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Chapter 7 Structural Organisation In Animals
The living world exhibits a fascinating variety of organisms, from simple unicellular beings where a single cell performs all life functions, to complex multicellular animals. In complex multicellular organisms, the same basic functions (like digestion, respiration, reproduction) are carried out by different groups of cells working together in a well-organized manner. These groups of similar cells, along with intercellular substances that perform a specific function, are called **tissues**.
Surprisingly, all complex animals are composed of only four basic types of tissues (epithelial, connective, muscular, and neural). These tissues are organized in specific proportions and patterns to form **organs** (like the stomach, lungs, heart, kidneys). When two or more organs interact physically or chemically to perform a common function, they form an **organ system** (like the digestive system, respiratory system). This hierarchical organization from cells to organ systems allows for **division of labour**, where different parts perform specific tasks, contributing to the survival and efficient functioning of the whole organism.
Organ And Organ System
In multicellular organisms, cells are organized into tissues, which in turn organize to form organs. Organs then associate to form organ systems. This structural organization is essential for the efficient and coordinated activities performed by millions or billions of cells in a complex organism.
Each organ is typically composed of one or more types of basic tissues. For instance, the human heart contains all four basic tissue types: epithelial (lining), connective (support), muscular (contraction), and neural (control). The complexity of organ and organ system organization displays evolutionary trends across different groups of animals.
**Morphology** refers to the study of the form or externally visible features of an organism or its parts. In animals, this typically means the external appearance of the body and its organs. **Anatomy**, conventionally, refers to the study of the internal structure or morphology of internal organs in animals, usually revealed through dissection or imaging techniques.
This chapter introduces the morphology and anatomy of the frog as a representative example of vertebrates, illustrating the structural organization at the organ and organ system level.
Question 1. Draw a neat diagram of digestive system of frog.
Answer:
Diagram of the digestive system of a frog (similar to Fig 7.2 in the textbook):
Question 2. Mention the function of the Ureters in frog.
Answer:
In frogs, the **ureters** are tubes that emerge from the kidneys. Their function is to transport urine from the kidneys to the cloaca (and temporarily stored in the urinary bladder). In male frogs, the ureters also serve as **urinogenital ducts**, transporting both urine and sperms from the testes to the cloaca.
Frogs
Frogs belong to the Class Amphibia of Phylum Chordata, capable of living both on land and in freshwater habitats. A common species in India is *Rana tigrina*.
Frogs are **cold-blooded (poikilotherms)**, meaning their body temperature fluctuates with the environmental temperature. They can change their skin colour for camouflage (mimicry) to protect themselves from predators. Frogs are not active during extreme summer (summer sleep or aestivation) and winter (winter sleep or hibernation), taking shelter in burrows.
Morphology (External Features)
The skin of a frog is smooth, slippery, and moist due to mucus glands, allowing cutaneous respiration (gas exchange through skin). The dorsal side is typically olive green with dark spots; the ventral side is uniformly pale yellow. Frogs absorb water through their skin and do not drink water.
The body is divided into a **head** and a **trunk**; a neck and tail are absent in adult frogs. Features on the head include a pair of nostrils above the mouth, bulged eyes covered by a protective nictitating membrane (useful in water), and a membranous tympanum on either side (receives sound signals).
The **forelimbs** (four digits) and **hind limbs** (five digits) aid in various movements: swimming, walking, leaping, and burrowing. Hind limbs are larger and more muscular. The webbed digits of the hind feet are particularly adapted for swimming.
Frogs exhibit **sexual dimorphism**, meaning males and females can be distinguished by external differences: male frogs have sound-producing **vocal sacs** and a **copulatory pad** (nuptial pad) on the first digit of the forelimbs, both of which are absent in females.
Anatomy (Internal Structure)
The body cavity of the frog houses well-developed organ systems:
- **Digestive System:** Consists of the alimentary canal and digestive glands. The alimentary canal is short as frogs are carnivores. Mouth opens into the buccal cavity, leading to the pharynx, oesophagus, stomach, intestine, rectum, and finally opening outside through the **cloaca**. Digestive glands include the liver (secretes bile stored in gall bladder) and pancreas (produces pancreatic juice with enzymes). Food is captured by the bilobed tongue. Digestion starts in the stomach (HCl and gastric juices). Partially digested food (chyme) enters the duodenum (first part of small intestine), receiving bile and pancreatic juice. Final digestion and absorption (via villi and microvilli) occur in the intestine. Undigested waste passes through rectum and out via cloaca.
- **Respiratory System:** Frogs have different respiratory methods for land and water:
- In water: **Cutaneous respiration** occurs through the moist skin by diffusion of dissolved oxygen.
- On land: **Pulmonary respiration** using lungs (a pair of pink, sac-like structures in the thorax), and also respiration through buccal cavity and skin. During aestivation and hibernation, respiration occurs solely through the skin.
- **Circulatory System:** Well-developed, **closed type**. Consists of blood vascular system (heart, blood vessels, blood) and lymphatic system (lymph, lymph channels, lymph nodes). Heart is muscular, located in the upper body cavity, covered by pericardium. It is **three-chambered** (two atria, one ventricle). Sinus venosus receives deoxygenated blood from vena cava and joins the right atrium. Ventricle pumps blood into conus arteriosus. Arteries carry blood away from heart; veins bring blood to heart. Frogs have hepatic portal system (between liver and intestine) and renal portal system (between kidney and lower body parts). Blood has plasma and cells (nucleated RBCs with haemoglobin, WBCs, platelets). Lymph lacks some proteins and RBCs. Circulation is achieved by heart's pumping action.
- **Excretory System:** Well-developed for eliminating nitrogenous wastes (urea, making them ureotelic). Consists of a pair of compact, dark red, bean-shaped **kidneys** (composed of nephrons/uriniferous tubules). **Ureters** emerge from kidneys; in males, they are urinogenital ducts (carrying urine and sperms) opening into the cloaca. In females, ureters and oviducts open separately into the cloaca. A thin-walled **urinary bladder** stores urine temporarily, opening into the cloaca. Waste is filtered from blood in kidneys and excreted as urine via the cloaca.
- **Nervous System:** Highly evolved system for control and coordination, includes neural system and endocrine glands. Neural system is divided into Central Nervous System (brain, spinal cord), Peripheral Nervous System (cranial and spinal nerves - 10 pairs of cranial nerves from brain), and Autonomic Nervous System (sympathetic and parasympathetic). Brain is enclosed in cranium, divided into forebrain (olfactory lobes, cerebral hemispheres, diencephalon), midbrain (optic lobes), and hindbrain (cerebellum, medulla oblongata). Medulla oblongata continues as spinal cord in vertebral column.
- **Sense Organs:** Include organs of touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes), and hearing/balance (tympanum with internal ear). Eyes and internal ears are well-organized; others are cellular aggregations around nerve endings. Eyes are spherical, simple eyes (one unit). External ear is only the tympanum; internal ear is for hearing and balance.
- **Reproductive System:** Well-organized male and female systems.
- **Male:** Pair of yellowish, ovoid testes (adhered to kidneys by mesorchium). Vasa efferentia (10-12 from testes) enter kidneys, open into Bidder’s canal, which communicates with urinogenital duct (from kidney), opening into cloaca. Cloaca is common chamber for faecal matter, urine, and sperms.
- **Female:** Pair of ovaries (near kidneys, no functional connection). Pair of oviducts (from ovaries) open separately into the cloaca. Mature female lays 2500-3000 ova at once.
Fertilisation is external, occurring in water. Development is indirect, involving a larval stage (tadpole) that undergoes metamorphosis to become an adult frog.
Frogs are beneficial as they eat insects (pest control) and maintain ecological balance as part of food chains/webs. In some cultures, their muscular legs are used as food.
Question 1. Draw a neat diagram of digestive system of frog.
Answer:
Diagram of the digestive system of a frog (similar to Fig 7.2 in the textbook):
Question 2. Mention the function of the Ureters in frog.
Answer:
In frogs, the **ureters** are tubes that emerge from the kidneys. Their function is to transport urine from the kidneys to the cloaca (and temporarily stored in the urinary bladder). In male frogs, the ureters also serve as **urinogenital ducts**, transporting both urine and sperms from the testes to the cloaca.
Exercises
Question 1. Draw a neat diagram of digestive system of frog.
Answer:
Question 2. Mention the function of the Ureters in frog.
Answer: