much 

 increasing the transverse axis of the cephalothorax and making 

 it longer than the longitudinal axis. This feature of its struc- 

 ture makes it easy for the crab to run sideways. Notice that 

 the ventral edge of the branchiostegite is closely applied to 

 the body, so that the respiratory water could hardly enter the 



