Anatomy of finfish and shellfish

Unit 2 - Cell structure, tissue and body organization

2.2. Symmetry
It refers to the similarity in size, shape and number of parts on the opposite sides of a median line. Parts of an animal body are often so arranged that it is possible to cut it into two similar halves by one or more planes. Such animals are said to be symmetrical. Some animals, like Amoeba and Snail, cannot be divided into equal parts by any plane. They are said to be asymmetrical. The symmetrical animals show one of the three types of symmetry: spherical, radial and bilateral.

Section – 1 Spherical (universal) symmetry:
Body is divisible into similar halves by any plane passing through the center. Example: Seaurchin

Section – 2 Radial symmetry:
Body is with a number of similar parts radiating out from a central axis. The boy can be split up into equal halves by any plane passing through the middle from top to bottom. The animals with radial symmetry are called Radiata. Examples: Sponges and most coelenterates.
In some animals, some parts of the body are single or paired rather than radial, so that only one or two planes through the longitudinal axis divide the animal into similar halves. Such a symmetry is called biradial. Examples: Comb-jellies, seanemones and seastars.

Section –3 Bilateral symmetry:
In rest of the Metazoa, chief organs of the body are paired and are arranged on the sides of a central axis connecting the head with the tail. With the result, the body can be divided into two similar halves only by one plane. Such are termed as bilateral. The bilaterally symmetrical animals are termed the bilateria.
A plane or section passing dorso-ventrally (vertically) through the middle of the anterior-posterior (longitudinal) axis is known as the Sagittal plane or section. This is the plane that divides the bilateral animal into similar and right and left halves.
A plane or section running antero-posteriorly at right angles to the sagittal plane is called the frontal plane or section. It divides the body into dorsal and ventral halves.
A plane or section passing dorso-ventrally at right angles to the antero-posterior axis is known as transverse plane or section. It divides the body into anterior and posterior halves.

Section – 4 Digestive tract:
The Mesozoa and parazoa lack a digestive tract. The Eumetazoa possess a digestive tract, and are termed the Enterozoa. Certain enterozoa, viz., coelenterata,Ctenophora and Platyhelminthes, have incomplete digestive tract, which has a single external opening, the mouth, that serves for intake of food as well as elimination of faeces. All other enterozoa have complete digestive tract, which has two external openings, the mouth for intake of food and the anus for elimination of faeces.

Section – 5 Coelom:
Some bilateria have no cavity in their body, except that in the digestive tract. Such forms are known as the acoelomates. All others possess a cavity in addition to the one in the digestive tract. The space between the two cavities is called the body cavity the nature and origin of this cavity varies in different groups. When lined by a mesodermal epithelium, the peritoneum, it is known as the true coelom, and when unlined by peritoneum, it is termed false coelom or pseudocoel. The animals with false and true coelom are respectively called the pseudocoelomates end eucoelomates. In Mollusca and Arthropoda the coelom is greatly reduced, and the blood fills the spaces between the internal organs. Such a body-cavity is called the haemocoel. The coelom is of great significance in animal evolution. It provides space for the development of various organ systems.

Section – 6 Metamerism or Segmentation:
Segments occur in three phyla: Annelida, Arthropoda and Chordata. Each segment is called a metamere or somite. Metamerism affects external and internal structures. It is said to be homonomous, if the somites are similar, as in earthworm (annelida); and heteronomous, is the segments are dissimilar as in prawn (Arthropoda).The segmentation in Chordates shows up clearly in the embryonic stage in te segmentally arranged mesodermal somites. In adult chordates segmentation is largely internal and is seen in their vertebrae, ribs, nerves and blood vessels.