In simpler organisms like unicellular animals, all essential life functions, such as digestion, respiration, and reproduction, are carried out within a single cell.

However, in complex multicellular animals, the body is organized into different groups of specialized cells that work together in a coordinated manner to perform these same basic functions. For example, even in a relatively simple multicellular organism like *Hydra*, there are thousands of cells organized into different types.

In highly complex animals, like humans, billions of cells cooperate. This cooperation is achieved through the organization of similar cells and their surrounding intercellular substances into functional units called tissues, which perform specific tasks.

Complex animals are built from just four basic types of tissues. These tissues are precisely arranged and proportioned to form organs (e.g., stomach, heart, kidney).

Furthermore, two or more organs interact physically and/or chemically to perform a common function, forming an organ system (e.g., digestive system, respiratory system).

The organization into cells, tissues, organs, and organ systems demonstrates a clear division of labour, ensuring efficiency and contributing to the overall survival of the organism.

Animal Tissues

The structure of animal cells is directly related to their specific functions, leading to different types of tissues. Animal tissues are broadly classified into four fundamental types:

1. Epithelial Tissue
2. Connective Tissue
3. Muscular Tissue
4. Neural Tissue

---

Epithelial Tissue

Commonly known as epithelium, this tissue forms coverings or linings for various parts of the body. It always has a free surface that is exposed either to a body fluid (like blood) or the external environment.

Epithelial cells are densely packed with very little intercellular space or matrix.

There are two main types of epithelial tissues:

1. Simple Epithelium: Composed of a single layer of cells. It functions primarily as linings for body cavities, ducts, and tubes.
2. Compound Epithelium: Consists of two or more cell layers. Its main role is to provide protection against chemical and mechanical stress, as seen in the skin.

---

Types of Simple Epithelium (Based on cell shape):

1. Squamous Epithelium: Made of a single, very thin layer of flattened cells with irregular boundaries. Forms diffusion barriers in structures like the walls of blood vessels and air sacs of the lungs.
2. Cuboidal Epithelium: Composed of a single layer of cube-shaped cells. Found in ducts of glands and kidney tubules (nephrons). Main functions are secretion and absorption. Some cells, like those in the proximal convoluted tubule (PCT) of the kidney, have microvilli to increase surface area for absorption.
3. Columnar Epithelium: Made up of a single layer of tall and slender cells. Nuclei are typically located at the base. The free surface may have microvilli. Lines the stomach and intestine, involved in secretion and absorption.

Ciliated Epithelium: If cuboidal or columnar cells bear cilia on their free surface, they form ciliated epithelium. Cilia help move particles or mucus in a specific direction over the epithelial surface. Found lining hollow organs like bronchioles and fallopian tubes.

---

Glandular Epithelium:

Modified columnar or cuboidal cells specialized for secretion.

• Unicellular glands: Consist of isolated glandular cells, like goblet cells in the alimentary canal that secrete mucus.
• Multicellular glands: Consist of clusters of cells, like those forming salivary glands.

Based on how they release their secretions, glands are:

• Exocrine glands: Secrete products (mucus, saliva, enzymes, milk, etc.) through ducts or tubes.
• Endocrine glands: Ductless glands that secrete hormones directly into the surrounding fluid (e.g., blood).

---

Compound Epithelium:

Made of more than one layer of cells.

Main function is protection against mechanical and chemical stress.

Limited role in secretion and absorption.

Found covering surfaces like the dry skin surface, moist lining of the buccal cavity and pharynx, and inner lining of ducts of salivary and pancreatic glands.

---

Cell Junctions:

Specialized junctions hold epithelial cells together and provide structural and functional connections. Three types are found in epithelium and other tissues:

• Tight junctions: Prevent substances from leaking across the tissue.
• Adhering junctions: Function as cement to keep neighboring cells attached.
• Gap junctions: Facilitate communication by connecting the cytoplasm of adjacent cells, allowing rapid transfer of ions and small/large molecules.

---

Connective Tissue

Connective tissues are the most abundant and widely distributed tissues in complex animals. Their primary role is to link and support other tissues and organs.

They range from soft tissues to specialized types.

Components: In most connective tissues (except blood), cells secrete protein fibers (like collagen or elastin) that provide strength, elasticity, and flexibility. Cells also secrete modified polysaccharides that form the matrix (ground substance) in which cells and fibers are embedded.

Classification:

1. Loose connective tissue: Cells and fibers are loosely arranged in a semi-fluid matrix.
   • Areolar tissue: Located beneath the skin, serving as a supporting framework for epithelium. Contains fibroblasts (secrete fibers), macrophages (phagocytic cells), and mast cells (release inflammatory mediators).
   • Adipose tissue: Another type of loose connective tissue, mainly found beneath the skin. Cells (adipocytes) are specialized for fat storage. Excess nutrients are converted and stored as fat in this tissue.
2. Dense connective tissue: Fibers and fibroblasts are densely packed.
   • Dense regular connective tissue: Collagen fibers are arranged in parallel rows between parallel bundles of fibers. Forms structures like tendons (connect skeletal muscle to bone) and ligaments (connect bone to bone).
   • Dense irregular connective tissue: Fibroblasts and many fibers (mostly collagen) are oriented in different, irregular patterns. Found in the skin.
3. Specialised connective tissue: Includes cartilage, bone, and blood.
• Cartilage: Has a solid, pliable intercellular matrix that resists compression. Cells (chondrocytes) are enclosed in small cavities (lacunae) within the matrix they secrete. Most cartilage in vertebrate embryos is replaced by bone in adults. Found in the tip of the nose, outer ear, between vertebrae, and in limb/hand joints in adults.
• Bone: Has a hard, non-pliable matrix rich in calcium salts and collagen fibers, providing strength. Forms the structural framework of the body, supporting and protecting tissues/organs. Bone cells (osteocytes) are located in spaces called lacunae. Limb bones support weight and enable movement via muscle attachment. Bone marrow in some bones produces blood cells.
• Blood: A fluid connective tissue. Components include plasma (fluid matrix) and blood cells: red blood cells (RBCs), white blood cells (WBCs), and platelets. It is the primary transport medium for various substances throughout the body.

---

Muscle Tissue

Muscle tissue is composed of long, cylindrical fibers called muscle fibers, arranged in parallel. These fibers contain fine fibrils called myofibrils.

Function: Muscle fibers are capable of contraction (shortening) in response to stimulation and relaxation (lengthening). This coordinated action enables movement of the body and maintenance of posture.

Three types of muscle tissue:

1. Skeletal Muscle: Closely associated with skeletal bones. Fibers are bundled together in a parallel manner and show characteristic striations (striped appearance) under the microscope. Skeletal muscles are generally under voluntary control (we can consciously control their contraction, e.g., biceps).
2. Smooth Muscle: Fibers are fusiform (tapering at both ends) and do not show striations. Cells are held together by cell junctions and bundled by connective tissue. Forms the walls of internal organs like blood vessels, stomach, and intestine. Smooth muscles are involuntary, meaning their function cannot be directly controlled consciously.
3. Cardiac Muscle: Contractile tissue found only in the heart. Cardiac muscle cells are joined by cell junctions that cause them to stick together. Intercalated discs (communication junctions) are present at fusion points, allowing cells to contract as a unit – a signal to one cell quickly spreads to neighboring cells. Cardiac muscle tissue is also striated and involuntary.

---

Neural Tissue

Neural tissue provides the body with the ability to respond rapidly to changing conditions and controls most body functions.

Primary Cells: The functional units of the neural system are neurons, which are excitable cells.

Support Cells: Neuroglial cells constitute the remaining part of the neural tissue. They protect and support neurons and make up more than half the volume of neural tissue in the body.

Function: When a neuron receives a stimulus, it generates an electrical impulse that travels along its plasma membrane (axon). At the neuron's endings, this disturbance triggers the release of chemical signals (neurotransmitters) that can stimulate or inhibit adjacent neurons or other cells.

Organ And Organ System

The four basic types of tissues (epithelial, connective, muscular, and neural) organize to form organs in multicellular organisms. These organs then associate to form organ systems.

This hierarchical organization (cells $\rightarrow$ tissues $\rightarrow$ organs $\rightarrow$ organ systems) allows for highly efficient and coordinated activities, enabling complex organisms to survive and function.

Each organ in the body is typically composed of one or more types of tissues. For example, the heart is made of all four basic tissue types (epithelial lining, connective tissue framework, cardiac muscle for contraction, and neural tissue for regulation).

The complexity observed in the organization of organs and organ systems often reflects evolutionary trends.

Morphology refers to the study of the form or external features of an organism or its parts. In animals, it describes the external appearance of organs or body parts.

Anatomy conventionally refers to the study of the morphology of internal organs in animals, typically involving dissection or imaging techniques to examine internal structures.

To illustrate the structural organization at different evolutionary levels, the morphology and anatomy of three representative organisms are often studied: Earthworm (an invertebrate), Cockroach (an invertebrate), and Frog (a vertebrate).

Earthworm

Earthworms are reddish-brown terrestrial invertebrates belonging to the phylum Annelida. They inhabit the upper layers of moist soil, creating burrows by consuming soil.

Their presence in gardens is often indicated by 'worm castings', which are fecal deposits.

Common Indian earthworms include *Pheretima* and *Lumbricus*.

---

Morphology

Earthworms have a long, cylindrical body that is distinctly segmented. These segments, called metameres, are numerous (about 100-120) and externally similar.

The dorsal surface is identifiable by a dark median line running along the body's length, which is the dorsal blood vessel visible through the body wall.

The ventral surface is distinguished by the presence of genital openings.

The anterior end has the mouth, covered by a lobe called the prostomium. The prostomium helps in opening cracks in the soil for burrowing and is sensory.

The first body segment is the peristomium (or buccal segment), which contains the mouth.

In mature worms, segments 14-16 are covered by a prominent, dark, glandular band called the clitellum.

Based on the clitellum, the body is divided into three regions: preclitellar (segments before 14), clitellar (segments 14-16), and postclitellar (segments after 16).

External Openings:

• Spermathecal apertures: Four pairs, ventro-lateral, in intersegmental grooves of segments 5/6, 6/7, 7/8, and 8/9.
• Female genital pore: Single, median-ventral, on segment 14.
• Male genital pores: A pair, ventro-lateral, on segment 18.
• Nephridiopores: Numerous minute pores opening on the body surface in almost all segments (except the first, last, and clitellum, depending on type).
• Anus: Small, rounded aperture at the posterior end.

Setae: S-shaped chitinous bristles embedded in epidermal pits. Present in rows in each segment (except the first, last, and clitellum). They can be extended or retracted and play a key role in locomotion.

---

Anatomy

Body Wall: From outside to inside, the body wall consists of a thin, non-cellular cuticle, followed by the epidermis (single layer of columnar epithelial and glandular cells), two muscle layers (outer circular, inner longitudinal), and an innermost coelomic epithelium.

Alimentary Canal: A straight tube extending from the first to the last segment (Figure 7.10).

• Mouth (terminal) $\rightarrow$ Buccal cavity (segments 1-3) $\rightarrow$ Muscular Pharynx $\rightarrow$ Oesophagus (segments 5-7) $\rightarrow$ Muscular Gizzard (segments 8-9; for grinding soil/leaves) $\rightarrow$ Stomach (segments 9-14; contains calciferous glands to neutralize humic acid) $\rightarrow$ Intestine (starts from segment 15, continues to the end).
• Intestinal caecae: A pair of short, conical projections from the intestine on segment 26.
• Typhlosole: An internal, median fold of the dorsal intestinal wall, present after segment 26 (except the last few). It significantly increases the surface area for absorption.
• Anus: External opening at the end of the intestine.

Digestion: Earthworms ingest organic-rich soil. Digestive enzymes break down complex organic matter. Simple molecules are absorbed through the intestinal wall.

Circulatory System: Closed type blood vascular system (Figure 7.11), meaning blood is always confined within blood vessels and hearts. Composed of hearts (pumping organs), blood vessels, and capillaries. Blood circulation is unidirectional due to contractions of vessels/hearts. Smaller vessels supply the gut, nerve cord, and body wall.

• Blood: Composed of plasma and blood cells. Haemoglobin is dissolved in the plasma (not in RBCs like vertebrates).
• Blood glands: Present in segments 4, 5, and 6, producing blood cells and haemoglobin.
• Blood cells: Phagocytic.

Respiration: Lack specialized respiratory organs. Gaseous exchange occurs directly through the moist body surface into the blood stream by diffusion.

Excretory System: Consists of segmentally arranged, coiled tubular structures called nephridia (Figure 7.12). They regulate fluid volume and composition by collecting excess coelomic fluid and removing wastes.

Types of Nephridia based on location:

• Septal nephridia: Present on both sides of intersegmental septa from segment 15 to the last. Open into the intestine.
• Integumentary nephridia: Attached to the body wall lining from segment 3 to the last. Open directly onto the body surface.
• Pharyngeal nephridia: Present as three paired tufts in segments 4, 5, and 6.

Nervous System: Consists of segmentally arranged ganglia joined by paired ventral nerve cords. In the anterior region (segments 3-4), the nerve cord splits, encircles the pharynx, and connects to dorsal cerebral ganglia to form a nerve ring (brain ring). The cerebral ganglia process sensory information and control muscular responses.

Sensory System: Lack eyes, but have light- and touch-sensitive receptor cells to sense light intensity and ground vibrations. Specialized chemoreceptors (taste receptors) respond to chemical stimuli, located on the anterior part of the body.

Reproductive System: Earthworms are hermaphrodite (bisexual), with both male and female organs in the same individual (Figure 7.13). They are protandrous, meaning the male organs mature earlier than the female ones.

• Male: Two pairs of testes (segments 10, 11). Vasa deferentia from each testis join prostatic ducts and open via paired male genital pores on segment 18. Accessory glands are in segments 17 and 19.
• Female: A pair of ovaries (at inter-segmental septum of 12/13). Ovarian funnels beneath ovaries continue into oviducts, which join and open via a single median female genital pore on segment 14.
• Spermathecae: Four pairs (segments 6-9) store sperm received during copulation.

Reproduction Process:

• Copulation: Mutual exchange of sperm between two worms. They align their bodies so opposite gonadal openings are juxtaposed, exchanging packets of sperm called spermatophores.
• Cocoon Formation: The clitellum secretes a mucus ring that forms a cocoon. Eggs, sperm (from spermathecae), and nutritive fluid are deposited into the cocoon.
• Fertilisation and Development: Fertilisation occurs within the cocoon. The cocoon is shed into the soil. Development is direct (no larval stage). Baby worms emerge from the cocoon after about 3 weeks. Each cocoon typically yields an average of four young worms (range 2-20).

Economic Importance:

• Known as 'friends of farmers'. Their burrowing action makes soil porous, improving aeration and root penetration.
• Help increase soil fertility through vermicomposting (decomposition of organic matter).
• Used as bait for fishing.

Cockroach

Cockroaches are insects belonging to the class Insecta of Phylum Arthropoda. Common species (*Periplaneta americana*) are brown or black, but other colors exist in tropical regions.

They range in size and possess long antennae, legs, and a flattened upper body wall extension (pronotum) covering the head.

Habitat and Behavior: They are nocturnal, omnivorous, and prefer damp places. They are commonly found in human dwellings, acting as serious pests and vectors of diseases by contaminating food.

---

Morphology

*Periplaneta americana* adults are 34-53 mm long. Wings extend beyond the abdomen tip in males.

Body is segmented and divided into three distinct regions: Head, Thorax, and Abdomen (Figure 7.14).

Exoskeleton: The entire body is covered by a hard, brown chitinous exoskeleton, providing protection and support. Segments have hardened plates called sclerites (tergites dorsally, sternites ventrally), connected by a flexible arthrodial membrane.

Head: Triangular, located anteriorly, oriented at right angles to the body axis. Formed by fusion of six segments, allowing great mobility via a flexible neck. Bears:

• A pair of large compound eyes dorsally.
• A pair of thread-like antennae arising from sockets in front of eyes. Antennae have sensory receptors.
• Mouthparts: At the anterior end, modified for biting and chewing. Consist of:
  • Labrum (upper lip)
  • Pair of Mandibles (jaws, with grinding and incising regions)
  • Pair of Maxillae (accessory jaws, for handling food)
  • Labium (lower lip)
  • Hypopharynx (median flexible lobe, acting as a tongue within the mouth cavity).

Thorax: Composed of three segments: prothorax, mesothorax, and metathorax. Connected to the head by the neck (an extension of the prothorax).

• Each thoracic segment bears a pair of walking legs (total 3 pairs).
• Wings:
  • First pair: Forewings (tegmina) arise from the mesothorax. They are opaque, dark, leathery, and cover the hindwings at rest, providing protection.
  • Second pair: Hindwings arise from the metathorax. They are transparent, membranous, and used for flight.

Abdomen: Consists of 10 segments in both males and females.

• Females: 7th sternum is boat-shaped, combined with 8th and 9th sterna to form a brood or genital pouch. This pouch contains the female gonopore, spermathecal pores, and collateral glands.
• Males: Genital pouch or chamber is at the hind end, bounded by the 9th and 10th terga dorsally and 9th sternum ventrally. It contains the dorsal anus, ventral male genital pore, and gonapophyses (external genitalia).
• Anal styles: A pair of short, thread-like structures present in males, absent in females.
• Anal cerci: A pair of jointed, filamentous structures borne on the 10th segment in both sexes.

---

Anatomy

Alimentary Canal: A tube divided into three regions within the body cavity (Figure 7.16):

1. Foregut: Lined by cuticle. Includes: Mouth $\rightarrow$ Pharynx (short tube) $\rightarrow$ Oesophagus (narrow tube) $\rightarrow$ Crop (sac for food storage) $\rightarrow$ Gizzard (Proventriculus) (outer thick circular muscles, inner chitinous plates/teeth for grinding food).
2. Midgut (Mesenteron): Not lined by cuticle. Site of digestion and absorption.
3. Hindgut: Broader than midgut. Differentiated into Ileum $\rightarrow$ Colon $\rightarrow$ Rectum. Rectum opens to the exterior through the anus.

Accessory Structures:

• Hepatic or gastric caeca: A ring of 6-8 blind tubules at the junction of foregut and midgut. Secrete digestive juice.
• Malpighian tubules: A ring of 100-150 thin, yellow, filamentous tubules at the junction of midgut and hindgut. Involved in excretion.

Blood Vascular System: Open type (Figure 7.17). Blood vessels are poorly developed, opening into spaces called the haemocoel. Visceral organs are bathed in blood (haemolymph).

• Haemolymph: Colourless plasma and haemocytes. Does not transport oxygen (haemoglobin is absent).
• Heart: Elongated muscular tube along the mid-dorsal line, differentiated into funnel-shaped chambers with openings (ostia) on either side. Blood enters heart through ostia and is pumped anteriorly.

Respiratory System: A network of air tubes (trachea) that transport oxygen directly to tissues.

• Openings: Tracheae open to the outside through 10 pairs of small holes called spiracles, located on the lateral sides of the body. Spiracles are regulated by sphincters.
• Tracheae branch into smaller tubes called tracheoles.
• Gas Exchange: Occurs by diffusion at the tracheoles.

Excretory System: Primary excretory organs are the Malpighian tubules. They are lined by glandular and ciliated cells that absorb nitrogenous waste products from the haemolymph and convert them into uric acid.

• Excretion: Uric acid is excreted out through the hindgut. Cockroaches are therefore uricotelic.
• Other organs assisting in excretion: Fat body, nephrocytes, and urecose glands.

Nervous System: Organized into a series of fused, segmentally arranged ganglia connected by paired ventral longitudinal connectives.

• Ganglia: Three in the thorax, six in the abdomen.
• Head Ganglion: The brain is represented by the supra-oesophageal ganglion in the head, which innervates the antennae and compound eyes.
• Significance of ventral nerve cord: A large part of the nervous system is located ventrally in the thorax and abdomen. This explains why a cockroach can survive for about a week even after its head is cut off.

Sense Organs: Include antennae, compound eyes, maxillary palps, labial palps, and anal cerci.

• Compound eyes: Located dorsally on the head. Each eye consists of about 2000 hexagonal units called ommatidia.
• Vision: Produce mosaic vision (multiple images), which has high sensitivity but lower resolution. This type of vision is especially effective in low light conditions (nocturnal vision).

Reproductive System: Cockroaches are dioecious (separate sexes) with well-developed reproductive organs (Figure 7.18).

• Male: Pair of testes (segments 4-6). Vasa deferentia lead to the ejaculatory duct via seminal vesicles (store sperm). Ejaculatory duct opens at the male gonopore (ventral to anus). Mushroom-shaped gland (segments 6-7) functions as accessory reproductive gland. External genitalia are chitinous, asymmetrical structures called gonapophyses or phallomeres surrounding the gonopore. Sperm are stored as bundles called spermatophores.
• Female: Two large ovaries (segments 2-6). Each ovary has 8 ovarian tubules (ovarioles) containing developing eggs. Oviducts from each ovary unite to form a single median oviduct (vagina) opening into the genital chamber. A pair of spermathecae (segment 6) also open into the genital chamber to receive sperm during mating.

Reproduction and Development:

• Sperm transfer: Via spermatophores.
• Fertilization: Internal.
• Eggs are encased in dark, reddish-brown capsules called oothecae (about 8 mm long). Females produce 9-10 oothecae, each containing 14-16 eggs. Oothecae are dropped or glued near a food source in high humidity.
• Development: Paurometabolous, involving gradual metamorphosis. Young stages called nymphs resemble adults but lack wings. Nymphs grow by moulting (about 13 times) to reach the adult stage. Wing pads appear in the second-to-last nymphal stage, but functional wings are only in adults.

Economic Impact: While many species are wild, those living with humans are pests. They spoil and contaminate food with excreta, causing bacterial diseases.

Frogs

Frogs belong to the class Amphibia of phylum Chordata, capable of living in both freshwater and on land. *Rana tigrina* is a common species in India.

Thermoregulation: Frogs are cold-blooded (poikilotherms), meaning their body temperature fluctuates with the environment.

Adaptations for Survival:

• Camouflage (Mimicry): Ability to change skin colour to blend with surroundings (e.g., grass, dry land), hiding from predators.
• Aestivation (Summer sleep): Taking shelter in deep burrows during peak summer to avoid extreme heat.
• Hibernation (Winter sleep): Similar behavior during winter to protect from extreme cold.

---

Morphology

Skin: Smooth and slippery due to mucus secretion, keeping it moist. Dorsal side is usually olive green with dark spots, ventral side is uniformly pale yellow. Frogs absorb water directly through their skin, not by drinking.

Body Division: Body is divisible into head and trunk (Figure 7.19). Neck and tail are absent in adults.

External Features:

• Nostrils: A pair located above the mouth.
• Eyes: Bulging, covered by a nictitating membrane (transparent eyelid) for protection in water.
• Tympanum: A membranous structure on either side of the head, representing the ear, receives sound signals.
• Limbs: Two pairs, adapted for various movements (swimming, walking, leaping, burrowing). Forelimbs (4 digits) are shorter than hindlimbs (5 digits). Hindlimb digits are webbed, aiding in swimming.

Sexual Dimorphism: Male and female frogs differ in appearance (Figure 7.19):

• Males have sound-producing vocal sacs.
• Males have a rough-skinned copulatory pad on the first digit of the forelimbs, which helps in holding the female during mating (amplexus).
• These structures are absent in females.

---

Anatomy

Body Cavity: Houses various well-developed organ systems.

Digestive System: Consists of the alimentary canal and digestive glands (Figure 7.20).

• Alimentary Canal: Relatively short due to their carnivorous diet (digestion of meat requires a shorter intestine than plant matter). Mouth $\rightarrow$ Buccal cavity $\rightarrow$ Pharynx $\rightarrow$ Oesophagus (short tube) $\rightarrow$ Stomach $\rightarrow$ Intestine $\rightarrow$ Rectum $\rightarrow$ Cloaca $\rightarrow$ Cloacal aperture (exterior opening).
• Tongue: Bilobed, used to capture prey.
• Digestive Glands: Liver (secretes bile, stored in gall bladder), Pancreas (secretes pancreatic juice with enzymes).
• Digestion: Begins in the stomach with HCl and gastric juices. Partially digested food (chyme) moves to the duodenum (first part of small intestine). Duodenum receives bile (emulsifies fat) and pancreatic juice (digests carbohydrates and proteins) via a common bile duct. Final digestion occurs in the intestine.
• Absorption: Digested food is absorbed through finger-like folds (villi and microvilli) in the intestinal lining.
• Waste Elimination: Undigested waste passes into the rectum and is eliminated through the cloaca.

Respiratory System: Uses different organs depending on habitat and activity.

• In water: Skin (cutaneous respiration) acts as the primary respiratory organ. Dissolved oxygen diffuses through the moist skin into blood.
• On land: Buccal cavity lining, skin, and lungs are used.
  • Pulmonary respiration: Breathing using lungs. Lungs are a pair of elongated, pink, sac-like structures in the trunk (thorax). Air enters via nostrils $\rightarrow$ buccal cavity $\rightarrow$ lungs.
• During aestivation/hibernation: Gaseous exchange primarily occurs through the skin.

Circulatory System: Well-developed closed type blood vascular system, also has a lymphatic system. Blood vascular system includes heart, blood vessels, and blood. Lymphatic system includes lymph, lymph channels, and lymph nodes.

• Heart: Muscular, three-chambered (two atria, one ventricle), located in the upper body cavity, enclosed by a pericardium membrane.
  • Sinus venosus: Triangular structure joining the right atrium, receives deoxygenated blood from major veins (vena cavae).
  • Conus arteriosus: Sac-like structure ventral to the ventricle, pumps oxygenated and deoxygenated blood to the body and lungs/skin.
• Blood Vessels: Arteries carry blood away from heart (arterial system). Veins collect blood from body parts back to heart (venous system).
• Portal Systems: Special venous connections:
  • Hepatic portal system: Between intestine and liver.
  • Renal portal system: Between kidneys and lower body parts.
• Blood: Plasma + blood cells. RBCs are nucleated and contain haemoglobin. Also have WBCs and platelets.
• Lymph: Fluid similar to blood plasma but lacks some proteins and RBCs.
• Circulation: Achieved by the pumping action of the heart, transporting nutrients, gases, and water. (Note: due to the single ventricle, there is mixing of oxygenated and deoxygenated blood).

Excretory System: Well-developed, eliminates nitrogenous wastes. Consists of:

• Kidneys: A pair of compact, dark red, bean-shaped structures located posteriorly on either side of the vertebral column. Structural and functional units are uriniferous tubules (nephrons).
• Ureters: Tubes emerging from kidneys. In male frogs, they also serve as urinogenital ducts, carrying both urine and sperm to the cloaca. In females, ureters and oviducts open separately into the cloaca.
• Urinary bladder: Thin-walled sac ventral to the rectum, also opening into the cloaca. Stores urine temporarily.
• Cloaca: Small median chamber receiving waste from rectum, ureters (and oviducts/urinogenital duct). Opens to the exterior via the cloacal aperture.

Nature of Excretion: Frogs excrete urea, hence are ureotelic. Waste is filtered from blood by kidneys and excreted.

Control and Coordination: Highly evolved system involving both neural system and endocrine glands.

• Endocrine Glands: Pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals, gonads. Secrete hormones for chemical coordination.
• Neural System: Organized into:
  • Central Nervous System (CNS): Brain and spinal cord.
  • Peripheral Nervous System (PNS): Cranial nerves (10 pairs from brain) and spinal nerves.
  • Autonomic Nervous System (ANS): Sympathetic and parasympathetic nervous system.
• Brain: Enclosed in a bony skull structure called the brain box (cranium). Divided into:
  • Forebrain: Olfactory lobes, paired cerebral hemispheres, unpaired diencephalon.
  • Midbrain: Characterized by a pair of optic lobes.
  • Hindbrain: Cerebellum and medulla oblongata.
• Spinal Cord: Medulla oblongata passes through the foramen magnum and continues as the spinal cord within the vertebral column.

Sense Organs: Include specialized organs for touch, taste, smell, vision, and hearing.

• Organs of touch: Sensory papillae.
• Taste: Taste buds.
• Smell: Nasal epithelium.
• Vision: Eyes.
• Hearing: Tympanum and internal ears.
• Eyes: Paired, spherical structures in skull orbits. They are simple eyes (unlike cockroach compound eyes).
• Ear: External ear absent, only tympanum visible externally. Ear is for both hearing and balance (equilibrium).

Reproductive System: Separate sexes with well-organized male and female systems.

• Male: Pair of yellowish, ovoid testes (Figure 7.21) attached to the upper part of kidneys by mesorchium (peritoneal fold). 10-12 vasa efferentia arise from testes, enter kidneys, and open into Bidder's canal. Bidder's canal communicates with the urinogenital duct (common passage for urine and sperm) which exits kidneys and opens into the cloaca.
• Female: Pair of ovaries (Figure 7.22) situated near kidneys (no functional connection to kidneys for eggs). A pair of oviducts arise from ovaries and open separately into the cloaca.

Reproduction Process:

• A mature female can lay 2500-3000 ova (eggs) at once.
• Fertilization is external, occurring in water.
• Development involves a larval stage called a tadpole.
• Tadpoles undergo metamorphosis to transform into adult frogs.

Importance:

• Beneficial to humans by eating insects, protecting crops.
• Maintain ecological balance as a link in food chains/webs.
• Muscular legs are consumed as food in some cultures.

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer: