Structural Organisation in Animals

7.1 Organ and Organ System

In multicellular animals, the body is composed of billions of cells, each specialized to perform specific functions essential for the organism's survival. Unlike unicellular organisms where a single cell carries out all life processes such as digestion, respiration, and reproduction, multicellular organisms exhibit division of labour among cells. Groups of similar cells along with intercellular substances that perform a common function form tissues. Remarkably, all complex animals have only four basic types of tissues: epithelial, connective, muscular, and neural tissues. These tissues are organized in specific proportions and patterns to form organs such as the stomach, lungs, heart, and kidneys. When two or more organs work together through physical and/or chemical interactions to perform a common function, they form an organ system, for example, the digestive system or respiratory system. This hierarchical organization from cells to tissues, organs, and organ systems ensures efficient and coordinated functioning of the body, contributing to the survival of the organism as a whole. Each organ in the body is made up of one or more types of tissues. For instance, the human heart contains all four tissue types: epithelial, connective, muscular, and neural. The complexity of organs and organ systems shows an evolutionary trend, which will be studied in detail in higher classes. This section introduces the concept of morphology and anatomy using the frog as a representative vertebrate. Morphology refers to the study of external form or visible features of an organism, while anatomy refers to the study of internal organs and their structure.

• Multicellular animals have specialized cells grouped into tissues.
• Four basic tissue types: epithelial, connective, muscular, and neural.
• Tissues form organs; organs work together as organ systems.
• Organ systems enable efficient division of labour.
• Each organ may contain multiple tissue types.
• Morphology studies external features; anatomy studies internal structures.
• 📌 Tissue: Group of similar cells with intercellular substances performing a specific function.
• 📌 Organ: Structure composed of different tissues performing specific functions.
• 📌 Organ system: Group of organs working together to perform a common function.

7.2 Frogs

Frogs are amphibians belonging to the class Amphibia and phylum Chordata. The most common species found in India is Rana tigrina. Amphibians are unique in that they can live both on land and in freshwater environments. Frogs are poikilothermic or cold-blooded animals, meaning their body temperature varies with the surrounding environment. This adaptation allows them to survive in diverse habitats but also makes them dependent on external temperatures. Frogs exhibit the ability to change their skin colour to blend with their surroundings, a protective mechanism called mimicry or camouflage that helps them avoid predators. During extreme environmental conditions such as peak summer and winter, frogs enter a state of dormancy to survive. In summer, this is called aestivation (summer sleep), and in winter, it is called hibernation (winter sleep). During these periods, frogs take shelter in deep burrows to escape heat or cold. These adaptations reflect the frog's ability to survive in variable environments and highlight their ecological importance.

• Frogs belong to class Amphibia and phylum Chordata.
• Common Indian species: Rana tigrina.
• Cold-blooded (poikilothermic) animals; body temperature varies with environment.
• Can change skin colour for camouflage (mimicry).
• Undergo aestivation in summer and hibernation in winter.
• Adapted to live both in water and on land.
• 📌 Poikilothermic: Animals whose body temperature fluctuates with the environment.
• 📌 Mimicry: Protective coloration to blend with surroundings.
• 📌 Aestivation: Dormant state during summer heat.