Structural Organisation in Animals

Structural Organisation in Animals explores how multicellular animals are constructed — from the simplest cellular level to the most complex organ systems. Every organism is an integrated system, where each cell, tissue, and organ plays a specific role to sustain life. This topic is fundamental to understanding the anatomy and physiology of animals, focusing primarily on the organisation of tissues and the morphological features of representative species, such as the earthworm, cockroach, and frog.

1.0Levels of Structural Organisation

Animals exhibit a hierarchical level of organization, which progresses as follows:

1. Cellular Level: Basic structural and functional unit — the cell.
2. Tissue Level: Group of similar cells performing a common function (e.g., epithelial, connective, muscular, nervous).
3. Organ Level: Different tissues combine to form organs (e.g., heart, stomach, brain).
4. Organ System Level: Organs working together to perform a function (e.g., digestive, circulatory systems).
5. Organism Level: All systems function collectively to maintain the life of the animal.

2.0Introduction

Animal tissues are broadly classified into four major types based on their structure and function.

Epithelial Tissue

Epithelial tissue forms the covering or lining of body surfaces, organs, and cavities. It provides protection, absorption, secretion, and sensory reception.

Types of Epithelial Tissue:

1. Simple Epithelium: Single layer of cells; specialized for absorption and filtration.

• Simple squamous: Flattened cells (e.g., alveoli).
• Simple cuboidal: Cube-shaped (e.g., kidney tubules).
• Simple columnar: Tall cells (e.g., intestine).
• Ciliated epithelium: With cilia for movement (e.g., trachea).

2. Compound Epithelium: Multiple layers of cells for protection (e.g., skin, oral cavity).
3. Glandular Epithelium: Modified for secretion (e.g., sweat glands, salivary glands).

Connective Tissue

Connective tissue binds, supports, and connects different tissues and organs. It’s the most abundant tissue in animals.

Types of Connective Tissue:

Loose Connective Tissue:

• Areolar tissue (supports organs)
• Adipose tissue (stores fat)

Dense Connective Tissue:

• Tendons (connect muscles to bones)
• Ligaments (connect bones to bones)

Skeletal Connective Tissue:

• Cartilage (flexible support, e.g., ear pinna)
• Bone (rigid structural framework)

Fluid Connective Tissue:

• Blood (transports nutrients and gases)
• Lymph (maintains fluid balance and immunity)

Muscular Tissue

Muscular tissue enables movement of the body and its parts. It consists of elongated cells called muscle fibers.

Types of Muscular Tissue:

1. Skeletal (Striated) Muscle: Voluntary control, striated appearance, attached to bones.
2. Smooth (Non-Striated) Muscle: Involuntary, found in walls of internal organs (e.g., intestine, bladder).
3. Cardiac Muscle: Striated and involuntary, found only in the heart; specialized for rhythmic contraction.

Nervous Tissue

Nervous tissue coordinates and controls body functions through electrical impulses. It consists of neurons (nerve cells) and neuroglial cells (supportive cells).

Neuron Structure

• Cell body (soma)
• Dendrites (receive impulses)
• Axon (transmits impulses)

Function: Transmits information rapidly between body parts and integrates responses through the brain and spinal cord.

3.0Morphology and Anatomy of Animals

Earthworm

Scientific Name: Pheretima posthuma

• Body Structure: Long, cylindrical, segmented body.
• Segmentation: 100–120 segments; mouth in first segment, anus in last.
• Digestive System: Straight alimentary canal from mouth to anus.
• Circulatory System: Closed type with blood vessels.
• Reproductive System: Hermaphrodite (both male and female reproductive organs).
• Special Feature: Nephridia for excretion and moist skin for respiration.

Cockroach

Scientific Name: Periplaneta americana

• Body Division: Head, thorax, abdomen.
• Exoskeleton: Hard chitinous cuticle for protection.
• Digestive System: Alimentary canal divided into foregut, midgut, and hindgut.
• Circulatory System: Open type with haemocoel.
• Respiratory System: Network of tracheae and spiracles.
• Reproductive System:
• • Male: Testes and vas deferens
  • Female: Ovaries and oviducts
• Special Feature: Compound eyes and antennae for sensory detection.

Frog

Scientific Name: Rana tigrina

• Habitat: Amphibious — lives both on land and in water.
• Body Division: Head and trunk.
• Digestive System: Mouth → Oesophagus → Stomach → Intestine → Cloaca.
• Circulatory System: Closed with three-chambered heart (two atria, one ventricle).
• Respiratory System: Lungs, skin, and buccal cavity.
• Excretory System: Kidneys, ureters, and cloaca.
• Reproductive System:
• • Male: Testes
  • Female: Ovaries
• Special Feature: Cold-blooded (poikilothermic) and shows hibernation in winter.