skeleton

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 of the surface forming a living or growing endoskeleton .
three types of skeletons develop in verterbrates -

1. epidermal/horny exoskeleton

these include hard and horny keratinized derivatives of epidermal layer of skin ,such as claws ,most reptilian's scales,birds feathers and mammalian hairs ,horns ,nails and hoof ,etc.all living amphibians lack an exoskeleton.

2.    dermal/bony exoskeleton -


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 .dermal scales become exposed due to wearing out of epidermis ,and form exokeleton

3 enodskeleton    

greater part of verterbrate skeleton lies more deeply,forming the endoskeleton .it develops from mesenchyme.endoskelet on is formed by bones in vertebrates .

Skeleton in different animals are as follown-

Invertebrate-

=========== 

Protozoa-no skeleton

Porifera-calcareous spicules+silicious spicules+spongin fibres in mesenchyme.

Coelenterata-calcareous(corals)and chitinous(perisarc).

Helminthes-no skeleton,cuticle present

Annelida-no skeleton ,cuticle present

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