Page 184 - Biology_F5

Comparative studies of natural groups of organisms

Distinctive features of class Crustacea Structure of a crab
Crustacea have distinctive features that A crab is covered with a thick exoskeleton
make them unique among other arthropods (carapace) composed of calcium carbonate.
as highlighted below: This means that they are well protected
(a) They possess two body parts, which against predators (Figure 3.48). The body
are cephalothorax (the head fused with is divided into two parts: the cephalothorax
FOR ONLINE READING ONLY
thorax) and abdomen. and abdomen. The abdomen is entirely
(b) They have carapace or an exoskeleton hidden under the carapace; it may not be
hardened with calcium salts, which visible at all, unless the crab is turned over.
acts as a protective shell. The head bears two pairs of antennae. The
(c) They have heads bearing two pairs of animal is equipped with a pair of claws
antennae. (pincers), which are the most important
(d) They have a pair of compound eyes at weapons with at least three functions:
the end of movable stalks. seizing, eating and subduing the prey. If
(e) They have at least three pairs of the food is a shellfish, then the pincers can
mouthparts. exert force to open or break the mollusc’s
(f) Gills are used for gas exchange, which shell. Males also use pincers for or during
are the outgrowth of the body wall or fighting.
limbs.

(g) Unlike other arthopods, the legs and
other appendages of Crustacea have
two branches (they are biramous).

Antenna

Cheliped

Eye Propodus
Carpus
Merus
Walking legs
Carapace

Figure 3.48: Structure of a crab

Form Five Student’s Book
177