This is the largest of kingdom animalia. About ¾ of all known animals are arthropods
Characteristics of phylum arthropoda
- They are triploblastic coelomates
- They are mathematically segment
- They are bilaterally symmetrical
- The coelom is much reduced, and the main body cavity is called a haemocoel in it, blood moves freely over the tissues through a series of spaces (i.e. they have open blood circulation).
- Their Central nervous system consists of a pairs pre-oral ganglion connected by nerves called commissures to a central nerve cord
- Their excretory system consists of coelomoducts instead of (nephridia as in annelids). The coelomoducts are tubes which open to the outside of the organism at the base of appendages.
- Arthropods posses Malpighian tubules which open to the mid and hind gut for excretion.
- Each segment bears a pair of jointed appendages (arthros = Jointed) (poda = legs) this serves purposes like locomotion feelings and sensation.
- There is a dorsal heart and their circulation, and the body is differentiated into a head thorax and abdomen
- Many larva forms exist within the phylum; there is indirect development or metamorphosis
- They have a chitinous exoskeleton.
Importance of exoskeleton to arthropods
- It offers structural support
- It prevents water loss since it is impermeable to water
- It provides a system of levers for the muscle attachment
- It is modified for the development of sensory structures like antennae
- It is modified to provide flexibility at the joints
- It undergoes ecdysis (moulting) for increase in the arthropods seize
- It is modified for gaseous exchange e.g. the trachea system in insects or book lungs in arachnids.