Characteristics of crustacean( L-crusta=hard shell)

1) They are mainly aquatic,generally marine but few freshwater and few live in moist places.

2) They are generally free living but few are parasitic forms.

3) Their head are often fused with thorax to form cephalothorax covered dorsally by carapace.

4) Head bears a pair of compound eyes and five pairs of appendages.

5) Thorax and abdomen are often with a pair of biramous appendages in each segment. The appendages are modified for various functions.

6) Respiration is either by gills or general body surface.  

7) Coelom is greatly reduced  in the form haemocoel.

8) Blood vascular system includes a dorsal contractile heart .

9) Excretory organs are either maxillary glands or antennary glands.
10) Sexes are usually separate. Sexual dimorphism is common.

11)Development occurs by metamorphosis with free larval stage.