In this article we will discuss about the physical organization of the animal body.
The animal body is composed of one or more cells. The simplest animals are the unicellular protozoa. In such a primitive group, a single cell forms the entire organism, and the living matter of the cell has to carry on all the vital activities, such as nutrition, respiration, excretion, growth and reproduction.
The higher animals are the multicellular metazoa. All metazoa are composed of a large number of cells. In such a creature, there is marvelous unity and division of labour among the various cells which form the body.
Metazoan cells differ in shape and size according to the special nature of the function which they are to perform. Some of the cells carry on respiration, others nutrition and still others reproduction. Thus, there is a physiological division of labour.
Tissues:
In higher metazoa, cells are aggregated to form tissues. A tissue may be defined as a collection of similar cells adapted to carry out the same general function. Four principal types of tissues are recognised in all higher animals.
These are:
(1) Epithelial tissue,
(2) Connective or supporting tissue,
(3) Muscular tissue, and
(4) Nervous tissue.
Epithelial tissues consist of cells which cover surfaces and line up cavities. Epithelial cells are usually arranged in such a way as to form a membrane. They may also be modified to form solid or hollow structures, called glands, which either secrete useful substances or excrete waste products.
Connective tissues include bones, cartilages, other supporting materials and blood. It is composed of only a few cells and fibres. The main bulk of the tissue is a non-living intercellular substance called matrix which is secreted by the cells. This ground substance hardens up in bones and cartilages.
Muscular tissues form the muscles of the body. A muscle is composed of muscle cells which are adapted to contraction and relaxation, thereby causing movement and locomotion. Nervous tissue is composed of nerve cells and nerve fibres. It is capable of transmitting impulses from one part of the body to another.
Thus with the increase of structural complexity, the body cells become more and more specialised and there is a greater tendency to develop tissues.
Organs:
A single tissue does not usually work alone. The efficiency is greatly increased when two or more tissues function together. An organ may be defined as an association of tissues for performing a special function.
The lung, for example, is an organ for carrying out respiration; it is composed mainly of epithelial and connective tissues. Similarly, the human hand is an organ for manipulating and grasping objects; it consists of all the four kinds of tissues.
Organ Systems:
An isolated organ cannot work satisfactorily. Several organs act together in the metazoan body for effecting a particular important function. Such a group of organs is known as an organ system. The body of a fish or a frog is comparable to a huge factory with several separate departments.
Each department is entrusted with a special kind of job and the various departments act in cooperation with one another. The organ systems, just mentioned, are the various departments of the animal machinery. Each system performs its own part and the benefit is shared by the others.
The organ systems which are found in the higher metazoa are:
(1) Integumentary,
(2) Skeletal,
(3) Muscular,
(4) Alimentary,
(5) Circulatory,
(6) Respiratory,
(7) Excretory,
(8) Reproductive,
(9) Nervous, and
(10) Endocrine.
Figure 4 illustrates the organ systems of the common toad.
1. Integumentary System:
Integumentary system includes the skin and its various outgrowths, such as hairs, nails, scales and feathers. The principal function of this system is protective. But-the skin is a ‘jack of all trades’ and often helps in the performance of other functions, such as respiration, excretion and regulation of body temperature.
2. Skeletal System:
Skeletal system consists of the main supporting framework of the body. It is composed of bones, cartilages, ligaments and other hard structures. A skeletal framework lying outside the body in animals, like crabs and snails is known as the exoskeleton. It protects soft parts from external injury.
In vertebrates, there is an endoskeleton inside the body which functions as the main prop for support and protects internal organs. Moreover, the shape of the animal depends, to some extent, on the shape of its skeleton.
3. Muscular System:
Muscular system is composed of muscles attached to bones, and also of muscles arranged on the walls of organs such as heart, digestive canal, etc. The muscles are the motive power in the physical machinery of an animal and serve to produce movement and locomotion.
The skeletal muscles are said to be voluntary because they are contracted and relaxed according to the wish of the animal possessing them. Heart muscles and those on the wall of other soft organs are said to be involuntary because they act independently of the wish of their possessor.
4. Alimentary or Digestive System:
Alimentary or digestive system is concerned with taking in of food, preparing it for absorption by all the cells of the body, and then egesting the indigestible residue. It consists of a long tube running through the body from end to end, and composed of mouth, pharynx, oesophagus, stomach, intestines and anus.
Associated with the alimentary tube, there are digestive glands such as liver, pancreas, etc., which secrete enzymes for digesting food.
5. Circulatory or Vascular System:
Circulatory or vascular system brings about transportation of food, oxygen, and waste products. It consists of organs such as heart, arteries, veins, etc. The transporting medium is the blood, which is pumped into the arteries by the heart. The veins bring the blood back to the heart, thus making a complete circuit.
6. Respiratory System:
Respiratory system constitutes the oxygen-intake apparatus of the animal body. Terrestrial’ animals inhale oxygen into organs called lungs: but most of the aquatic animals breathe oxygen dissolved in water by special organs known as gills.
Blood absorbs oxygen from the lungs or gills and transports the gas to the tissues, where combustion and release of energy occurs and carbon dioxide is formed as a by-product. This carbon dioxide is carried by the blood back to the lungs or gills, from where it is expelled.
7. Excretory System:
Excretory system removes waste products of metabolism from the animal body. Blood carries away waste products from all tissues to the excretory organs, which collect and throw them out. In animals like earthworms and leeches, the process is accomplished by a set of tubes called nephridia. It vertebrates, the principal excretory organs are the kidneys which expel nitrogenous waste products in the form of urine.
8. Reproductive System:
Reproductive system essentially consists of the gonads. The male gonads are the testes (singular, testis) and the female gonads are the ovaries (singular, ovary).
The gonads are often associated with the excretory organs and when this happens, the two separate systems are described together as the urinogenital system. The gonads by manufacturing and releasing germ cells (spermatozoa in males and ova or eggs in females) are responsible for the/production of young individuals of the same species.
9. Nervous System:
Nervous system enables the different parts of the body to work together, so that the organism can function as a whole. It is composed of organs such as brain, spinal cord, nerves, etc. Associated with the nervous system there are receptor organs, such as eyes, ears, nose, etc., each of which is expert in the perception of a special kind of sensation.
The nerves are of two kinds:
(1) Afferent or sensory and
(2) Efferent or motor.
An afferent or sensory nerve connects a receptor organ with either the brain or the spinal cord. A motor nerve runs the opposite way; it connects the brain or cord with an effector organ, that is, either a muscle or a gland.
The manner in which nervous co-ordination occurs is best explained by a simple example. The smell or sight of food is perceived by the brain via sensory nerves. The brain promptly decides the steps that are to be taken and sends down its orders along the motor nerves.
The hands are directed to grasp the food and put it into the mouth; the teeth, tongue and pharynx are then prompted to chew and swallow the food step by step in a co-ordinated manner. Lastly, the digestive glands are ordered by the brain to secrete enzymes for digesting the food that has been swallowed.
10. Endocrine System:
Endocrine system consists of a number of ductless glands, such as thyroid, pituitary, adrenal, etc., Much release chemical substances called hormones. The hormones are washed away by the blood and carried to every part of the body. By the combined effort of these chemical substances the animal body regulates its growth and metabolism.
Thus a higher animal possesses two coordinating systems:
(1) The nervous system for producing nervous co-ordination, and
(2) The endocrine system for effecting chemical co-ordination.
The organ systems as listed above, are best studied in a familiar animal. The common toad., Bufo melanostictus, is very suitable for this purpose. It is abundantly found in all parts of Indian subcontinent, and can, therefore, be procured easily for laboratory work almost throughout the year.
The toad is a typical vertebrate. The animal is large enough for a beginner to dissect easily. It has, therefore, become a convention to begin zoology with the study of toad, which, in fact, is called the grammar of Zoology. The anatomical peculiarities of the frog are somewhat similar and hence frogs are good substitutes in places where they are easily available.
