The invertebrate animals do not have a back bone. They function well without an internal skeleton (endoskeleton), but some species of animals are included because they do not have a spinal column. The majority of the animals are invertebrates.
The molluscs like the slug, snail ... the insects like the fly, the spiders, the worms, shellfish and a wealth of others do not have skeletons and are invertebrates. Many have a solid carapace in which the muscles move the various solid sections of their bodies, like crab.
The insects are often like crab, a solid exoskeleton to provide structure like a beetle or a tick. But others, like the flies, are not made of a rigid outer body. The principle remains the same one, it is a rigid or semi-rigid body which keeps a form and contains mobile structure and other parts.
The ground worms are an example of invertebrate without skeleton or rigid carapace, it is completely soft. It moves by contractions, to and from of its body. It can contract and grow bigger or the reverse while lengthening, which enables it to move.
The sea urchins, starfish and other marine animals of the group of Echinodermata are invertebrates. The sea urchins for example are only one shell which contains all the bodies. The starfishes move with small feet with suction cups.
The sponges and the anemones are invertebrates. The anemone is in fact the best example. It resembles a gelatine mass which floats and one can even see with through... obviously it is an invertebrate animal.
