The following points highlight the ten main types of systems found in the body of an animal. The types are: 1. Integumentary System 2. Skeletal System 3. Muscular System 4. Digestive or Alimentary System 5. Respiratory System 6. Blood Vascular System 7. Excretory System 8. Nervous System 9. Endocrine System 10. Reproductive System.

Type # 1. Integumentary System:

It forms the skin in most animals. The outermost layer of the skin may be uni- or multilayered and known as epidermis.

a. In many animals the epidermal cells secrete a non-cellular substance, the cuticle, covering the outer surface. In some the cuticle thickens and hardens to chitin.

b. Protective structures like hairs (mam­mals), feathers (birds), scales (snakes, lizards) develop from the epidermis.

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c. Sensory cells present in the epidermis act as receptors for stimuli.

d. Glands—protective and of vital func­tions, develop from the dermis, the layer below epidermis. In fishes the scales are dermal in origin.

Type # 2. Skeletal System:

It includes all the hard structures of ani­mal body, either protecting the soft parts or adding rigidity to the body or serving for attachment of muscles. The skeleton situated outside the body and covering it is called external skeleton or exoskeleton. That present within the body is internal skeleton or endoskeleton.

The exoskeleton is formed by the thickening and hardening of cuticle secreted by the epidermis or dermis. The endoskeleton consists of cartilage and bone. Both the exo- and endoskeleton are movably jointed and the muscles are attached to them for movement.

Type # 3. Muscular System:

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A muscle is the aggregate of a large num­ber of muscle fibres into a bundle which has the capacity to contract lengthwise.

In some cases the two ends of a muscle are attached to skeletal system and with the contraction of muscles corresponding parts are drawn nearer. In others the muscles are not con­nected with the skeleton but arranged one layer after another in such a fashion that the direction of contraction of different muscular layers are different.

Type # 4. Digestive or Alimentary System:

It is a system of cavities in which food is received, digested, nutritive substances absorbed and the faeces thrown out. The gastrointestinal tract is lined with epithelial secretory cells, and it can be divided into three distinct regions—ingestive, digestive and absorptive and egestive. Mouth cavity to gullet which receives food and prepare it for digestion is the ingestive part.

Stomach to small intestine in which the food is digested and nutritive materials absorbed is the diges­tive and absorptive part. Various digestive glands—liver, pancreas open into this region. The posterior portion of the digestive tract or rectum, in which undigested portion of food or faeces is temporarily stored in its passage to outside through the anus, is the egestive part.

Type # 5. Respiratory System:

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The system responsible for gaseous exchange between the animal body and the environment is called respiratory system. The phenomenon is respiration.

The gaseous exchange, i.e., absorption of O2 and release of CO2, takes place in respiratory organs. In aquatic forms, gills and in terrestrial animals, lungs are the chief respiratory organs. In addition, the skin and other accessory struc­tures take part in respiration in some ani­mals. Animals without definite respiratory structures respire through body surface.

Type # 6. Blood Vascular System:

It consists of a heart or central pumping station and narrow tube-like structures, the blood vessels. Vessels carrying blood to dif­ferent body parts from the heart are arteries and collectively named arterial system. The vessels carrying blood from body to the heart are veins and collectively called venous sys­tem. Such organised vascular system is ab­sent in lower animals.

Type # 7. Excretory System:

Organs meant for throwing out nitro­genous wastes from, the animal body are excretory organs, and the system is excretory system. The system varies widely in structure in different animals, starting from simple tubes, one end opening inside and the other end outside the body, to a complex kidney in vertebrates.

Type # 8. Nervous System:

It is composed of nerve cells (neurons) and supporting cells (neuroglial cells).

Ner­vous system maintains communication be­tween different parts of the body and also between the body and the environment. Nervous system assumes a net-like structure in lower animals (Hydra) but it is highly organised in higher animals. It consists of a central nervous system (brain and spinal cord), and a peripheral nervous system with afferent and efferent nerve fibres.

The brain is the controlling centre. Messages are sent to the brain through afferent (receptor) nerve fibres and impulses are carried to different body parts through efferent (motor) neurons. Eyes, nose, tongue and ears are highly specialised sense organs. They are intimately associated with nervous system.

Type # 9. Endocrine System:

Hormones secreted by endocrine glands constitute endocrine system in animals. Endocrine glands are highly specialized struc­tures and ductless and the hormones are extremely complex chemical compounds. Hormones are capable of inducing physio­logical changes in the target organs located at a distance.

They are moved from the sites of origin to targets through blood or through axons of the neurons or through diffusion.

Type # 10. Reproductive System:

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The system or set of organs by which reproduction takes place in animals. The organs are ovaries and testes, associated struc­tures, and their ducts. Animals having both the sexes in an individual are hermaphrodite and those bearing only one sex are unisexual.

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