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The hardened tissues of the body together form the skeleton (sclero = hard). Skeleton of invertebrates is most often secreted on the surface, forming a lifeless or dead exoskeleton. Whereas skeleton of vertebrates develops most often underneath the surface forming a living or growing endoskeleton. Three types of skeletons develop in vertebrates :

(1) Epidermal/Horny exoskeleton : These include hard and horny of keratinized derivatives of epidermal layer of skin, such as claws, most reptilian’s scales, bird feathers and mammalian hairs, horns, nails and hoofs, etc. All living amphibians lack an exoskeleton.

(2) Dermal/Bony skeleton : Dermal bony skeleton is derived from the dermis of skin. It includes bony scales and plates or scutes (osteoderms), finrays and antlers of fishes, some reptiles (crocodiles, turtles and tortoises) and mammals. In fishes, dermal scales become exposed due to wearing out of epidermis, and form exoskeleton.

(3) Endoskeleton : Greater part of vertebrate skeleton lies more deeply, forming the endoskeleton. It develops from mesenchyme. Endoskeleton is formed by bones in vertebrates. Skeleton in different animals are as follows -

Invertebrate -

(i) Protozoa - No skeleton.

(ii) Porifera – Calcarius spicules + silicious spicules + spongin fibre in mesenchyme.

Spicules in porifera represent endoskeleton.

(iii) Coelentrata - Calcareous (corals) and chitinous (perisarc).

(iv) Helminth - No skeleton, cuticle present.

(v) Annelida - No skeleton, cuticle present.

In earthworm and ascaris is hydrostatic skeleton is found that is fluid is filled in coelom and form turgid skeleton.

(vi) Arthropoda - Dead Chitinus exoskeleton, shed at intervals, called ecdysis or moulting. Cuticle made up of non chitinous outer epicuticle and chitinous, inner endocuticle.

(vii) Mollusca - Calcarius shell, may be external or internal or absent.

(viii) Echinodermata - Dermal calcareous plates are present.

(ix) Hemichordates - Endoskeleton in form of proboscis skeleton, pygochord.

In vertebrates exoskeleton may be epidermal or dermal.

Vertebrates : In vertebrates dermal skeleton is formed by bones. Bone is the connective tissue with intercellular spaces filled with ossein matrix composed of 25% water, 25% protein fibers, 50% mineral salts. The inner most region is full of bone marrow having various types of cells. In mammals the bone is full of haversian canals. The bones are of following types -

(i) Cartilage bones : The bones which are formed by the ossification of preexisting cartilage are called cartilage bones or replacing bones. e.g., vertebra, Girdles, limbs bones, basioccipital, supraoccipital, sphenoid, Incus, malleus, stapes.

(ii) Membrane or dermal bones : The bones which are formed by independent ossification in connective tissue are called dermal, membrane or investing bones. e.g., Ribs, sternum, clavicle, Nasal, vomer, palatine, maxilla.

(iii) Sesamoid bones : Ossification takes place on Ligament cotyloid bone of Rabbit and Tendons e.g., Patella, Pisiform.

(iv) Pneumatic bones : Bones with hollow spaces containing air e.g., bones of bird, Frontal, sphenoid ethmoid, maxilla of human.

(v) Irregular bones : Vertebrae are irregular bone.

(vi) Flat bones : Cranial bone, scapula, Ribs.

(vii) Short bones : Carpals and tarsals.

Functions of endoskeleton : Chief function of vertebrate endoskeleton can be enumerated as follows –

(i) To provide physical support to body by forming a firm and rigid internal framework.

(ii) To give definite body shape and form.

(iii) To protect by surrounding delicate internal organs like brain, heart, lungs etc.

(iv) To permit growth of huge body size (whale, elephant, extinct dinosaurs), since it is living and growing.

(v) To provide surface for attachment of muscles.

(vi) To serve as levers on which muscles can act.

(vii) To manufacture blood corpuscles in bone marrow.

(viii) To help in breathing (tracheal rings, ribs).