On earth we see two types of skeletons: exoskeletons (think of exit (outside) to remember which is which) and endoskeletons.
Exoskeletons are outside the body and encase it like armor: a good example is a crab in its shell. Exoskeletons protect the body. They are light and strong, and provide attachment places for the muscles inside. They protect the body from dehydration, predators, and excessive sunlight.
Endoskeletons lead to some difficulties, too. The shell does not grow, so it has to be shed periodically. It looks to be a struggle to get out of an exoskeleton, and the animal is vulnerable while the soft new shell hardens. This may take several hours. Exoskeletons are also difficult to repair if they have been damaged.
Endoskeletons are bones inside the body. They support the body so that it can stand and walk, and some structures, like the skull and rib cage, protect delicate and important organs. Endoskeleton bones provide a storehouse of calcium for the body, and the calcium can be drawn on and used in other ways if necessary. Endoskeleton bones do heal if they get broken, though a doctor's help is often needed to ensure that they fuse together properly.
Some bodily tissues are supported by cartilage. This is stiff and rubbery tissue. You have cartilage in your ears and the tip of your nose. Some sea animals have cartilage instead of bone. The shark is a good example of an animal with a skeleton of cartilage instead of bone.
Your aliens may need skeletons. Think carefully about this. Do you want the skeletons to be exoskeletons or endoskeletons?
Let's do an example of the kind of thinking an d write-up that you should do for your aliens. Start with a sketch of a diagram, and use the diagram on your page. Remember, diagrams help with visualization and clarification: although they are sometimes beautiful, it is their clarity, rather than their artistic merit, on which they are judged.
The Snorvep and Its Skeleton
Here is the outline of the snorveps' body.
Where does it need support?
Well, obviously the blue hearing organs need to be attached to something. Let's put some cartilage plates under them. In fact, let's put cartilage in the whole front of the body (the "head" part). That will keep the sensory organs fixed in their relationship to each other and will also provide support for anchoring the muscles that move the front tentacles and run down the body to the back end.
Here we see the body with the skeleton in it. At the front end there are plates to support the hearing organs and two bent spines to support the antennae. The muscles that run down the body from "head to tail" can also be attached here.
At the other end of the body there is a tough cartilage plate. The muscles that run down the body can be attached to it, as can the rear tentacles.
The rest of the body is made of muscles and
organs, and held in shape somewhat by the tough outer skin.
Here we see two diagrams of the snorveps' head. In front you can see the two hearing organ supports and the sockets for the antennae, which can be moved. The central part also provides an attachment site for the tongue (at the top of the arch under the antennae sockets) and attachment sites for muscles, both at the top and on the downward-pointing ridges that also support the hearing organs.
The back view shows that the top of the head is covered. The powerful muscles that let the snorvet arch its back connect to this point. Part of the brain connects to nerves from the hearting organs under this cap of cartilage (the bones that are in front are colored gray.)
Note: When you are planning your own animals, remember that muscles can pull but not push.
Reproduction, Respiration, Circulation, the Skeletal and the Nervous