Phylum – Arthropoda

This is the largest phylum in the animal kingdom, and it includes the insects. In fact, more than two-thirds of all the named species on earth are arthropods, which makes this by far the most successful group of animals.

Body organisation, symmetry, layers and cavity

• They show an organ-system level of organisation.
• They are bilaterally symmetrical.
• They are triploblastic.
• They are segmented and coelomate, with a true body cavity.

Exoskeleton, body regions and jointed appendages

The body is covered by a hard outer covering made of chitin, called a chitinous exoskeleton. The body is divided into three regions: a head, a thorax and an abdomen. A key feature is the presence of jointed appendages, and this is what the name means, since arthros stands for joint and poda for appendages.

Respiration, circulation and sense organs

• Breathing is carried out by gills, book gills, book lungs or a system of air tubes called the tracheal system, depending on the group.
• The circulatory system is of the open type, in which blood is not fully enclosed within vessels.
• Sense organs are well developed and include antennae, both compound and simple eyes, and balancing organs called statocysts.

Excretion and reproduction

• Excretion takes place through malpighian tubules.
• They are mostly dioecious, with separate sexes.
• Fertilisation is usually internal. …