What is Endoskeleton?

An endoskeleton is an internal support structure of an animal. In three phyla and one subclass of animals, endoskeletons of various complexity are found: Chordata, Echinodermata, Porifera, and Coleoidea. An endoskeleton allows the body to move and gives the body structure and shape. A true endoskeleton is derived from mesodermal tissue. Such a skeleton is present in echinoderms and chordates. The poriferan 'skeleton' consists of microscopic calcareous or siliceous spicules or a spongin network. The Coleoidae do not have a true endoskeleton in the evolutionary sense; here, a mollusc exoskeleton evolved into several sorts of internal structure, the "cuttlebone" of cuttlefish being the best-known version. Yet they do have cartilaginous tissue in their body, even if it is not mineralized, especially in the head, where it forms a primitive cranium. An important advantage of an endoskeleton over an exoskeleton is that it provides more structural support.

Vertebrates have a more or less rigid group of structures composed of cartilage or bone or of a combination of these two connective tissues. The most primitive of these structures is the notochord, which is a backbone of cartilage occurring in fishes. Animals higher on the evolutionary scale have an axial skeleton, consisting of the skull, spinal column, and ribs, and an appendicular skeleton, made up of the pelvic and pectoral girdles and the appendages.
source: wiki/encarta encyclopedia