Structural Organisation in Animals

1. Chapter at a Glance

In unicellular organisms, all functions like digestion, respiration and reproduction are performed by a single cell. In the complex body of multicellular animals, the same basic functions are carried out by different groups of cells in a well-organised manner. Cells that are similar in structure and function, together with intercellular substances, form tissues. Tissues are organised in specific proportions and patterns to form organs like the stomach, lung, heart and kidney. When two or more organs perform a common function by their physical and/or chemical interaction, they together form an organ system, e.g. the digestive and respiratory systems. This chapter covers the four basic animal tissues — epithelial, connective, muscular and neural — and the morphology and anatomy of the frog as a representative vertebrate. NEET asks one or two questions every year, mostly on types of epithelia, cell junctions, connective tissues and frog features.

2. Epithelial Tissue

We commonly refer to an epithelial tissue as epithelium (plural: epithelia). This tissue has a free surface, which faces either a body fluid or the outside environment, and thus provides a covering or a lining for some part of the body. The cells are compactly packed with little intercellular matrix. There are two types of epithelial tissues, namely simple epithelium and compound epithelium. Simple epithelium is composed of a single layer of cells and functions as a lining for body cavities, ducts and tubes. Compound epithelium consists of two or more cell layers and has a protective function, as it does in our skin.

Some of the columnar or cuboidal cells get specialised for secretion and are called glandular epithelium. They are mainly of two types: unicellular, consisting of isolated glandular cells (goblet cells of the alimentary canal), and multicellular, consisting of clusters of cells (salivary gland). On the basis of the mode of pouring of their secretions, glands are divided into two categories, namely exocrine and endocrine glands.

• Exocrine glands secrete mucus, saliva, earwax, oil, milk, digestive enzymes and other cell products. These products are released through ducts or tubes.
• Endocrine glands do not have ducts. Their products, called hormones, are secreted directly into the fluid bathing the gland.

Cell junctions

All cells in epithelium are held together with little intercellular material. In nearly all animal tissues, specialised junctions provide both structural and functional links between its individual cells. Three types of cell junctions are found in the epithelium and other tissues:

3. Connective Tissue

Connective tissues are the most abundant and widely distributed in the body of complex animals. They are named connective tissues because of their special function of linking and supporting other tissues/organs of the body. They range from soft connective tissues to specialised types, which include cartilage, bone, adipose and blood. In all connective tissues except blood, the cells secrete fibres of structural proteins called collagen or elastin. The fibres provide strength, elasticity and flexibility to the tissue. These cells also secrete modified polysaccharides, which accumulate between cells and fibres and act as matrix (ground substance). Connective tissues are classified into three types: (i) loose connective tissue, (ii) dense connective tissue and (iii) specialised connective tissue.

4. Muscle Tissue

Each muscle is made of many long, cylindrical fibres arranged in parallel arrays. These fibres are composed of numerous fine fibrils, called myofibrils. Muscle fibres contract (shorten) in response to stimulation, then relax (lengthen) and return to their uncontracted state in a coordinated fashion. Their action moves the body to adjust to changes in the environment and to maintain the positions of the various body parts. In general, muscles play an active role in all the movements of the body. Muscles are of three types: skeletal, smooth and cardiac.

5. Neural Tissue

Neural tissue exerts the greatest control over the body's responsiveness to changing conditions. Neurons, the unit of neural system, are excitable cells. The neuroglial cells, which constitute the rest of the neural system, protect and support neurons. Neuroglia make up more than one-half the volume of neural tissue in our body. When a neuron is suitably stimulated, an electrical disturbance is generated which swiftly travels along its plasma membrane. Arrival of the disturbance at the neuron's endings, or output zone, triggers events that may cause stimulation or inhibition of adjacent neurons and other cells (studied in detail in Neural Control and Coordination).

6. Frog: Habit and Habitat

Frogs can live both on land and in fresh water and belong to class Amphibia of phylum Chordata. The most common species of frog found in India is Rana tigrina. They do not have constant body temperature, i.e. their body temperature varies with that of the environment. Such animals are called cold-blooded or poikilotherms. You might have also noticed changes in the colour of the frogs while they are in grasses and on dry land. They have the ability to change colour to hide them from their enemies (camouflage). This protective coloration is called mimicry. You may also know that frogs are not seen during the peak summer and winter. During this period they take shelter in deep burrows to protect them from extreme heat and cold. This is called summer sleep (aestivation) and winter sleep (hibernation), respectively.

7. Morphology of Frog

• The skin is smooth and slippery due to the presence of mucus. The skin is always maintained in a moist condition.
• The colour of the dorsal side of the body is generally olive green with dark irregular spots. On the ventral side, the skin is uniformly pale yellow.
• The frog never drinks water but absorbs it through the skin.
• The body of a frog is divisible into head and trunk. A neck and tail are absent.
• Above the mouth, a pair of nostrils is present. Eyes are bulged and covered by a nictitating membrane that protects them while in water.
• On either side of the eyes, a membranous tympanum (ear) receives sound signals.
• The forelimbs and hind limbs help in swimming, walking, leaping and burrowing. The hind limbs end in five digits and they are larger and muscular than the forelimbs, which end in four digits. Feet have webbed digits that help in swimming.
• Frogs exhibit sexual dimorphism. Male frogs can be distinguished by the presence of sound-producing vocal sacs and also a copulatory pad on the first digit of the forelimbs, which are absent in female frogs.

8. Anatomy of Frog

The body cavity of frogs accommodates different organ systems such as digestive, circulatory, respiratory, nervous, excretory and reproductive systems, with well-developed structures and functions.

Digestive system

• The digestive system consists of the alimentary canal and digestive glands. The alimentary canal is short because frogs are carnivores, and hence the length of the intestine is reduced.
• The mouth opens into the buccal cavity, which leads to the oesophagus through the pharynx. The oesophagus is a short tube that opens into the stomach, which in turn continues as the intestine, rectum and finally opens outside by the cloaca.
• The liver secretes bile that is stored in the gall bladder. The pancreas, a digestive gland, produces pancreatic juice containing digestive enzymes.
• Food is captured by the bilobed tongue. Digestion of food takes place by the action of HCl and gastric juices secreted from the walls of the stomach. Partially digested food called chyme is passed from the stomach to the first part of the intestine, the duodenum. The duodenum receives bile from the gall bladder and pancreatic juices from the pancreas through a common bile duct. Bile emulsifies fat, and pancreatic juices digest carbohydrates and proteins. Final digestion takes place in the intestine. Digested food is absorbed by the numerous finger-like folds in the inner wall of the intestine called villi and microvilli. The undigested solid waste moves into the rectum and passes out through the cloaca.

Respiratory system

Frogs respire on land and in water by two different methods. In water, the skin acts as an aquatic respiratory organ (cutaneous respiration). Dissolved oxygen in the water is exchanged through the skin by diffusion. On land, the buccal cavity, skin and lungs act as the respiratory organs. The respiration by lungs is called pulmonary respiration. The lungs are a pair of elongated, pink-coloured sac-like structures present in the upper part of the trunk region (thorax). Air enters through the nostrils into the buccal cavity and then to the lungs. During aestivation and hibernation, gaseous exchange takes place through the skin.

Circulatory system

• The vascular system of the frog is well developed, closed type. Frogs also have a lymphatic system.
• The blood vascular system involves heart, blood vessels and blood. The lymphatic system consists of lymph, lymph channels and lymph nodes.
• The heart is a muscular structure situated in the upper part of the body cavity. It has three chambers: two atria and one ventricle, and is covered by a membrane called pericardium. A triangular structure called sinus venosus joins the right atrium. It receives blood through the major veins called vena cava. The ventricle opens into a sac-like conus arteriosus on the ventral side of the heart.
• The blood from the heart is carried to all parts of the body by the arteries (arterial system). The veins collect blood from different parts of the body to the heart and form the venous system.
• Special venous connections between the liver and intestine as well as the kidney and lower parts of the body are present in frogs. The former is called the hepatic portal system and the latter is called the renal portal system.
• The blood is composed of plasma and cells. The blood cells are RBC (red blood cells) or erythrocytes, WBC (white blood cells) or leucocytes, and platelets. RBCs are nucleated and contain the red-coloured pigment, namely haemoglobin. The lymph is different from blood: it lacks a few proteins and RBCs. The blood carries nutrients, gases and water to the respective sites during circulation. The circulation of blood is achieved by the pumping action of the muscular heart.

Excretory system

The elimination of nitrogenous wastes is carried out by a well-developed excretory system. The excretory system consists of a pair of kidneys, ureters, cloaca and urinary bladder. These are compact, dark red and bean-like structures situated a little posteriorly in the body cavity on both sides of the vertebral column. Each kidney is composed of several structural and functional units called uriniferous tubules or nephrons. Two ureters emerge from the kidneys in the male frogs. The ureters act as urinogenital ducts, which open into the cloaca. In females the ureters and oviduct open separately in the cloaca. The thin-walled urinary bladder is present ventral to the rectum, which also opens in the cloaca. The frog excretes urea and thus is a ureotelic animal. Excretory wastes are carried by blood into the kidney, where they are separated and excreted.

Control and coordination

The system for control and coordination is highly evolved in the frog. It includes both the neural system and endocrine glands. The chemical coordination of various organs of the body is achieved by hormones, which are secreted by the endocrine glands. The prominent endocrine glands found in a frog are pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads.

The nervous system is organised into a central nervous system (brain and spinal cord), a peripheral nervous system (cranial and spinal nerves) and an autonomic nervous system (sympathetic and parasympathetic). There are ten pairs of cranial nerves arising from the brain. The brain is enclosed in a bony structure called the brain box (cranium). The brain is divided into fore-brain, mid-brain and hind-brain. The forebrain includes olfactory lobes, paired cerebral hemispheres and an unpaired diencephalon. The midbrain is characterised by a pair of optic lobes. The hind-brain consists of the cerebellum and medulla oblongata. The medulla oblongata passes out through the foramen magnum and continues into the spinal cord, which is enclosed in the vertebral column.

Sense organs

The frog has different types of sense organs, namely organs of touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes) and hearing (tympanum with internal ears). Out of these, eyes and internal ears are well-organised structures and the rest are cellular aggregations around nerve endings. Eyes in a frog are a pair of spherical structures situated in the orbit in the skull. These are simple eyes (possessing only one unit). External ear is absent in frogs and only the tympanum can be seen externally. The ear is an organ of hearing as well as balancing (equilibrium).

Reproductive system

• Frogs have well-organised male and female reproductive systems. Male reproductive organs consist of a pair of yellowish ovoid testes, which are found adhered to the upper part of the kidneys by a double fold of peritoneum called mesorchium. Vasa efferentia are 10–12 in number; they arise from the testes, enter the kidneys on their side and open into the Bidder's canal. Finally it communicates with the urinogenital duct that comes out of the kidneys and opens into the cloaca. The cloaca is a small, median chamber that is used to pass faecal matter, urine and sperms to the exterior.
• The female reproductive organs include a pair of ovaries. The ovaries are situated near the kidneys, and there is no functional connection with kidneys. A pair of oviducts arising from the ovaries opens into the cloaca separately. A mature female can lay 2500 to 3000 ova at a time.
• Fertilisation is external and takes place in water. Development involves a larval stage called tadpole. The tadpole undergoes metamorphosis to form the adult.

9. Economic Importance of Frogs

Frogs are beneficial to humankind because they eat insects and protect the crop. Frogs maintain ecological balance because they serve as an important link of the food chain and food web in the ecosystem. In some countries, the muscular legs of frogs are used as food by man.

10. Master Summary Table

11. Mind Map

12. Tips and Tricks

13. Important One-liners

• Squamous epithelium forms a diffusion boundary in blood vessels and alveoli.
• Goblet cells are unicellular glands of the alimentary canal.
• Exocrine glands have ducts; endocrine glands are ductless.
• Connective tissue is the most abundant tissue in complex animals.
• Fibroblasts, macrophages and mast cells occur in areolar tissue.
• Bone marrow is the site of blood cell production.
• Intercalated discs let cardiac muscle cells contract as a unit.
• The common Indian frog is Rana tigrina.
• The nictitating membrane protects the frog's eyes in water.
• Frogs have ten pairs of cranial nerves.
• Vasa efferentia (10–12) open into Bidder's canal in male frogs.
• A female frog lays 2500–3000 ova at a time.

14. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

15. Quick Revision Summary