Types of Skeletal System

Types

Skeletal system is of two types, exoskeleton and endoskeleton.

Exoskeleton

It is rigid protective and supporting covering framework present outside the body. Exoskeleton is made up of hard structures or dead tissues. It is present both in invertebrates as well as vertebrates.

• Invertebrates. In invertebrates, exoskeleton is composed of pellicle, lorica, theca, cuticle, shell, corals and sclerites.
• Vertebrates. In vertebrates, exoskeleton consists of scales, feathers, hair, claws, nails, hoofs and horns. Regarding their origin, they are of two types—epidermal and mesodermal.
• (i) Epidermal or Ectodermal Exoskeleton. The exoskeleton develops from ectoderm or epidermis, e.g., mammals, birds, snakes and many lizards.
• (ii) Mesodermal or Dermal Exoskeleton. The exoskeleton develops from mesoderm, e.g., pisces (scales) and few reptiles like crocodiles, turtles and tortoises.

Endoskeleton

It is the hard supporting structural framework which occurs inside the body of an organism. It is present in invertebrates as well as vertebrates.

• Invertebrates. Some invertebrates have hard endoskeleton e.g. spicules (Porifera), internal shell (Cuttlefish).
• Vertebrates. In case of vertebrates, it is mainly composed of bones and cartilage.

Bone is the hardest connective tissue which has a solid matrix made of mineral matter (Ca2+, Mg2+, Cl–) and organic matter containing protein ossein. Matrix has a large number of irregular spaces or lacunae in which bone cells or osteocytes reside.

Cartilage is firm but flexible skeletal connective tissue which has a matrix made of mucoprotein chondrin. It also possesses regular spaces or lacunae in which cartilage cells or chondrocytes occur. In human

skeleton cartilage occurs in ear, nose, larynx, conducting region of respiratory tract, ends of many bones. Rest of the skeleton is made of bones. In chondrichthyes the entire skeleton is formed of cartilage.

Fig. Human Skeleton (ventral view)

Differences between Exoskeleton and Endoskeleton