A CRAB 33 



Class : Crustacea. Division : Malacostraca. Order : Decapoda 



A CRAB 



The crab is a representative of the more highly speciaUzed 

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

 clude 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 cephalo thorax and the abdomen bent under 

 it. The abdomen of the male crab is narrow and 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 ap- 

 plied closely to the body as they are in the lobster and the cray- 

 fish, but stand out from it, very much increasing the transverse 

 axis of the cephalothorax and making it longer than the longi- 

 tudinal axis. This feature of its structure makes it easy for the 

 crab to run sideways. Notice that the ventral edge of the bran- 

 chiostegite is closely applied to the body, so that the respiratory 

 water could hardly enter the gill chamber along this edge as it 

 does in the crayfish and the lobster. An opening is present, how- 

 ever, at the base of the chehped through which the water enters. 

 Pass a probe into the branchial chamber through this opening. 

 Notice the prominent stalked eyes ; also the two pairs of delicate 

 antennae. Examine and identify the mouth parts and the thoracic 

 legs ; they will be found to correspond to those of the crayfish or 



