Introduction of Symmetry

Animal  Organization :-

The evolution of tissues is the first key transition in the animal body plan. Eumetazoans exhibit higher levels of organization. The embryonic tissue layers of eumetazoans are called germ layers.

The lowest level of organization is cellular grade of organization, which is exhibited by sponges.

Division of labor seen among the cells in sponges and such cells have little tendency to become into tissues. Nerve cells and sensory cell are absent; hence cells are functionally isolated in sponges. In cellular grade of organization cells demonstrate division of labor but are not strongly associated to perform a specific collective function.

Diploblastic animals such as cniderians exhibit tissue grade of organization or diploblastic organization. In tissue grade of organization germs layers such as ectoderm and endoderm are developed. Cells are aggregated inti tissues. Nerve cells and sensory cells are present in the epidermis and the gastrodermis. Ectoderm gives rise to the epidermis; endoderm gives rise to the gastrodermis. The cells of a tissue together perform their common functions as a highly coordinates unit. Tiploblastic animals exhibit organ system grade of organization or tiploblastic organization. In organ-system grade of organization all the three germ layers are present such as ectoderm, endoderm and mesoderm. The evolution of mesoderm resulted in structural complexity. The tissues are assembled into larger functional units called organs. Organs working together to perform one or more specific functions constitute an organ system. Eleven kinds of organ systems are observed in metazoans such as integumentary, skeletal, muscular, digestive, respiratory, circulatory, excretory, nervous, endocrine, immune and reproductive. Tiploblastic animals have highly specialized sensory cells and nerve cells which bring about a higher level of coordination and integration.

Symmetry :-

The symmetry of an animal generally fits its life-style. Radial symmetry is an advantage to sessile or planktomic or slow moving organisms. Evolution of bilateral symmetry is the second key transition in the animal body plan. In symmetrical animals are body lacks definite form or geometrical arrangement of parts and can not divided into mirror image halves by any plane. Gastropods are primarily bilaterally symmetrical and secondary asymmetrical. Most of the sponges and some protozoan like Amoeba exhibit asymmetry. In a symmetrical animal similar body parts occur on the opposite sides of a plane passing through the principle axis of the animal. A symmetrical animal can be cut into two mirror image halves or antimeres by one or more planes of symmetry passing along the principle axis. The term axis refers to an imaginary straight line joining two opposite points at the ends, or on the surfaces of the body. The term principle axis means an imaginary straight line joining the midpoint at one end or surface and the midpoint at the opposite end or surface. The term plane means a flat area that runs through any axis. Bilaterally symmetrical animals possess anterior (cephalic) end posterior (caudal) end dorsal surface, ventral surface and lateral surfaces. Frontral plane is the plane that runs through the anterior- posterior and sagittal axis. Transverse plane(cross section) passes through sagittal and transverse axis. Frontral plane divides a bilaterally symmetrical body into dorsal and ventral portions. Sagittal plane divides bilaterally symmetrical animal into right and left halves. Transverse plane divides the animal into anterior and posterior portions. Each of these three planes is at right angles to the other two planes.

Fig :- Example of symmetry  

Fig :- Example of symmetry

Fig :- Example of symmetry

Fig :- Example of symmetry