42 INVERTEBRATE 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 
