Chapter Notes

Organ and Organ System

In the world of living things, there's a beautiful order to how bodies are built, especially in complex multicellular animals like us. This organization allows for an incredible division of labour, where different parts of the body have specialized jobs, all working together for the survival of the whole organism.

Let's look at this hierarchy of organization:

1. Cells: The basic building blocks of life.
2. Tissues: When a group of similar cells, along with the substances between them, work together to perform a specific function, they form a tissue. All complex animals are built from just four basic types of tissues.
3. Organs: Tissues are then organized in specific ways to form an organ, like the stomach, heart, or lungs. Each organ is made of one or more types of tissues. For instance, the human heart is a powerful organ that contains all four basic tissue types: epithelial, connective, muscular, and neural.
4. Organ Systems: When two or more organs coordinate through physical or chemical interactions to perform a common function, they form an organ system. Examples include the digestive system and the respiratory system.

This organized structure makes the activities of millions of cells more efficient and better coordinated. The increasing complexity we see in organ systems as we study different animals is an evolutionary trend.

To understand this organization better, we study two key aspects of an animal's body:

• Morphology: This is the study of the form or externally visible features of an organism. For animals, it refers to the external appearance of the body and its parts.
• Anatomy: This term is used for the study of the morphology of internal organs.

In this chapter, we will explore the morphology and anatomy of the frog as a representative example of a vertebrate animal.

Frogs

Frogs are fascinating creatures that belong to the Class Amphibia and Phylum Chordata. This means they can live both on land and in freshwater. The most common species found in India is the Indian Bullfrog, Rana tigrina.

Here are some key characteristics and survival strategies of frogs:

• Cold-Blooded (Poikilotherms): Frogs do not have a constant body temperature. Instead, their body temperature changes with the temperature of their surroundings.
• Camouflage: Frogs can change their color to blend in with their environment, whether it's green grass or dry land. This helps them hide from predators. The text refers to this protective coloration as mimicry.
• Aestivation (Summer Sleep): During the extreme heat of peak summer, frogs are not seen. They take shelter in deep burrows to protect themselves.
• Hibernation (Winter Sleep): Similarly, during the cold of peak winter, frogs take shelter in burrows to survive. This is known as winter sleep.

Morphology

Let's explore the external features of a frog.

• Skin: A frog's skin is smooth, moist, and slippery due to a layer of mucus. It always stays moist, which is crucial for respiration. Frogs don't drink water; they absorb it directly through their skin.
• Coloration: The dorsal side (back) is typically olive green with dark, irregular spots, while the ventral side (belly) is a uniform pale yellow.
• Body Structure: The body is divided into a head and a trunk. A neck and tail are absent in adult frogs.
• Head:
  • A pair of nostrils is present above the mouth.
  • The eyes are large and bulged. They are protected by a nictitating membrane, a transparent third eyelid that covers the eye when the frog is in water.
  • On each side of the eyes, there is a membranous tympanum, which acts as an eardrum and receives sound signals.
• Limbs:
  • Forelimbs: Shorter and end in four digits.
  • Hind limbs: Larger and more muscular, ending in five digits. These powerful limbs are used for swimming, walking, leaping, and burrowing. The digits on the feet are webbed, which helps propel the frog through water.
• Sexual Dimorphism: Male and female frogs have distinct external differences.
  • Male frogs can be identified by the presence of sound-producing vocal sacs and a copulatory pad on the first digit of their forelimbs.
  • These features are absent in female frogs.

Anatomy

The body cavity of a frog contains several well-developed organ systems, including the digestive, circulatory, respiratory, nervous, excretory, and reproductive systems.

Digestive System

The digestive system consists of an alimentary canal and digestive glands.

• Alimentary Canal: The frog's alimentary canal is relatively short because frogs are carnivores, and meat is easier to digest than plants.
  • The path of food is: Mouth → Buccal Cavity → Pharynx → Oesophagus (a short tube) → Stomach → Intestine → Rectum → Cloaca.
  • The cloaca is a small, common chamber at the end of the digestive tract that is also used to pass urine and reproductive cells (sperms or eggs) to the outside.
• Digestive Glands:
  • Liver: Secretes bile, which is stored in the gall bladder.
  • Pancreas: Produces pancreatic juice containing digestive enzymes.
• Process of Digestion:
  1. Food is captured by a sticky, bilobed tongue.
  2. In the stomach, digestion begins with the help of HCl and gastric juices. The partially digested food is called chyme.
  3. The chyme moves to the duodenum (the first part of the small intestine). Here, it mixes with bile from the gall bladder and pancreatic juices from the pancreas, delivered through a common bile duct.
  4. Bile emulsifies fats (breaks them into smaller droplets), while pancreatic juices digest carbohydrates and proteins.
  5. Final digestion occurs in the intestine.
  6. Nutrients are absorbed through numerous finger-like folds in the inner wall of the intestine called villi and microvilli.
  7. Undigested solid waste moves to the rectum and is passed out through the cloaca.

Respiratory System

Frogs have two different methods of respiration for their dual life on land and in water.

• In Water (Cutaneous Respiration): The skin acts as the respiratory organ. Dissolved oxygen from the water diffuses directly through the moist skin into the blood.
• On Land (Pulmonary Respiration): The buccal cavity, skin, and lungs are used.
  • The lungs are a pair of elongated, pink, sac-like structures in the upper trunk (thorax).
  • Air enters through the nostrils, goes into the buccal cavity, and is then pushed into the lungs.
• During aestivation and hibernation, gaseous exchange happens exclusively through the skin.

Vascular System

The frog has a well-developed, closed-type vascular system, meaning blood flows within vessels. It also has a lymphatic system.

• Blood Vascular System: Includes the heart, blood vessels (arteries and veins), and blood.
  • Heart: A muscular, three-chambered organ with two atria and one ventricle. It is covered by a membrane called the pericardium.
  • Blood Flow:
    • The sinus venosus, a triangular structure, collects deoxygenated blood from the body via major veins (vena cava) and delivers it to the right atrium.
    • The ventricle pumps blood into a sac-like conus arteriosus, which distributes it to the rest of the body through arteries.
  • Portal Systems: Frogs have special venous connections.
    • Hepatic Portal System: Connects the intestine and the liver.
    • Renal Portal System: Connects the lower parts of the body and the kidneys.
• Blood: Composed of plasma and cells (RBCs, WBCs, and platelets).
  • Red Blood Cells (RBCs) in frogs are nucleated (unlike in mammals) and contain the red pigment haemoglobin for oxygen transport.
• Lymphatic System: Consists of lymph, lymph channels, and lymph nodes. Lymph is different from blood as it lacks RBCs and some proteins.

Excretory System

This system eliminates nitrogenous wastes from the body.

• Organs: A pair of kidneys, ureters, a cloaca, and a urinary bladder.
• Kidneys: These are compact, dark red, bean-shaped structures located on both sides of the vertebral column. Each kidney is made of structural and functional units called uriniferous tubules or nephrons.
• Ureters: Tubes that emerge from the kidneys.
  • In male frogs, the ureters act as a urinogenital duct, carrying both urine and sperm to the cloaca.
  • In female frogs, the ureters and oviducts open separately into the cloaca.
• Urinary Bladder: A thin-walled sac that stores urine and also opens into the cloaca.
• The frog excretes urea, making it a ureotelic animal.

Control and Coordination

The frog has a highly evolved system for control and coordination, involving both the nervous system and the endocrine system.

• Endocrine System: This system uses chemical messengers called hormones, secreted by endocrine glands (pituitary, thyroid, adrenals, etc.), to coordinate various body functions.
• Nervous System: It is organized into three parts:
  1. Central Nervous System (CNS): The brain and spinal cord.
  2. Peripheral Nervous System (PNS): Cranial nerves (10 pairs arising from the brain) and spinal nerves.
  3. Autonomic Nervous System (ANS): Sympathetic and parasympathetic nerves.
• Brain: The brain is protected in a bony box called the cranium. It is divided into:
  • Fore-brain: Includes olfactory lobes, cerebral hemispheres, and diencephalon.
  • Mid-brain: Characterized by a pair of optic lobes.
  • Hind-brain: Consists of the cerebellum and medulla oblongata. The medulla oblongata connects to the spinal cord.

Sense Organs

Frogs have various sense organs to interact with their environment.

• Organs of touch (sensory papillae), taste (taste buds), and smell (nasal epithelium).
• Eyes: A pair of spherical, simple eyes located in orbits in the skull.
• Ears: The external ear is absent. A tympanum is visible externally, which is the eardrum. The ear serves both for hearing and for balancing (equilibrium).

Reproductive System

Frogs have separate, well-organized male and female reproductive systems.

• Male Reproductive System:
  • A pair of yellowish, ovoid testes are attached to the upper part of the kidneys by a membrane called the mesorchium.
  • About 10-12 fine tubes called vasa efferentia arise from the testes.
  • These tubes enter the kidneys and open into Bidder's canal.
  • From there, sperm travels through the urinogenital duct (the ureter) and is released outside through the cloaca.
• Female Reproductive System:
  • A pair of ovaries are located near the kidneys but have no functional connection to them.
  • A pair of oviducts arise near the ovaries and open separately into the cloaca.
• Reproduction and Development:
  • A mature female can lay 2500 to 3000 eggs (ova) at a time.
  • Fertilisation is external and occurs in water.
  • Development includes a larval stage known as the tadpole.
  • The tadpole undergoes metamorphosis (a series of changes) to transform into an adult frog.

Economic Importance of Frogs

Frogs play a significant and beneficial role in the ecosystem and for humans.

• Pest Control: They eat insects and help protect agricultural crops.
• Ecological Balance: Frogs are a vital link in the food chain and food web, serving as both predator and prey.
• Food Source: In some countries, the muscular legs of frogs are consumed as food by humans.