Nematoda
The phylum nematoda is one of the most diverse groups of animals. There are thousands of different nematodes, and more than half of these documented animals are parasitic. They are worm-like and microscopic. They have no real skeletal system, just a thick cell layer known as a cuticle.
An example of an animal in the phylum Nematoda is Ascaris lumbricoides, a parasitic nematode that infects people in sub tropical areas. It causes the disease Ascariasis in areas of poor hygiene. It has no real skeletal system beyond the cuticle, a thick cell layer.
An example of an animal in the phylum Nematoda is Caenorhabditis elegans, a non-parasitic, free living nematode that lives in bacteria and nutrient rich environment. It, like all nematodes has no real skeleton, but instead, a thick cell layer known as a cuticle.
An example of an animal in the phylum Nematoda is Necator americanus, a type of hookworm. Hookworms are parasitic and can infect humans as well as some domesticated animals. It only has a thick layer of cells, known as the cuticle, to call a skeletal system.