The Platyhelminthes Skeletal System: Platyhelminthes are characterized as being either soft-bodied or lacking cuticles and don’t have the rigid skeletal support system found in a more complex phylum. Most Platyhelminthes come in flat ribbion or leaf like shapes. They are usually unsegmented and invertebrate animals.

They are bilaterally symmetrical (i.e., the right and left sides are similar) and lack specialized respiratory, skeletal, and circulatory systems; no body cavity (coelom) is present.

Exoskeleton and endoskeleton are absent in members of phylum Platyhelminthes. These are the first group of animals where three primary germ layers develop, i.e. ectoderm, mesoderm and endoderm.

The nematode body is encased in a cuticle, a flexible but tough exoskeleton, or external skeleton, which offers protection and support. The cuticle contains a carbohydrate-protein polymer called chitin.

Flatworms are acoelomate, triploblastic animals. They lack circulatory and respiratory systems, and have a rudimentary excretory system. Cestodes, or tapeworms, infect the digestive systems of primary vertebrate hosts. Nematodes are pseudocoelomate members of the clade Ecdysozoa.

The earthworm’s body is also known as a hydrostatic skeleton, which is a flexible skeleton filled with fluid. A common earthworm (L.

Earthworms do not have an internal skeleton as we do, and they do not have a protective hard exoskeleton as does an insect. They are flexible, long bundles of muscle, especially designed for life underground.

It is generally thought snakes evolved from lizards. Over time, their legs got smaller and eventually they were lost altogether. Some snake species, including pythons and boas, still retain the remnants of their legs with tiny digits they use to grasp with while mating.

Snakes used to wander the Earth on legs about 150 million years ago, before they shifted from strut to slither. Some snakes, such as pythons, retain tiny vestiges of legs in the form of two small bumps on either side of their pelvis.