Structural Organization in Animals

Learning Outcomes:

1. Understanding the basic types of animal tissues and their functions.
2. Gaining knowledge about the structure and function of organs and organ systems.
3. Exploring the morphology and anatomy of earthworms, cockroaches, and frogs.
4. Analyzing the role of epithelial, connective, muscular, and neural tissues in maintaining bodily functions.

Introduction to Structural Organization in Animals

In the animal kingdom, organisms exhibit varying levels of complexity, from unicellular to multicellular forms. Unicellular organisms perform all life functions within a single cell. On the other hand, multicellular animals have specialized cells, tissues, organs, and organ systems working together. Each of these components plays a distinct role, contributing to the overall functioning and survival of the animal body.

Animal Tissues

Tissues are groups of cells with a common origin, performing a specific function. Multicellular animals contain only four basic types of tissues, which are crucial for the body’s operation: epithelial, connective, muscular, and neural tissues.

Epithelial Tissue

Epithelial tissue, or epithelium, forms the covering or lining of body surfaces, cavities, ducts, and tubes. Cells are closely packed with little intercellular matrix. There are two main types of epithelial tissue: simple and compound.

1. Simple Epithelium: Made of a single layer of cells. It can be subdivided based on the structure of the cells:
   • Squamous epithelium: Flattened cells with irregular boundaries, involved in diffusion (e.g., in blood vessels and lungs).
   • Cuboidal epithelium: Cube-shaped cells found in ducts and tubular structures (e.g., kidney tubules) for secretion and absorption.
   • Columnar epithelium: Tall, slender cells with nuclei at the base. They are found in the lining of the stomach and intestines, aiding in absorption and secretion.
   • Ciliated epithelium: A subtype of cuboidal or columnar cells that bear cilia. These are present in respiratory and reproductive tracts.
2. Compound Epithelium: Comprising multiple layers of cells, its primary role is protection. Found on the skin surface and in the mouth, pharynx, and ducts of glands, this tissue guards against mechanical and chemical stresses.

Note: Glandular epithelium, specialized for secretion, includes unicellular (e.g., goblet cells) and multicellular glands (e.g., salivary glands).

Connective Tissue

Connective tissues link and support other tissues and organs. They are the most abundant tissues and can be classified into three main types.

1. Loose Connective Tissue: This type has loosely arranged cells and fibers in a semi-fluid ground substance.
   • Areolar tissue: Found under the skin, it provides support and flexibility.
   • Adipose tissue: Specialized in storing fat, it serves as an energy reservoir and insulation layer.
2. Dense Connective Tissue: Dense regular connective tissues contain collagen fibers arranged in parallel, providing tensile strength. Examples include tendons (which connect muscles to bones) and ligaments (which connect bones to each other).
3. Specialized Connective Tissue:
   • Cartilage: Solid yet pliable, it is found in areas like the tip of the nose and ear. Chondrocytes (cartilage cells) are enclosed in the matrix.
   • Bone: The hardest tissue, rich in calcium salts, forming the body’s framework and aiding movement.
   • Blood: A fluid connective tissue responsible for transporting substances like oxygen and nutrients. It consists of plasma, RBCs, WBCs, and platelets.

Comparison of Connective Tissues

Muscular Tissue

Muscle tissues are specialized for contraction and help in body movements. There are three types of muscular tissues:

1. Skeletal Muscle: Striated muscle attached to bones, responsible for voluntary movements.
2. Smooth Muscle: Involuntary, non-striated muscle found in organs like the stomach and intestines.
3. Cardiac Muscle: A striated, involuntary muscle found exclusively in the heart, functioning to pump blood.

Neural Tissue

Neural tissue forms the basis of the nervous system. Neurons are excitable cells that transmit electrical signals throughout the body. Neuroglia support and protect neurons.

Organ and Organ Systems

Complex animals have organs formed by the organization of tissues, and these organs work together to form organ systems. For example, the digestive system consists of organs like the stomach, intestines, and liver, each performing specific roles in the breakdown and absorption of food.

Earthworm

Earthworms (e.g., Pheretima and Lumbricus) are invertebrates living in moist soil.

Morphology

Earthworms have a long, cylindrical body divided into segments, each marked by setae aiding in locomotion. The prostomium covers the mouth and acts as a sensory lobe. The clitellum secretes mucous and helps in reproduction.

1. Segments: Earthworm bodies are divided into over 100 segments, with specialized genital pores in specific segments.
2. Setae: S-shaped bristles found in all but the first, last, and clitellar segments aid movement.
3. Clitellum: A thick band of glandular tissue found on the 14th to 16th segments.

Anatomy

1. Body Wall: Consists of a thin cuticle, epidermis, and two muscle layers. Below this is the coelomic epithelium.
2. Alimentary Canal: Earthworms ingest decaying matter mixed with soil. Food passes through the buccal cavity, pharynx, esophagus, and gizzard, where it is ground up.
3. Circulatory System: A closed system with blood vessels, blood glands, and hearts. Earthworms have hemoglobin dissolved in their blood.
4. Excretory System: The earthworm has segmentally arranged nephridia for removing nitrogenous waste.

Note: Earthworms are hermaphrodites, possessing both male and female reproductive organs.

Cockroach

Cockroaches belong to the class Insecta, with Periplaneta americana being a common species.

Morphology

The body of the cockroach is divided into three regions: head, thorax, and abdomen. Cockroaches possess antennae for sensory perception and wings for locomotion.

1. Head: Triangular with compound eyes and antennae.
2. Thorax: Comprises prothorax, mesothorax, and metathorax, with legs attached to each.
3. Abdomen: Divided into ten segments, with genital structures located in the last few segments.

Anatomy

1. Digestive System: The alimentary canal consists of a pharynx, esophagus, crop, gizzard, and intestines. Malpighian tubules help with excretion.
2. Circulatory System: Open type, where the blood (hemolymph) bathes internal organs.
3. Respiratory System: A network of tracheae and spiracles aids in gas exchange.
4. Reproductive System: Cockroaches are dioecious, with separate sexes. Females produce oothecae, each containing several eggs.

Note: Cockroaches have mosaic vision, with each compound eye having multiple ommatidia for enhanced light sensitivity.

Frog

Frogs (e.g., Rana tigrina) are amphibians capable of living both in water and on land.

Morphology

The body is divided into head and trunk, with webbed feet aiding in swimming. Frogs have sexual dimorphism, where males have vocal sacs and copulatory pads.

1. Skin: Moist and covered with mucus, enabling cutaneous respiration.
2. Limbs: Hind limbs are muscular and used for jumping.

Anatomy

1. Digestive System: Frogs have a short alimentary canal due to their carnivorous diet, consisting of a stomach, intestines, and cloaca.
2. Respiratory System: Frogs respire through skin in water and lungs on land.
3. Circulatory System: A closed system with a three-chambered heart. RBCs in frogs are nucleated.
4. Excretory System: Frogs are ureotelic, excreting urea through kidneys.
5. Reproductive System: Fertilization occurs externally in water, with frogs laying 2,500–3,000 ova.

Important Concept: Frogs exhibit metamorphosis, where a larval stage (tadpole) transforms into an adult frog.

Comparison of Reproductive Systems in Earthworms, Cockroaches, and Frogs

MCQ: Which of the following animals exhibits external fertilization?

• a) Earthworm
• b) Cockroach
• c) Frog
  Answer: c) Frog