Structural Organisation in Animals

Edited By Irshad Anwar | Updated on Jun 06, 2025 09:37 AM IST

The chapter Animal Structural Organisation deals with the process of cells aggregating to form tissues, which aggregate to form organs and organ systems, setting the stage for the important functions in multicellular animals. It gives you a detailed understanding of the four basic categories of animal tissues: epithelial, connective, muscular, and nervous, all of which are important to animal physiology and pathology. Histological investigations reveal that tissue differentiation plays an important role in the specialisation and effectiveness of function, with epithelial tissues contributing to protection and absorption, connective tissues providing structural support, muscular tissues enabling movement, and nervous tissues facilitating communication and control.

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Important Topics of Structural Organisation in Animals
Structural Organisation in Animals
Morphology and Anatomy of the Earthworm
Structure and Anatomy of the Cockroach
Structure and Anatomy of the Frog
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Structural Organisation in Animals

This chapter also contains comparative anatomical investigations of animals such as the earthworm (Pheretima posthuma), cockroach (Periplaneta americana), and frog (Rana tigrina), which belong to different phyla and modes of organ system organisation (e.g., closed vs. open circulatory systems, segmental vs. fused body plans). This information is for disciplines such as zoology, veterinary sciences, and biomedical research, where structure-function relationships are the basis of diagnostic and therapeutic knowledge.

Important Topics of Structural Organisation in Animals

This chapter discusses some important concepts concerning the structure and function of animal tissues, organs, and body systems. It further encompasses comparative anatomy of a few organisms for studying evolutionary adaptations. The following are the topics to understand:

Animal tissues: Epithelial, connective, muscular, and nervous tissues
Morphology and anatomy of the earthworm (Pheretima posthuma)
Structure and anatomy of the cockroach (Periplaneta americana)
External and internal features of the frog (Rana tigrina)

While all animals are multicellular, the arrangement and functional organisation of their cells differ between groups. Depending on the degree of complexity in cellular arrangements and functions, animals show four main levels of structural organisation:

Cells of these organisms can either be loose groups, tissues, organs, or highly organised organ systems. These levels show body design evolutionary progress.

Level of Organisation	Description	Example Organism(s)
Cellular Level	Cells are loosely arranged with minimal coordination No true tissues are formed.	Sponges (Porifera)
Tissue Level	Similar cells are grouped to form functional tissues Division of labour begins.	Coelenterates (e.g., Hydra)
Organ Level	Different tissues combine to form specific organs with defined functions.	Platyhelminthes (Flatworms)
Organ-System Level	Organs work together as part of systems for complex body functions.	Annelids, Arthropods, Vertebrates

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Classification of Tissues

Tissues are classified into four types. All of them, along with their feature and level of organisation, are discussed below:

Component	Key Features	Examples/Types
Epithelial Tissue	Lacks blood vessels nourished by diffusion Closely packed cells with minimal intercellular space Lines of internal organs and external surfaces	Simple epithelium Compound epithelium Shapes: Squamous, Cuboidal, Columnar
Connective Tissue	Most abundant and widely distributed in mammals Produces collagen/elastin Connects and supports other tissues	Blood, Bone, Cartilage, Ligaments, Tendons
Muscular Tissue	Made of long fibres arranged in parallel arrays Composed of myofibrils Responsible for movement and contraction	Skeletal muscles Smooth muscles Cardiac muscles
Nervous Tissue	Composed of neurons and neuroglial cells Neurons are excitable and conduct impulses Neuroglia support and outnumber neurons	Brain Spinal cord Nerves
Organ Level of Organisation	Similar tissues combine to form organs with specific functions	Platyhelminthes
Organ System Level of Organisation	Organs are organised into systems performing specific physiological functions	Annelids, Arthropods, Echinoderms, Molluscs, Chordates

Morphology and Anatomy of the Earthworm

Earthworms are invertebrates of the phylum Annelida found in damp soil. The species, Pheretima posthuma, is widely used for study purposes in India. Earthworms contribute significantly to soil fertility and are referred to as 'friends of farmers' because their burrowing activities and their mode of feeding enrich the soil. Some important Morphological Characters are discussed below:

The body is cylindrical, bilaterally symmetrical, long, and segmented.
The back is darker and has a median dorsal line.
Segmentation: The body is divided into approximately 100-120 segments or metameres.
Clitellum: A well-developed glandular band found between segments 14 to 16, which plays a role in reproduction.
Does not have eyes, yet reacts to light, touch, and chemicals with the help of sensory cells.
Setae: Small chitinous bristles found on all segments except the first, last, and clitellum segments, which are used for locomotion.

Anatomy of the Earthworm

The anatomical features are discussed below in the table:

System	Description
Digestive System	Complete and straight Consists of mouth, pharynx, oesophagus, gizzard, intestine, and anus.
Circulatory System	Closed type with blood vessels and capillaries, blood is red due to haemoglobin dissolved in plasma.
Respiratory System	Gaseous exchange occurs through the moist body surface.
Excretory System	Excretion through nephridia is present in almost all segments.
Nervous System	A nerve ring and a paired ventral nerve cord with segmental ganglia.
Reproductive System	Hermaphroditic cross-fertilisation occurs. Testes in the 10th & 11th segments, ovaries in the 13th segment.

Structure and Anatomy of the Cockroach

Cockroaches are of the phylum Arthropoda and class Insecta. The Indian cockroach is Periplaneta americana. They are nocturnal, omnivorous, and swift insects that typically live in dark and moist areas. Their body is externally protected with a hard exoskeleton made of chitin, which is used for protection as well as for muscle attachment.

Cockroaches have bilateral symmetry and a segmented body that is separated into the head, thorax, and abdomen. They possess well-developed sensory organs such as compound eyes, antennae, and cerci that enable them to react rapidly to changes in their environment. Because of their strong adaptability and reproductive nature, cockroaches are regarded as hardy survivors and have been on Earth for more than 300 million years.

Key Morphological Characteristics:

Body dorsoventrally flattened, elongated, and segmented; bilaterally symmetrical.
Size: About 34–53 mm long and reddish-brown.
Composed of three segments: Head, Thorax, and Abdomen.
Triangular head with a pair of compound eyes, antennae, and chewing-type mouthparts.
The thorax with three segments: prothorax, mesothorax, and metathorax, each with a pair of walking legs.
Wings: Two pairs of forewings (tegmina) are leathery; hindwings are membranous and employed for flying.
The abdomen is 10-segmented. Both sexes possess anal cerci (sensory organs), and males possess anal styles.

Anatomy of the Cockroach

The anatomical features are discussed below in the table:

System	Description
Digestive System	Includes foregut (pharynx, oesophagus, crop, gizzard), midgut, and hindgut. Digestive glands include salivary glands.
Circulatory System	Open type with haemolymph (no haemoglobin)the heart is tubular with 13 chambers.
Respiratory System	Network of tracheae opening outside through spiracles, oxygen is directly delivered to tissues.
Excretory System	Uses Malpighian tubules for the removal of nitrogenous wastes.
Nervous System	Consists of a nerve ring and paired ventral nerve cords with segmental ganglia.
Reproductive System	Sexually dimorphic internal fertilisation. Males have testes, Females have ovaries and produce egg cases called oothecae.

Structure and Anatomy of the Frog

Frogs are amphibians of the phylum Chordata, class Amphibia. Rana tigrina (previously renamed Hoplobatrachus tigerinus) is the species generally used in India. Frogs are cold-blooded, have moist skin, and can survive on land as well as in water. They are carnivorous and help regulate insects in their food cycle.

Key Morphological Characteristics:

The body is streamlined, bilaterally symmetrical, and consists of two halves: head and trunk (no tail or neck).
Skin is glandular and moist, assisting in cutaneous respiration; no scales.
Eyes are bulging and shielded by nictitating membranes.
The tympanum (eardrum) found behind the eyes is utilised for hearing.
Forelimbs are short (for support), and hindlimbs are muscular and lengthy (modified for swimming and jumping).
Feet are webbed, aiding in swimming.
Sexes are distinct; males possess vocal sacs and nuptial pads to facilitate mating.

Anatomy of the Frog

The anatomical features are discussed below in the table:

System	Description
Digestive System	The complete body includes the mouth, buccal cavity, pharynx, oesophagus, stomach, intestine, and cloaca.
Circulatory System	A closed-type heart is three-chambered (two atria and one ventricle) Blood is red with nucleated RBCs.
Respiratory System	Respiration occurs through the lungs, skin, and buccal cavity.
Excretory System	Excretion via a pair of kidneys that open into the cloaca through the ureters.
Nervous System	The central nervous system includes the brain and spinal cord; the peripheral and autonomic systems are present.
Reproductive System	Sexes separate males have testes, females have ovaries. Fertilisation is external, and development is indirect via the tadpole stage.