In vertebrates the skeleton forms a framework of the body to which muscles are found attached. The skeleton is made of cartilage or bone or of both cartilage and bone. The presence of an endoskeleton is a characteristic of vertebrates, though many vertebrates also have a dermal exoskeleton.
Dermal Bony Skeleton:
The dermal exoskeleton consists of dermal scales and fin rays in fishes. The dermis forms bony dermal plates of fossil ostracoderms, placoderms and amphibians. It also forms the bony armour (plates and scutes) of sturgeons, turtles, crocodilians, and some mammals (armadillos). Thus, it is somewhat artificial to make a distinction between dermal exoskeleton and endoskeleton because dermal bones contribute to the formation of some parts of the endoskeleton.
It lies deep in the body and develops from mesenchyme. In embryonic stage it is formed of cartilage but in adults it is replaced by bones, i.e., bones developed in place of existing cartilages are called cartilage or replacing bones. If the bones develop in the dermis then these are called dermal or membrane bones.
But both the above formed bones are similar in structure. The notochord forms the endoskeleton in Amphioxus, but in a majority of living vertebrates the notochord is only embryonic and in the adult it is replaced largely by a vertebral column.
Subdivisions of Endoskeleton:
The endoskeleton of vertebrates has an axial skeleton made of skull, vertebral column, ribs, and sternum, and an appendicular skeleton consisting of pectoral and pelvic girdles and the skeleton of paired limbs and unpaired appendages.
These two types are included in somatic skeleton. A visceral skeleton forms the support of jaws and gills in fishes, but in higher forms it contributes to the skull and hyoid and is included in the axial skeleton.
Functions of Endoskeleton:
1. The endoskeleton forms a framework for the support of the body.
2. It permits growth because it is living and grows in size with the rest of the body.
3. It maintains a definite shape and form of an animal.
4. It provides protection for delicate vital organs of the body.
5. It furnishes a firm and adequate surface for attachment of muscles by means of tendons.
6. In bone marrow blood corpuscles are manufactured.
7. Ear ossicles aid in hearing.
8. Tracheal rings and ribs help in breathing.
There are joints in the endoskeleton so that movement of parts is possible. In transition from water to land levers in the form of limbs were provided which not only bring about locomotion, but also lift the body above the ground. The vertebral column became a rigid axis to support the heavy head and to resist the thrust exercised by the limbs.
The long bones of vertebrates contain yellow marrow which stores fats, but in mammals the long bones and skull bones also have red marrow or myeloid tissue besides yellow marrow.
In amphibians, reptiles, and birds the bone marrow forms all kinds of corpuscles, while in mammals the red marrow forms erythrocytes, blood platelets, and granular leucocytes only, the other white corpuscles are formed in the lymphatic tissues. Embryos of mammals have only red marrow, later some of this is replaced by yellow marrow.