University:
University of Phoenix
Course:
BIO 101T | Principles of Biology 3
Academic year:

2021
Views:

342

Pages:

7
Author:

Arnav Newman

Endoskeleton in Vertebrates or Axial and Appendicular Skeleton Introduction: The hardened tissue of the body together forms the skeleton. In vertebrates, the skeleton is placed on the surface, forming a dead exoskeleton. But the skeleton of vertebrates develops below the surface, forming a living and growing endoskeleton. The endoskeleton of vertebrates develops from mesenchyme. At an early embryonic stage, it is composed of cartilage, but in adults, it is replaced by bone. Some parts retain cartilage, and this cartilage is termed replacing bone. Thus, they are distinguished from membrane bones. Types of Vertebrate Endoskeleton: The presence of an endoskeleton is a characteristic of vertebrates. It forms the framework of the body, to which muscles are found attached. It gives definite shape to the body and protects vital organs like the brain, heart, lungs, etc. The endoskeleton of vertebrates is divided into two major categories: Axial skeleton: Includes the skull, vertebral column, ribs, and sternum. Appendicular skeleton: Includes girdle bones and limb bones. Skull: The skeletal framework of the vertebrate head is called the skull. The skull of vertebrates is derived from three embryonic components: Neurocranium (Chondrocranium): This is also called the chondrocranium. It encloses the cartilaginous cranium (brain box). It includes 2 pairs of olfactory capsules containing special sense organs of smell (olfactory), sight (vision), and hearing (otic). Dermatocranium: This includes membranes or dermal bones attached to the chondrocranium or splanchnocranium. The dermatocranium is absent in cyclostomes and elasmobranchs but present in higher vertebrates like fish, amphibians, reptiles, birds, and mammals. Splanchnocranium: This derives from the visceral or pharyngeal skeleton. This skeleton is cartilaginous but becomes largely replaced by bones in higher forms. It forms the jaws and suspension of the jaws. Skull in Different Vertebrates: The comparative study of the skull in different vertebrates shows that the basic architectural plan for the 3 major components of the skull (neurocranium, dermatocranium, splanchnocranium) is essentially the same. However, there are many differences in the general form and detailed structure of the skull. Cyclostomata: The skull in living cyclostomes is the most primitive type, and the visceral skeleton is modified as a branchial basket. Chondrichthyes: In elasmobranchs, the neurocranium is cartilaginous. Olfactory and optic capsules are fused with the neurocranium. Osteichthyes: In ganoids and primitive bony fishes, the neurocranium is partially ossified. Amphibia: The skull of amphibians shows modifications due to shifting life from water to land. There are fewer bones and much more cartilage in the skull of modern amphibians. Reptilia: The neurocranium in reptiles shows extensive ossification, and there are more dermal bones present than in amphibians. Aves: The skull of birds is essentially reptilian in structure. The neurocranium is well-developed, and the skull is light with thin dermal bones. Appendicular Skeleton: Girdle and Limb Bones: The first group of the appendicular skeleton consists of girdle bones, and the second group consists of limb bones. The shoulder girdle connects the bones of the upper limb to the axial skeleton, while the pelvic girdle connects the bones of the lower limb to the axial skeleton.