Class 1. Crustacea
1. Crustacea are mainly aquatic, mostly marine, few freshwater and few live in moist places on land while few parasitic forms.
2. Body is covered externally by chitinous cuticle. 3. Body is bilaterally symmetrical, triplob-lastic and divisible into three regions hea4tho-rax and abdomen.
4. Head consists ofsix fused anterior seg-ments and bears a median eye, a pair of com-pound eyes and five pairs of appendages i.e., a pair of antennules. a pair of antenna. a pair of mandibles and two pairs of maxillae.
5. Head is often fused with few or all the thoracic segments to form the ccphalothorax.
6. Thorax and abdomen bear a pair of biramous appendages in each segment.
7. Respiration takes place either by the general surface of the body or by gills.
8. Body cavity is divided into compart-ments. most ofwhich contain blood. True coe-lom is greatly reduced and is represented by the cavities of reproductive organs.
9. Blood vascular system comprises a dorsal contractile heart communicating by val-vular ostia with an enclosing pericardial sinus.
10. Excretory organs arc particularly modified coelomoducts which may eidwa- max-illary glands or antennary (green) glands.
II. Nervous system consists of a brain united by oesophageal connectives with a ven-tral nerve cord.
12. Sexes are separate or united. Sexual dimorphism is common.
13. Development includes metamorpho-sis with free larval stage.
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