Animals without backbones are called invertebrates, and they make up more than 95% of all animals on Earth. Let us organize the characteristics of their diverse groups.
Basic Knowledge
Main groups of invertebrates:
① Arthropods: have segmented bodies and an exoskeleton. They are the largest group.
・Insects (three body parts—head, thorax, and abdomen; six legs; usually two pairs of wings): grasshoppers, butterflies
・Crustaceans (cephalothorax and abdomen; ten legs): shrimp, crabs
・Arachnids (cephalothorax and abdomen; eight legs; no wings): spiders, scorpions
・Myriapods (long, slender bodies and many legs): centipedes, millipedes
② Mollusks: have unsegmented bodies, with internal organs covered by a mantle. Many have shells.
Examples: clams, turban shells, octopuses, squid, snails
③ Others: earthworms and leeches (annelids), sea urchins, starfish, and sea cucumbers (echinoderms), jellyfish and corals (cnidarians), and sponges (poriferans).
Invertebrates (the general term for animals without backbones)
Arthropods (animals with segmented bodies and exoskeletons; the largest group in the animal kingdom)
Exoskeleton (a hard shell covering the outside of the body; growth occurs through molting)
Spiracles (small openings for respiration on the abdomen of insects)
Mollusks (animals whose internal organs are covered by a mantle; clams, octopuses, and squid)
Mantle (a muscular membrane surrounding the internal organs of mollusks)
Further Study (Background and Significance)
Arthropods make up about 75% of animal species on Earth and are the largest group. Reasons for their success include body protection by an exoskeleton, flight ability provided by wings, division of niches through metamorphosis (larvae and adults eat different foods and therefore do not compete), and efficient living with small bodies.
The insect body is divided into head, thorax, and abdomen, and six legs extend from the thorax (which is why spiders are not insects). Insects respire through tracheae, taking in air through spiracles.
Mollusks protect their soft bodies with shells, while some species, such as octopuses and squid, have internalized their shells to increase mobility. Squid and octopuses are considered to have the highest intelligence among invertebrates.
- Invertebrates=more than 95% of all animals
- Arthropods=general term for insects, crustaceans, arachnids, and myriapods
- Insects=three body parts, six legs, respiration through spiracles
- Crustaceans=shrimp and crabs (ten legs, aquatic)
- Arachnids=eight legs and no wings (not insects)
- Mollusks=mantle and shell (clams, octopuses, and squid)
- Exoskeletons grow through molting
Points of Caution (Easily Confused)
① Spiders are not insects (they have eight legs). They belong to the arachnid group. ② Octopuses and squid are mollusks. Octopuses have eight arms and squid have ten, but they are not arthropods. ③ The distinction between arthropods and mollusks is whether they have segments and an exoskeleton or are covered by a mantle. ④ Earthworms are invertebrates, but they are not arthropods; they are annelids.
Practice
- Give two characteristics of the arthropod body.
- What kind of animals are squid and octopuses classified as?
- Why are spiders not insects?