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Biology for Advanced Level Secondary Schools
Examine the external features of (c) They have segmented bodies, which
specimens S , S and S , (Figure 3.47) are arranged into regions, called
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and then answer the questions that tagmata, comprising of the head,
follow. thorax, and abdomen. The head of
1. Identify the specimens by their some arthropods such as arachnids and
common names. crustaceans is fused with the thorax to
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2. In what kind of habitat would you form a structure called cephalothorax
expect to find specimen S ? (prosoma).
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3. What are the observable features (d) Their body plan is bilaterally symmetrical.
that make specimens S , S and S (e) They have a ventral nervous system
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different from each other? and open circulatory system, which is
4. Name the phylum and classes dorsally positioned.
to which each of the observed (f) They are triploblastic coelomate
specimens belongs. animals.
(g) Their coelom is reduced and confined
to cavities of excretory organs and
3.7.4 Phylum Arthropoda
This is the largest group in the Kingdom reproductive ducts during their
Animalia, with high species diversity. development and form another cavity
Arthropods represent about three-quarters called haemocoel.
of all known animal species. Some of the Classification of Arthropods
well-known arthropods include insects, Arthropods are classified into five classes
crustaceans, and arachnids. The word namely, class Crustacea, which includes
arthropoda comes from two Greek words crabs, class Arachnida, which includes
arthro that means ‘joint’ or 'jointed' and spiders and scorpions, class Chilopoda,
podos that means ‘foot’. Therefore, all which include centipedes, class Diplopoda,
arthropods have jointed appendages. which inlude millipedes, and class
Arthropods are found in almost every Insecta, which includes cockroaches and
known environment, including marine, grasshoppers.
fresh water, and terrestrial ecosystems.
They vary extremely in their habitats, life Class Crustacea
histories, and feeding habits. Crustaceans form a large group of
arthropods that include familiar animals
Characteristics of phylum Arthropoda such as crabs, lobsters, prawns, shrimps,
(a) They possess a chitinous and barnacles, and crayfish. Most crustaceans
sometimes calcareous exoskeleton, are marine aquatic animals, others are
which may be rigid, stiff or flexible; found in freshwater habitats, and a few are
shed during growth in some organisms. terrestrial. For example, woodlice (Isopoda)
(b) They have jointed, and paired are found in leaf litter.
appendages such as legs and antennae.
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