Comprehensive Study Notes on Structural Organisation in Animals and the Biology of Frogs

Levels of Structural Organisation in Multicellular Animals

• Biological Variability: The animal kingdom consists of a vast range of organisms, spanning from unicellular to complex multicellular forms.

• Unicellular Organisms: In single-celled organisms, all vital biological functions such as digestion, respiration, and reproduction are executed by a single cell.

• Multicellular Organisms: In complex bodies, functions are carried out by different groups of cells in a highly organised manner.

  • Simple Multicellularity (e.g., Hydra): The body is composed of different types of cells, where the number of cells of each type can reach into the thousands.

  • Complex Multicellularity (e.g., Human Body): Composed of billions of cells that work together to perform various functions.

• Definition of Tissue: In multicellular animals, a group of similar cells along with intercellular substances that perform a specific function is called a tissue.

• The Four Basic Tissues: All complex animals are comprised of only $4$ basic types of tissues. These tissues are organised in specific proportions and patterns to form organs.

• Organ Formation: Multiple tissues integrate to form an organ (e.g., stomach, lung, heart, and kidney).

• Organ System: When two or more organs perform a common function through physical or chemical interaction, they form an organ system (e.g., digestive system, respiratory system).

• Division of Labour: The hierarchical arrangement of cells, tissues, organs, and organ systems ensures a division of labour that contributes to the survival of the body as a whole.

Organ and Organ System Complexity

• Tissue Composition of Organs: Each organ is typically composed of one or more types of tissues.

  • Example (Human Heart): The heart consists of all $4$ types of tissues: epithelial, connective, muscular, and neural.

• Evolutionary Trend: The complexity observed in organs and organ systems displays a discernible trend known as the evolutionary trend.

• Nomenclature in Animal Study:

  • Morphology: Refers to the study of the form or externally visible features. In animals, this specifically refers to the external appearance of organs or body parts.

  • Anatomy: Conventionally used for the study of the morphology of internal organs in animals.

Biology and Ecology of the Frog (Rana tigrina)

• Taxonomy and Habitat: Frogs belong to the class Amphibia of the phylum Chordata. They are capable of living both on land and in freshwater.

• Species in India: The most common species of frog found in India is Rana tigrina.

• Thermoregulation: Frogs are cold-blooded or poikilotherms, meaning they do not have a constant body temperature; their internal temperature varies with the environmental temperature.

• Defense Mechanisms:

  • Camouflage: The ability to change skin colour to blend in with surroundings (e.g., grasses or dry land) to hide from enemies.

  • Mimicry: The term used for this protective coloration.

• Dormancy Periods (Survival Strategies): Frogs are not visible during peak summer or winter. They take shelter in deep burrows to protect themselves from temperature extremes.

  • Aestivation: Known as summer sleep.

  • Hibernation: Known as winter sleep.

External Morphology of the Frog

• Skin Characteristics:

  • The skin is smooth and slippery due to the presence of mucus.

  • It is always maintained in a moist condition.

  • Coloration: The dorsal side is generally olive green with dark irregular spots, while the ventral side is uniformly pale yellow.

  • Water Absorption: The frog never drinks water; instead, it absorbs water directly through the skin.

• Body Divisions: The body is divided into the head and trunk. A neck and tail are absent.

• Head Features:

  • Nostrils: A pair of nostrils is located above the mouth.

  • Eyes: The eyes are bulged and covered by a nictitating membrane that provides protection while the frog is underwater.

  • Tympanum: A membranous ear (tympanum) is present on either side of the eyes to receive sound signals.

• Appendages:

  • Forelimbs: End in $4$ digits.

  • Hind Limbs: These are larger and more muscular than the forelimbs and end in $5$ digits.

  • Feet: Feature webbed digits that assist in swimming.

  • Functions: Limbs are used for swimming, walking, leaping, and burrowing.

• Sexual Dimorphism: Male frogs are distinguished from females by:

  • The presence of sound-producing vocal sacs.

  • The presence of a copulatory pad on the first digit of the forelimbs.

Internal Anatomy: Digestive and Respiratory Systems

• Digestive System:

  • Structure: Consists of the alimentary canal and digestive glands.

  • Alimentary Canal: It is short because frogs are carnivores, leading to a reduced length of the intestine.

  • Pathway: Mouth $\rightarrow$ buccal cavity $\rightarrow$ pharynx $\rightarrow$ oesophagus (short tube) $\rightarrow$ stomach $\rightarrow$ intestine $\rightarrow$ rectum $\rightarrow$ cloaca.

  • Feeding: Food is captured using a bilobed tongue.

  • Digestion Process: Takes place in the stomach via $HCl$ and gastric juices. Partially digested food is called chyme.

  • Duodenum: The first part of the small intestine which receives bile (from the gall bladder) and pancreatic juice (from the pancreas) via a common bile duct.

  • Enzymatic Activity: Bile emulsifies fats, while pancreatic juices digest carbohydrates and proteins. Final digestion occurs in the intestine.

  • Absorption: Digested food is absorbed by finger-like folds called villi and microvilli in the inner intestinal wall.

  • Waste: Undigested solid waste passes through the rectum and out via the cloaca.

• Respiratory System:

  • Cutaneous Respiration: In water, the skin acts as the respiratory organ. Dissolved oxygen is exchanged via diffusion.

  • Pulmonary Respiration: On land, frogs use lungs (a pair of elongated, pink, sac-like structures in the thorax). Air enters through nostrils into the buccal cavity and then to the lungs.

  • Dormancy Respiration: During aestivation and hibernation, gaseous exchange occurs exclusively through the skin.

Vascular and Excretory Systems

• Circulatory System: Frogs have a well-developed, closed vascular system and a lymphatic system.

  • Blood Vascular System: Includes the heart, blood vessels, and blood.

  • Heart Structure: A muscular organ in the upper body cavity covered by a pericardium. It has $3$ chambers: $2$ atria and $1$ ventricle.

  • Venous Structures: Sinus venosus (triangular structure) joins the right atrium and receives blood from the vena cava.

  • Arterial Structures: The ventricle opens into the conus arteriosus on the ventral side.

  • Portal Systems:

    1. Hepatic Portal System: Venous connection between liver and intestine.

    2. Renal Portal System: Venous connection between kidney and lower body parts.

  • Blood Composition: Plasma and cells (RBCs, WBCs, and platelets). RBCs are nucleated and contain haemoglobin.

  • Lymph: Differs from blood by lacking RBCs and certain proteins.

• Excretory System:

  • Organs: A pair of kidneys, ureters, cloaca, and urinary bladder.

  • Kidneys: Compact, dark red, bean-like structures located posteriorly near the vertebral column. Composed of uriniferous tubules or nephrons.

  • Ureters: In males, they act as urinogenital ducts. In females, ureters and oviducts open separately into the cloaca.

  • Physiology: The frog is a ureotelic animal, excreting urea as its primary nitrogenous waste.

Control, Coordination, and Sense Organs

• Neural System: Includes the Central Nervous System (CNS - brain and spinal cord), Peripheral Nervous System (PNS - cranial and spinal nerves), and Autonomic Nervous System (ANS - sympathetic and parasympathetic).

  • Cranial Nerves: There are $10$ pairs of cranial nerves.

  • The Brain: Housed in a bony cranium. Divided into:

    • Fore-brain: Olfactory lobes, paired cerebral hemispheres, and unpaired diencephalon.

    • Mid-brain: Optic lobes.

    • Hind-brain: Cerebellum and medulla oblongata. The medulla passes through the foramen magnum into the spinal cord.

• Endocrine System: Chemical coordination is achieved via hormones from glands including the pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals, and gonads.

• Sense Organs:

  • Touch: Sensory papillae.

  • Taste: Taste buds.

  • Smell: Nasal epithelium.

  • Vision: Simple eyes (one unit) located in the skull orbit.

  • Hearing/Balance: Tympanum and internal ear. Use for hearing and maintaining equilibrium.

Reproductive Systems and Ecology

• Male Reproductive System:

  • Testes: Pair of yellowish ovoid structures adhered to kidneys by the mesorchium (peritoneum fold).

  • Vasa Efferentia: $10-12$ ducts that enter kidneys and open into Bidder’s canal.

  • Urinogenital Duct: Path for both urine and sperm into the cloaca.

• Female Reproductive System:

  • Ovaries: Situated near kidneys with no functional connection to them.

  • Laying Eggs: A mature female lays $2500$ to $3000$ ova at a time.

• Development:

  • Fertilisation: External, occurring in water.

  • Larval Stage: Tadpoles hatch from eggs and undergo metamorphosis to become adults.

• Ecological Significance:

  • Frogs protect crops by eating harmful insects.

  • They serve as a vital link in food chains and food webs, maintaining ecological balance.

  • In some cultures, frog legs are consumed as food.

Questions & Discussion

• Exercise 1: Draw a neat diagram of the digestive system of a frog. (Refer to Figure 7.2 in the original text for the diagram including liver, stomach, intestine, and cloaca).

• Exercise 2: Mention the function of the Ureters in a frog.

  • Answer: In male frogs, the ureters serve as urinogenital ducts that transport both urine and sperm to the cloaca. In female frogs, they transport urine separately from the oviducts, both opening into the cloaca.