Structural Organisation in Animals
Welcome to HSLC Guru! This chapter explores the levels of organisation in animal bodies — from cells grouped into tissues, tissues forming organs, and organs working together as organ systems. We will study the four basic types of animal tissues and examine the morphology and anatomy of three representative animals prescribed in the ASSEB Class 11 Biology syllabus: earthworm (Pheretima), cockroach (Periplaneta), and frog (Rana). Detailed question-answers, MCQs, fill-in-the-blanks, true/false statements and a glossary are included to help you master the chapter.
Summary
Animal Tissues: A tissue is a group of similar cells along with intercellular substances performing a specific function. Animal tissues are classified into four basic types — epithelial, connective, muscular and neural. Epithelial tissue has compactly packed cells with little intercellular matrix. It is divided into simple epithelium (single layer) and compound epithelium (two or more layers). Simple epithelium includes squamous (flat, found in walls of blood vessels and air sacs of lungs), cuboidal (cube-like, in ducts of glands and tubular parts of nephron), columnar (tall pillar-like, lining stomach and intestine), ciliated (bearing cilia, in inner surface of bronchioles and fallopian tubes) and glandular epithelium. Glandular epithelium forms exocrine glands (secrete through ducts — saliva, milk, mucus) and endocrine glands (ductless, secrete hormones into blood). Compound epithelium is multi-layered (e.g. dry skin surface, lining of buccal cavity) and provides protection.
Connective Tissue: The most abundant and widely distributed tissue. Cells are loosely spaced in an abundant matrix. Types include — Loose connective tissue: areolar (filler tissue between skin and muscles, supports epithelium) and adipose (stores fat beneath skin); Dense connective tissue: dense regular (tendons connect muscle to bone, ligaments connect bone to bone) and dense irregular (skin); Specialised connective tissue: cartilage (solid, pliable matrix; in nose tip, ear pinna, intervertebral discs), bone (hard calcified matrix; femur, ribs etc., supports the body), blood (fluid connective tissue with plasma, RBC, WBC and platelets) and lymph. Muscular Tissue: consists of long contractile cells called muscle fibres. Skeletal (striated) muscle is attached to bones, voluntary, with striations and multiple peripheral nuclei. Smooth (visceral) muscle is found in walls of internal organs (stomach, intestine, blood vessels), spindle-shaped, involuntary, non-striated. Cardiac muscle is exclusive to the heart wall, branched, striated, involuntary, with intercalated discs.
Neural Tissue: exerts the greatest control over body responsiveness. The unit of neural tissue is the neuron, an excitable cell with a cell body, dendrites and an axon. Neurons transmit nerve impulses from one part of the body to another. Supporting cells called neuroglia (glial cells) make up more than half of the volume of neural tissue and protect and support neurons. An organ is made of two or more tissues organised together to perform a specific function (e.g. stomach, heart, lungs). An organ system is a group of organs working together to perform broader functions (e.g. digestive, respiratory, circulatory systems).
Earthworm (Pheretima posthuma): a reddish-brown terrestrial invertebrate with elongated cylindrical body of 100–120 segments. Mouth lies in segment 1; clitellum (glandular thickening) lies in segments 14–16; genital openings are present. Anatomy: alimentary canal is a straight tube — mouth, buccal cavity, pharynx, oesophagus, gizzard, stomach, intestine and anus. Closed circulatory system with blood vessels, capillaries and hearts (lateral, lateral-oesophageal, supra-oesophageal). Respiration is through moist skin (cutaneous). Excretion via segmental nephridia. Nervous system has nerve ring and ventral nerve cord. Hermaphrodite — testes in 10th and 11th segments, ovaries in 13th. Cross-fertilisation occurs through cocoons. Cockroach (Periplaneta americana): reddish-brown insect, body divided into head, thorax (pro-, meso-, metathorax) and abdomen (10 segments). Has compound eyes, antennae, biting mouthparts, two pairs of wings (mesothoracic tegmina and metathoracic membranous), three pairs of legs. Digestive system: mouth, pharynx, oesophagus, crop, gizzard, mesenteron, hepatic caecae, ileum, colon, rectum and anus. Open circulatory system with 13-chambered tubular heart. Respiration through tracheae and spiracles. Excretion by Malpighian tubules. Nervous system — supra-oesophageal ganglion (brain), nerve ring and ventral ganglionated nerve cord. Sexes are separate; sexual dimorphism is marked.
Frog (Rana tigrina): a cold-blooded amphibian. Body divisible into head and trunk. Skin is moist, smooth, slippery, with mucous glands; aids in cutaneous respiration. Forelimbs have four digits, hind limbs five webbed digits for swimming and leaping. Anatomy: alimentary canal short — mouth, buccal cavity, pharynx, oesophagus, stomach, small intestine, large intestine and cloaca. Liver and pancreas are digestive glands. Closed circulatory system with three-chambered heart (two atria, one ventricle), arteries, veins and lymphatic system. Respiration occurs through skin (cutaneous), buccal cavity (buccal) and lungs (pulmonary). Excretion by paired mesonephric kidneys; ureotelic. Nervous system has brain, spinal cord and nerves; brain divided into forebrain, midbrain and hindbrain. Sexes are separate; fertilisation external; development through tadpole stage (metamorphosis). The frog plays a vital ecological role and is a model organism for biology.
Short Answer Questions (1 Mark)
Q1. What is a tissue?
Answer: A tissue is a group of similar cells along with intercellular substances that perform a specific function in the body.
Q2. Name the four basic types of animal tissues.
Answer: Epithelial tissue, connective tissue, muscular tissue and neural tissue.
Q3. What is the function of squamous epithelium?
Answer: Squamous epithelium is involved in functions such as forming a diffusion boundary in the walls of blood vessels and air sacs of the lungs.
Q4. Differentiate between exocrine and endocrine glands.
Answer: Exocrine glands secrete products (mucus, saliva, oil, milk, digestive enzymes) through ducts, while endocrine glands are ductless and secrete hormones directly into the blood.
Q5. Where is adipose tissue located?
Answer: Adipose tissue is mainly located beneath the skin and around internal organs; its cells are specialised to store fats.
Q6. What are tendons and ligaments?
Answer: Tendons are dense regular connective tissues that attach skeletal muscles to bones; ligaments attach one bone to another.
Q7. Name the muscle present in the wall of the heart.
Answer: Cardiac muscle, characterised by branching fibres and intercalated discs.
Q8. What is the structural unit of neural tissue?
Answer: The neuron (nerve cell) is the structural and functional unit of neural tissue.
Q9. In which segments of the earthworm is the clitellum present?
Answer: The clitellum of Pheretima is present in segments 14, 15 and 16.
Q10. What is the excretory organ of cockroach?
Answer: Malpighian tubules are the principal excretory organs of the cockroach.
Short Answer Questions (2-3 Marks)
Q1. Distinguish between simple and compound epithelium.
Answer: Simple epithelium consists of a single layer of cells and functions as a lining for body cavities, ducts and tubes — for example, alveoli of lungs, ducts of glands, and inner lining of intestine. Compound epithelium has two or more cell layers and is mainly protective in nature, providing limited role in secretion or absorption. It covers dry surface of the skin, moist surface of the buccal cavity, pharynx, inner lining of ducts of salivary glands and pancreatic ducts.
Q2. Describe the three types of muscular tissue with one location each.
Answer: (i) Skeletal muscle — striated, voluntary, multinucleated and attached to skeletal bones (e.g. biceps of arm). (ii) Smooth muscle — non-striated, involuntary, spindle-shaped with single nucleus, found in walls of internal organs such as the stomach, intestine and blood vessels. (iii) Cardiac muscle — striated, branched, involuntary fibres joined by intercalated discs, exclusively found in the wall of the heart.
Q3. Write a short note on connective tissue with examples.
Answer: Connective tissues are widely distributed and link or support different tissues and organs. They have abundant matrix between loosely arranged cells. Loose connective tissues include areolar (in skin) and adipose (fat storage). Dense connective tissues include tendons and ligaments. Specialised connective tissues include cartilage (ear pinna, nose), bone (long bones), blood and lymph (fluid connective tissues that transport substances throughout the body).
Q4. Explain the structure and function of neuron.
Answer: A neuron has three parts — the cell body (cyton) contains the nucleus and cytoplasm with Nissl granules; dendrites are short processes that receive impulses from sensory cells or other neurons; the axon is a long process that conducts impulses away from the cell body to the next neuron, muscle or gland. Neurons are excitable cells that generate and transmit nerve impulses, enabling rapid communication and coordination within the body.
Q5. Describe briefly the digestive system of cockroach.
Answer: The alimentary canal of the cockroach is divided into three regions — foregut (mouth, pharynx, oesophagus, crop and gizzard/proventriculus), midgut (mesenteron with hepatic caecae for enzyme secretion) and hindgut (ileum, colon and rectum opening at the anus). Salivary glands open into the buccal cavity. Food is stored temporarily in the crop and ground in the gizzard, while digestion and absorption occur in the midgut.
Q6. Mention the modes of respiration in frog.
Answer: The frog respires by three methods — (i) Cutaneous respiration through the moist skin, especially when in water and during hibernation/aestivation; (ii) Buccal respiration through the mucous membrane of the buccal cavity when at rest on land; (iii) Pulmonary respiration through paired sac-like lungs during active life on land.
Long Answer Questions (5-7 Marks)
Q1. Describe the morphology and anatomy of earthworm (Pheretima posthuma).
Answer: Morphology: The earthworm has a long, narrow, cylindrical, reddish-brown body of about 100–120 segments (metameres). The dorsal surface is darker due to a dorsal blood vessel, and the ventral surface is paler. The first segment, the peristomium, bears the mouth covered by a fleshy lobe, the prostomium. Segments 14–16 are covered by a glandular thickening called the clitellum. Two pairs of male genital openings lie ventrally on segment 18, and a single female genital pore lies on segment 14. Four pairs of spermathecal apertures lie on segments 5–9. Each segment except the first, last and clitellum bears S-shaped setae that aid locomotion.
Anatomy: The body wall is covered by cuticle and epidermis with circular and longitudinal muscles. The alimentary canal is a long, straight tube — mouth, buccal cavity (1–3), pharynx (3–4), oesophagus (5–7), muscular gizzard (8–9), stomach (9–14), intestine (15 onwards) and anus. The circulatory system is closed, with blood vessels, capillaries and hearts (lateral hearts in segments 7 and 9 and lateral-oesophageal hearts in segments 12 and 13). Respiration occurs through the moist skin. Excretion is by segmentally arranged nephridia — septal, integumentary and pharyngeal. The nervous system has a nerve ring around the pharynx and a ventral nerve cord with segmental ganglia. The earthworm is hermaphrodite — testes in segments 10–11, ovaries in 13 — but cross-fertilisation occurs and cocoons are formed by the clitellum.
Q2. Describe the morphology and anatomy of the cockroach.
Answer: Morphology: Periplaneta americana measures 34–53 mm and is reddish-brown in colour. The body is divided into head, thorax and abdomen and is covered by a hard chitinous exoskeleton. The head bears a pair of compound eyes, a pair of long thread-like antennae and biting and chewing mouthparts (labrum, mandibles, maxillae, hypopharynx, labium). The thorax has three segments — prothorax, mesothorax and metathorax — each bearing a pair of legs; the mesothorax bears leathery forewings (tegmina) and the metathorax bears membranous hindwings used for flight. The abdomen has 10 segments; in females the 7th sternum forms a brood pouch, while in males the 9th sternum bears a pair of short anal styles.
Anatomy: The alimentary canal is divided into foregut, midgut and hindgut, with salivary glands opening into it. Respiration occurs through a network of tracheae, which open externally through ten pairs of spiracles. The circulatory system is open, with a 13-chambered tubular heart and haemolymph. Excretion is by 100–150 yellow Malpighian tubules attached at the junction of midgut and hindgut. The nervous system has a brain (supra-oesophageal ganglion), sub-oesophageal ganglion and a double ventral ganglionated nerve cord. Sexes are separate; reproduction is sexual and the female lays eggs in protective egg cases (oothecae).
Q3. Describe the morphology and anatomy of the frog (Rana tigrina).
Answer: Morphology: The frog has a streamlined body divisible into head and trunk; neck and tail are absent. The skin is moist, slimy and rich in mucous glands. The dorsal side is olive-green with dark spots; the ventral side is pale-yellow. The head bears a pair of bulging eyes covered by nictitating membranes, a pair of external nostrils and a tympanum (ear membrane). The forelimbs are short with four digits and the hind limbs are long, muscular with five webbed digits adapted for leaping and swimming. Sexes are separate — males have vocal sacs and copulatory pads on the first digit of the forelimb.
Anatomy: The digestive system consists of mouth, buccal cavity, pharynx, oesophagus, stomach, small intestine (duodenum and ileum), large intestine, rectum and cloaca. The liver secretes bile stored in the gall bladder, and the pancreas secretes pancreatic juice. Respiration is cutaneous, buccal and pulmonary. The circulatory system is closed with a three-chambered heart (two atria and one ventricle), arteries, veins and a well-developed lymphatic system; RBCs are nucleated. Excretion is by paired mesonephric kidneys; the frog is ureotelic. The nervous system comprises the central (brain and spinal cord), peripheral and autonomic systems; the brain has forebrain, midbrain and hindbrain. Reproduction is sexual; fertilisation is external in water, and development involves a tadpole stage that undergoes metamorphosis.
Q4. Compare the body plans (organ systems) of earthworm, cockroach and frog.
Answer: All three animals exhibit organ-system level of body organisation but differ in the structural complexity of each system. Digestive system: the earthworm has a simple straight tube with gizzard for grinding soil and detritus; the cockroach has a more specialised gut with crop, gizzard and hepatic caecae for omnivorous feeding; the frog has a short tube with well-developed liver and pancreas for carnivorous digestion. Circulatory system: closed in earthworm and frog, but open in cockroach with a tubular heart. Respiratory system: earthworm uses moist skin, cockroach uses tracheae and spiracles, while frog uses skin, buccal lining and lungs. Excretory system: nephridia in earthworm, Malpighian tubules in cockroach, and kidneys in frog. Nervous system: ventral ganglionated cord in earthworm and cockroach but a dorsal tubular cord with well-developed brain in frog. Reproduction: earthworm is hermaphrodite; cockroach and frog have separate sexes with internal and external fertilisation respectively.
Q5. Discuss the structural and functional differences among the four types of animal tissues.
Answer: Epithelial tissue has compactly packed cells with little intercellular substance, resting on a basement membrane; functions include protection, absorption, secretion and excretion (e.g. skin, intestinal lining). Connective tissue has loosely spaced cells in an abundant matrix; functions include linking, supporting, packing and transport (e.g. bone, blood, cartilage). Muscular tissue consists of long contractile cells (muscle fibres) specialised for movement of body parts and locomotion; types are skeletal, smooth and cardiac. Neural tissue consists of neurons and neuroglia; it generates and transmits electrical impulses to coordinate body activities. Together these four tissues build all organs and organ systems, demonstrating division of labour at the cellular level.
Multiple Choice Questions (MCQs)
Q1. Which tissue forms the lining of blood vessels?
(a) Cuboidal epithelium (b) Squamous epithelium (c) Columnar epithelium (d) Ciliated epithelium
Answer: (b) Squamous epithelium.
Q2. Which of the following is a fluid connective tissue?
(a) Cartilage (b) Bone (c) Blood (d) Tendon
Answer: (c) Blood.
Q3. Tendons connect —
(a) Bone to bone (b) Muscle to muscle (c) Muscle to bone (d) Skin to muscle
Answer: (c) Muscle to bone.
Q4. Intercalated discs are characteristic of —
(a) Smooth muscle (b) Skeletal muscle (c) Cardiac muscle (d) None
Answer: (c) Cardiac muscle.
Q5. The clitellum of earthworm lies in segments —
(a) 5–9 (b) 10–13 (c) 14–16 (d) 17–19
Answer: (c) 14–16.
Q6. Malpighian tubules are excretory organs of —
(a) Earthworm (b) Cockroach (c) Frog (d) Hydra
Answer: (b) Cockroach.
Q7. The heart of frog is —
(a) Two-chambered (b) Three-chambered (c) Four-chambered (d) Tubular
Answer: (b) Three-chambered.
Q8. Hepatic caecae are present in —
(a) Earthworm (b) Frog (c) Cockroach (d) Hydra
Answer: (c) Cockroach.
Q9. Which gland is ductless?
(a) Salivary (b) Sweat (c) Pancreas (d) Pituitary
Answer: (d) Pituitary (endocrine).
Q10. Neuroglia are —
(a) Nerve cells (b) Supporting cells of nervous system (c) Muscle cells (d) Connective tissue cells
Answer: (b) Supporting cells of nervous system.
Fill in the Blanks
Q1. The fundamental unit of neural tissue is __________.
Answer: the neuron.
Q2. __________ epithelium has cilia on its free surface.
Answer: Ciliated.
Q3. The respiratory pigment in earthworm blood is __________.
Answer: haemoglobin (dissolved in plasma).
Q4. The heart of cockroach has __________ chambers.
Answer: 13.
Q5. The frog excretes nitrogenous waste mainly as __________.
Answer: urea (ureotelic).
True or False
Q1. Cardiac muscle is voluntary in nature.
Answer: False — cardiac muscle is involuntary.
Q2. Earthworm is hermaphrodite.
Answer: True.
Q3. Cockroach has a closed circulatory system.
Answer: False — it has an open circulatory system.
Q4. Frog has a four-chambered heart.
Answer: False — frog has a three-chambered heart.
Q5. Adipose tissue is a type of loose connective tissue.
Answer: True.
Glossary
| Term | Meaning |
|---|---|
| Tissue | Group of similar cells with intercellular substance performing a specific function. |
| Epithelium | Tissue covering body surfaces and lining cavities and ducts. |
| Squamous | Flat, scale-like epithelial cells. |
| Cuboidal | Cube-shaped epithelial cells lining ducts and tubules. |
| Columnar | Tall, pillar-like epithelial cells lining the gut. |
| Glandular epithelium | Epithelium specialised for secretion (exocrine and endocrine). |
| Connective tissue | Tissue that links and supports other tissues; cells in abundant matrix. |
| Areolar tissue | Loose connective tissue under the skin acting as packing. |
| Adipose tissue | Connective tissue specialised for fat storage. |
| Tendon | Dense regular connective tissue connecting muscle to bone. |
| Ligament | Dense connective tissue connecting bone to bone. |
| Cartilage | Specialised connective tissue with flexible matrix. |
| Bone | Hard calcified connective tissue forming skeleton. |
| Neuron | Structural and functional unit of neural tissue. |
| Neuroglia | Supporting cells of nervous tissue. |
| Clitellum | Glandular region of earthworm in segments 14–16. |
| Setae | S-shaped chitinous bristles aiding earthworm locomotion. |
| Nephridia | Excretory organs of earthworm. |
| Tracheae | Respiratory tubes of cockroach opening through spiracles. |
| Malpighian tubules | Excretory organs of cockroach. |
| Ootheca | Egg case produced by female cockroach. |
| Tympanum | External ear membrane of frog. |
| Nictitating membrane | Transparent eyelid protecting frog’s eyes. |
| Cloaca | Common chamber for digestive, urinary and reproductive openings in frog. |
| Metamorphosis | Transformation of tadpole to adult frog. |
| Ureotelic | Animal that excretes nitrogenous waste mainly as urea. |
| Organ | Group of tissues performing a specific function. |
| Organ system | Group of organs working together to perform broader functions. |