The following points highlight the fourteen types of organs found in animal body. The types are: 1. Digestive Tract 2. Digestion 3. Heterotrophic Mode of Nutrition 4. Active Movement 5. Blood Vascular System 6. Respiration 7. Lungs 8. Anaerobic Respiration 9. Excretion 10. Neural System 11. Receptors 12. Endocrine Glands 13. Skeleton 14. Notochord.
Type # 1. Digestive Tract:
Digestive tract is the passage where food is taken for digestion, absorption and elimination of undigested food. Cnidarians and flat-worms have a digestive tract with a single opening termed mouth that takes food as well as eliminates the undigested food.
This type of digestive tract is called incomplete. Digestive tract of many animals has two openings; mouth for intake of food and anus for the elimination of faecal matter. Such a digestive tract is termed complete. It is found from round worms to mammals.
Type # 2. Digestion:
Digestion is the breaking down of complex organic food molecules (carbohydrates, fats, proteins and nucleic acids) into simpler organic molecules by hydrolysis. Digestion is of two types; intracellular digestion and extracellular digestion.
Type # 3. Heterotrophic Mode of Nutrition:
Heterotroph is an organism that cannot use inorganic materials to synthesize the organic compounds needed for growth but obtains them by feeding on other organisms or their products. Examples: Carnivore, herbivore, omnivore, parasite, scavenger or saprophyte (All animals, fungi and most bacteria are heterotrophs). The nutrition is called heterotrophic nutrition.
Type # 4. Active Movement:
As compared to members of other kingdoms, animals perform more rapid and complex way of movement. Movement of animals is due to the flexibility of their cells, which is perhaps the most characteristic feature of the animals. Some animals can swim (e.g. jelly fish, squids, fishes, whales, etc.), some can fly (butterflies birds bats) and others can run or walk on land (e.g. horses, cattle, tigers, lions, men, etc.)
Type # 5. Blood Vascular System:
It is a system that takes part in continuous flow of blood in the body. Blood is not present in porifera, cnidaria, ctenophora, Platyhelminthes and Aschelminthes (Nemathelminthes).
Blood vascular system is of two types, closed and open:
(i) In closed type, the blood circulates inside the blood vessels without ever coming in direct contact with the body cells, e.g. annelids, chordates.
(ii) In open type the blood flows in open spaces like lacunae and sinuses.
It bathes the cells directly, e.g. arthropods, molluscs. Blood may be colorless (e.g. insects), bluish due to a respiratory pigment, the haemocyanin (e.g., prawn, pila) or red due to respiratory pigment, the hemoglobin (e.g. earthworm, vertebrates).
Type # 6. Respiration:
Aerobic animals utilize oxygen which is obtained from air or water. Certain aquatic animals like Hydra take oxygen and give up carbon dioxide through body surface. This is termed body-surface respiration.
Many animals such as prawn, unio, and fish have gills for exchange of gases. This type of respiration is called branchial respiration. In earthworm, leech, frog, etc., exchange of gases takes place through skin; this is termed as cutaneous respiration.
In insects and other arthropods, there are present tubular structures, the tracheae for exchange of gases, this is called tracheal respiration. In arachnids (e.g., spiders) exchange of gases takes place through book-like structures, the book-lungs. Book gills are respiratory organs in king crabs.
Type # 7. Lungs:
Lungs are meant for exchange of gases in many animals (e.g. amphibians, reptiles, birds, mammals). This type of respiration is termed pulmonary respiration. In certain animals like frogs and toads exchange of gases also takes place through the epithelial lining of the buccopharyngeal cavity.
This is called buccopharyngeal respiration. The frogs have three modes of respiration namely cutaneous, buccopharyngeal and pulmonary. Lung fishes respire through both gills and lungs.
Type # 8. Anaerobic Respiration: (in absence of oxygen) occurs in various parasitic animals.
Type # 9. Excretion:
Excretion is the removal of the metabolic wastes from the body. Sponges, cnidarians, ctenophores and echinoderms lose metabolic wastes by diffusion through body surface. Many animals have definite excretory structures. The flat worms have flame cells as excretory structures. In Ascaris (round worm), ‘H’ shaped excretory system of canals and complicated giant cell called renette cell is present.
Annelids have nephridia as excretory organs and crustaceans such as prawn contain antennary (green) glands in the antennae as excretory organs. Insects and some other arthropods have Malpighian tubules as excretory organs. Molluscs and vertebrates have kidneys as excretory organs. Excretory organs not only remove metabolic wastes but also maintain water and salt content in the body (osmoregulation).
Animals are also classified on the basis of the removal of nitrogenous wastes.
1. Ammonotelic Animals:
They excrete ammonia. Examples: protozoans (e.g. Amoeba, Paramecium), sponges (e.g. Sycon), cnidarians or coelenterates (e.g. Hydra), Liver fluke, Tape worm, Ascaris, Nereis, Earthworm, Leech, Prawn, Pila, Bony fish (e.g. Labeo), Amphibian tadpoles, tailed amphibians (e.g. Salamanders), Crocodiles.
2. Ureotelic Animals:
They excrete urea. Examples: Cartilaginous fishes (e.g., Sharks and Rays), semi-aquatic amphibians (e.g., frogs and toads), turtles, alligators, mammals including man. Ascaris and Earthworm are both ammotelicno and ureotelic.
3. Uricotelic Animals:
They excrete uric acid, Examples: Most insects, some land crustaceans (e.g., Ouiscus- commonly known as wood louse), land snails (e.g., Helix- commonly Called “Land snail”), land reptiles (lizards and snakes), birds.
4. Amino-telic Animals:
They excrete excess amino acids. Examples: Some molluscs (e.g., Limnaea, Unio), some echinoderms (e.g. Asterias— star fish).
5. Guano-telic Animals:
They excrete guanine. Example: Spider.
Dual Excretion:
Some animals perform two modes of excretion. That is called dual excretion. Earthworms excrete ammonia when sufficient water is available while they excrete urea instead of ammonia in drier surroundings. When lung fishes and Xenopus (African toad) live in water they are normally ammonotelic but they become ureotelic when they lie immobile in moist air or mud during their metamorphosis.
Crocodiles spend most of their time in water and are normally ammonotelic but when kept out of water the excretion of urea and uric acid increases. Frog’s tadpoles excrete ammonia in water but semi-aquatic frogs excrete urea.
Type # 10. Neural System:
Neural system is a system of neurons (nerve cells) that take part in conduction of impulses and coordination of body activities. Sponges do not possess nerve cells. Cnidarians have network of nerve cells that form “primitive type” of neural system. In flat worms and round worms the neural system consists of nerve ring around the anterior part of the alimentary canal and many nerve cords.
In annelids and arthropods it comprises a nerve ring around the anterior part of the alimentary canal and a double ventral solid nerve cord containing ganglia. In molluscs, the neural system consists of ganglia, commissures and connectives. A commissure joins two similar ganglia. A connective connects two different ganglia. In echinoderms there are usually present oral and aboral nerve rings and radial nerves.
Chordates have dorsally placed hollow nerve cord. In vertebrates the neural system consists of three parts; central neural system, peripheral neural system and autonomic neural system. The central neural system comprises the brain and spinal cord. The brain is situated in the cranium (brain box) of the skull and the spinal cord lies in the neural canal of the vertebral column (back bone).
The nerves which arise from the central neural system constitute the peripheral neural system. The latter includes the cranial nerves arising from the brain and the spinal nerves originating from the spinal cord. The autonomic neural system comprises the sympathetic and parasympathetic neural systems.
Type # 11. Receptors:
A receptor is a sensory cell or organ which receives stimuli (changes in the environment) from outside or inside the animal and passes impulses to the neural system. The receptors are always connected with the central neural system by means of sensory nerve fibres. Different types of receptors such as eyes, ear, skin etc. are found in various animals.
Type # 12. Endocrine Glands:
These glands are also called ductless glands. Their secretions are known as hormones. The latter are mostly transported by blood from the endocrine glands to the target cells or organs for their physiological activity.
Type # 13. Skeleton:
Hard external or internal structures of animal body constitute the skeleton. Skeleton supports and protects soft parts of the body. It is of two types; exoskeleton and endoskeleton.
Exoskeleton lies outside the body of an animal and made up of non-living materials. Examples of exoskeleton are chitinous cuticle of arthropods, calcareous shell of molluscs, and scales, feathers, hair, horns, hoofs, claws, nails and hair of vertebrates.
Endoskeleton lies entirely within the body of the animal. In vertebrates, it is composed of living hard connective tissues, namely cartilages and bones. In invertebrates such as sponges it is made up of calcareous and siliceous spicules. In some echinoderms (e.g., star fish) calcareous plates, the ossicles, which form the endoskeleton, are found in the dermis of the body wall.
Type # 14. Notochord:
It is a solid flexible rod like structure found in all chordates. It is derived from the mesoderm.