Structural Organisation in Animals: Morphology and Anatomy of the Frog (Rana tigrina)

Levels of Structural Organization

In the animal kingdom, there is a vast diversity of organisms, ranging from unicellular to complex multicellular forms. In unicellular organisms, a single cell is responsible for performing all vital life functions, including digestion, respiration, and reproduction. Conversely, in the complex bodies of multicellular animals, these basic functions are carried out by different groups of cells organized in a structured manner. For instance, a simple organism like Hydra is composed of different types of cells, where the count of each cell type can reach into the thousands. In contrast, the human body is composed of billions of cells that work together to perform various specialized functions. This organization ensures that multicellular animals can perform specific tasks through a division of labour, contributing to the survival of the organism as a whole.

Concept of Tissues, Organs, and Organ Systems

A tissue is defined as a group of similar cells along with intercellular substances that perform a specific function. In complex animals, all body structures are derived from only four basic types of tissues. These tissues are organized in specific patterns and proportions to form organs, such as the stomach, lungs, heart, and kidneys. When two or more organs collaborate to perform a common function through physical or chemical interaction, they form an organ system. Examples include the digestive system and the respiratory system. Each organ in the body is typically composed of more than one type of tissue; the heart, for example, contains all four basic types: epithelial, connective, muscular, and neural tissues. This hierarchical complexity—from cells to tissues, organs, and organ systems—displays a discernable trend known as an evolutionary trend.

Introduction to Frog Morphology and Taxonomy

Frogs are vertebrates belonging to the class Amphibia and the phylum Chordata. They are capable of living both on land and in freshwater. The most common species found in India is Rana tigrina. Frogs are poikilotherms, also known as cold-blooded animals, meaning their internal body temperature varies with the temperature of their environment. They exhibit protective coloration called mimicry, which allows them to change color (camouflage) to hide from enemies while in grass or on dry land. To survive extreme environmental conditions, frogs enter states of dormancy: they take shelter in deep burrows during peak summer, known as summer sleep or aestivation, and during peak winter, known as winter sleep or hibernation.

External Features and Morphology of Frog

Morphology refers to the study of the form or externally visible features. In animals, this involves the external appearance of organs or body parts. The skin of a frog is smooth, slippery, and constantly moist due to the presence of mucus. The dorsal side of the body is generally olive green with dark, irregular spots, while the ventral side is uniformly pale yellow. Notably, frogs do not drink water; they absorb it through their skin. The body is divided into a head and a trunk; a neck and tail are absent. The head features a pair of nostrils above the mouth and bulged eyes covered by a nictitating membrane that protects them while underwater. Sound signals are received by a membranous tympanum (ear) located on either side of the eyes. The frog possesses forelimbs and hind limbs used for swimming, walking, leaping, and burrowing. The hind limbs end in five digits and are larger and more muscular than the forelimbs, which end in four digits. The feet have webbed digits to assist in swimming. Frogs exhibit sexual dimorphism: males possess sound-producing vocal sacs and a copulatory pad on the first digit of the forelimbs, both of which are absent in females.

Anatomy of the Frog: The Digestive System

Anatomy is the study of the morphology of internal organs. The frog's body cavity accommodates well-developed systems including the digestive, circulatory, respiratory, nervous, excretory, and reproductive systems. The digestive system consists of the alimentary canal and digestive glands. Since frogs are carnivores, their alimentary canal is relatively short, and the length of the intestine is reduced. Food is captured using a bilobed tongue. The path of food starts at the mouth, leads to the buccal cavity, passes through the pharynx into the oesophagus (a short tube), and enters the stomach. The stomach continues into the intestine, then the rectum, and finally opens to the exterior via the cloaca. Digestion involves HCl and gastric juices secreted by the stomach walls. Partially digested food, called chyme, moves into the duodenum (the first part of the small intestine). The duodenum receives bile from the gall bladder (secreted by the liver) and pancreatic juice from the pancreas via a common bile duct. Bile emulsifies fats, while pancreatic juice digests carbohydrates and proteins. Final digestion occurs in the intestine, where nutrients are absorbed by finger-like folds called villi and microvilli. Undigested waste is expelled through the cloaca.

Respiratory System of the Frog

Frogs utilize different respiratory methods depending on their environment. In water, they perform cutaneous respiration, where the skin acts as the aquatic respiratory organ and dissolved oxygen is exchanged via diffusion. On land, they use the buccal cavity, skin, and lungs. Respiration via lungs is termed pulmonary respiration. The lungs are a pair of elongated, pink, sac-like structures located in the thorax (upper trunk). Air enters through the nostrils into the buccal cavity and then to the lungs. During aestivation and hibernation, gaseous exchange occurs exclusively through the skin.

Vascular and Lymphatic Systems

The frog has a well-developed, closed-type vascular system and a lymphatic system. The blood vascular system includes the heart, blood vessels, and blood. The heart is a muscular, three-chambered organ (two atria and one ventricle) located in the upper body cavity and covered by the pericardium. Structure-wise, a triangular sinus venosus joins the right atrium and receives blood from the vena cava. The ventricle opens into a sac-like conus arteriosus on the ventral side. Blood is distributed via the arterial system and collected by the venous system. Special connections exist: the hepatic portal system (between liver and intestine) and the renal portal system (between kidney and lower body parts). Blood consists of plasma and cells, including nucleated Red Blood Cells (RBCs/erythrocytes) containing haemoglobin, White Blood Cells (WBCs/leucocytes), and platelets. The lymphatic system consists of lymph, lymph channels, and lymph nodes; lymph differs from blood as it lacks RBCs and certain proteins.

Excretory System and Osmoregulation

The excretory system eliminates nitrogenous wastes and consists of a pair of kidneys, ureters, a cloaca, and a urinary bladder. The kidneys are compact, dark red, bean-like structures situated posteriorly near the vertebral column. Each kidney contains structural and functional units called uriniferous tubules or nephrons. In males, two ureters emerge as urinogenital ducts and open into the cloaca. In females, ureters and oviducts open separately into the cloaca. A thin-walled urinary bladder is located ventral to the rectum and also opens into the cloaca. Frogs are ureotelic animals because they excrete urea. Waste is carried by the blood to the kidneys for separation and excretion.

Control and Coordination: Neural and Endocrine Systems

Control and coordination are achieved through a highly evolved neural system and endocrine glands. Chemical coordination involves hormones secreted by glands such as the pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals, and gonads. The nervous system is divided into:

1. Central Nervous System (CNS): Includes the brain and spinal cord.
2. Peripheral Nervous System (PNS): Includes $10$ pairs of cranial nerves and spinal nerves.
3. Autonomic Nervous System (ANS): Includes sympathetic and parasympathetic divisions.

The brain is housed in a bony cranium (brain box) and is divided into the fore-brain (olfactory lobes, paired cerebral hemispheres, unpaired diencephalon), mid-brain (characterized by optic lobes), and hind-brain (cerebellum and medulla oblongata). The medulla oblongata exits through the foramen magnum into the spinal cord, which is protected by the vertebral column.

Sense Organs

Frogs possess various sense organs: organs of touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes), and hearing (tympanum with internal ears). The eyes are simple, spherical structures located in the skull orbits. External ears are absent; only the tympanum is visible externally. The ear functions for both hearing and maintaining equilibrium (balancing).

Reproductive System of the Frog

Frogs have well-organized reproductive systems.

• Male System: Consists of a pair of yellowish ovoid testes adhered to the kidneys by the mesorchium (a double fold of peritoneum). $10-12$ vasa efferentia arise from the testes, enter the kidneys, and open into Bidder’s canal. This communicates with the urinogenital duct, which opens into the cloaca.
• Female System: Consists of a pair of ovaries situated near the kidneys (with no functional connection to them). Oviducts open into the cloaca separately. A mature female can lay $2500$ to $3000$ ova at a time.
• Fertilization and Development: Fertilization is external and occurs in water. The life cycle includes a larval stage called a tadpole, which undergoes metamorphosis to become an adult.

Ecological and Economic Importance

Frogs are beneficial to humans as they prey on insects, protecting crops. They serve as a vital link in food chains and webs, maintaining ecological balance. Additionally, in some countries, the muscular legs of frogs are consumed as food.

Questions & Discussion

1. Draw a neat diagram of the digestive system of the frog. (The text refers to Figure 7.2 which shows the Alimentary Canal including Oesophagus, Liver, Gall bladder, Stomach, Pancreas, Intestine, Rectum, and Cloaca.)

2. Mention the function of the Ureters in frog. In male frogs, the ureters act as urinogenital ducts that carry both urine and sperms to the cloaca. In female frogs, the ureters carry only urine to the cloaca, as the oviducts open separately.