NEET Structural Organisation in Animals Previous Year Questions with Solutions

Frogs exhibit three distinct modes of respiration depending on their environmental state: 1. Cutaneous Respiration: Gas exchange across the thin, moist, highly vascularized skin. This is the SOLE mode of respiration when the frog is submerged in water, as well as during winter sleep (hibernation) and summer sleep (aestivation). 2. Buccopharyngeal Respiration: Occurs on land when the mouth remains closed while the buccal floor moves up and down. 3. Pulmonary Respiration: Occurs on land using a pair of sac-like lungs.

• • Mucus glands keep the skin moist to facilitate diffusion of dissolved oxygen.

1. Buccal cavity: Used only on land; cannot draw dissolved oxygen underwater. 2. Lungs: Frogs cannot breathe water into their lungs; lungs function exclusively in atmospheric air. 3. Trachea: Frogs lack a true trachea (they have a laryngotracheal chamber leading directly to lungs).

• • Option (1) is the only scientifically correct medium-specific organ.

In water, frogs respire using skin.

• • Matches Option (1).

The nervous system of the frog (Rana tigrina) is organized into a central nervous system (brain and spinal cord), a peripheral nervous system (cranial and spinal nerves), and an autonomic nervous system (sympathetic and parasympathetic).

• • Brain is enclosed in a cartilaginous or bony cranium (brain box).

In anamniotes (fishes and amphibians including frogs), exactly 10 pairs of cranial nerves arise from the brain (Olfactory I, Optic II, Oculomotor III, Trochlear IV, Trigeminal V, Abducens VI, Facial VII, Auditory VIII, Glossopharyngeal IX, and Vagus X). The 11th (Accessory) and 12th (Hypoglossal) nerves are absent.

• • NCERT verbatim: "10 pairs of cranial nerves arise from the brain."

In amniotes (reptiles, birds, and mammals including humans), 12 pairs of cranial nerves are present. Candidates who mistakenly apply human anatomy will select 12, which is incorrect for frog.

• • Option (3) is the correct answer.

The vascular system of frog is closed, complete, and contains a three-chambered heart (two atria and one ventricle):

• • Deoxygenated blood from the head, forelimbs, and body is collected by the anterior and posterior vena cava.

Evaluate the physiological role of the other options:

• • Aorta delivers systemic blood outward from the conus arteriosus.
• • Pulmonary veins carry oxygen-rich blood from the lungs into the left atrium.
• • Pulmonary arteries transport blood from heart to lungs for oxygenation.
• • Therefore, the vessel carrying deoxygenated blood from body to heart is the Vena cava, matching Option (4).

Male and female frogs can be distinguished externally during the breeding season by specific dimorphic structures:

• • Vocal sacs: Present in male frogs below the mouth to amplify mating calls.
• • Copulatory pad (nuptial pad): A specialised glandular swelling present on the ventral side of the first digit of the forelimb in male frogs.
• • Females lack both vocal sacs and copulatory pads.

During copulatory clasping (amplexus), fertilisation is external in water. The copulatory pad on the 1st digit of the forelimb provides tactile friction and a firm grip on the smooth, mucus-coated body of the female frog.

The pad is located strictly on the first digit of the forelimb (not on the second digit, and never on the hindlimbs). Thus, Option (4) is 100% correct.

A portal venous system is a vascular arrangement in which blood collected from one capillary bed is conveyed via a portal vein into a second capillary network in an intermediate organ before returning to the heart.

• • In amphibians such as frogs, two well-developed portal venous circuits exist: the hepatic portal system and the renal portal system.
• • Humans and mammals retain the hepatic portal system and hypophysial portal system, but the renal portal system is absent (reduced/vestigial).

Tracing the blood vessels in the posterior body:

• • Deoxygenated venous blood from the hindlimbs and pelvic region is collected by the femoral and sciatic veins.
• • These bifurcate to form the renal portal veins, which enter the kidneys and break up into a secondary capillary plexus.
• • This allows the renal tubular epithelium to clear nitrogenous waste and filter blood directly from the lower extremities.
• • The blood is subsequently collected by renal veins into the postcaval vein (posterior vena cava).
• • Thus, the renal portal system specifically links the kidney and lower part of the body.

Evaluating Option (1):

• • The venous connection that links the intestine (alimentary canal) to the liver is the hepatic portal system.
• • Conflating the hepatic and renal portal systems is an examiner's deliberate trap.

The renal portal system acts to link the kidney and lower parts of the body, matching Option (4).

When submerged underwater or during aestivation/hibernation:

• • Cutaneous respiration is the exclusive mode of respiration in water.
• • The skin is highly vascularized, thin, and kept perpetually moist by mucus glands.
• • Dissolved oxygen in the aquatic medium diffuses directly across the moist epidermis into cutaneous capillaries, while carbon dioxide diffuses out.
• • The mouth and external nares remain tightly closed underwater; buccal ventilation and pulmonary ventilation do not occur in water.
• • Therefore, the claim that frogs respire in water by "skin and buccal cavity" is factually FALSE.

When on land, the frog utilizes three integrated anatomical modalities:

• • 1. Cutaneous respiration: Oxygen continues to diffuse through the moist skin.
• • 2. Buccopharyngeal respiration: The lining of the buccal cavity is highly vascular; rhythmic raising and lowering of the hyoid apparatus draws air through external nares into the buccal cavity for gas exchange.
• • 3. Pulmonary respiration: A pair of elongated, pink, sac-like lungs perform pulmonary ventilation.
• • Therefore, the claim that on land frogs respire by "skin, buccal cavity and lungs" is completely TRUE.

Comparing the two clauses of the test statement:

• • Clause 1 (Water): "Frogs respire in water by skin and buccal cavity" -> FALSE.
• • Clause 2 (Land): "...and on land by skin, buccal cavity and lungs" -> TRUE.
• • Hence, the proposition is false for water but true for land.

The correct option is Option (3): The statement is false for water but true for land.

Skeletal muscle tissue is closely attached to skeletal bones. Striated muscle fibers are bundled together in a parallel fashion with multiple peripheral nuclei. Triceps and biceps are classic examples.

Smooth muscle fibers taper at both ends (fusiform) and do not show striations. Cell junctions hold them together and they are bundled in a connective tissue sheath. The wall of internal organs such as blood vessels, stomach and intestine contains this type of muscle tissue.

Cardiac muscle tissue is a contractile tissue present only in the heart. Cell junctions fuse the plasma membranes of cardiac muscle cells and make them stick together. Communication junctions (intercalated discs) at some fusion points allow the cells to contract as a unit.

The abdomen in both male and female cockroach consists of 10 segments.

NCERT explicitly describes: 1. Anal cerci: In both sexes, the 10th segment bears a pair of jointed filamentous structures called anal cerci (sensory to sound and vibrations). 2. Anal styles: Males bear a pair of short, threadlike anal styles on the 9th sternum which are absent in females.

Anal cerci are located on the 10th segment (Option 4).

Glandular epithelium is classified into two types based on the number of cells: - Unicellular: consisting of isolated glandular cells, like goblet cells of the alimentary canal (A -> III). - Multicellular: consisting of cluster of cells, like salivary glands (C -> I).

Compound epithelium is made of more than one layer (multi-layered) of cells and thus has a limited role in secretion and absorption. Its main function is to provide protection against chemical and mechanical stresses. They cover the dry surface of the skin, the moist surface of buccal cavity, pharynx, inner lining of ducts of salivary glands and of pancreatic ducts (B -> IV). Endocrine glands do not have ducts; their products called hormones are secreted directly into the fluid bathing the gland, exemplified by the endocrine islets of the pancreas (D -> II).

A-III, B-IV, C-I, D-II corresponds to Option (1).

The oesophagus opens into a sac like structure called crop used for storing of food (A -> IV). The crop is followed by gizzard or proventriculus. It has an outer layer of thick circular muscles and thick inner cuticle forming six highly chitinous plates called teeth. Gizzard helps in grinding the food particles (D -> I).

A ring of 6-8 blind tubules called hepatic or gastric caeca is present at the junction of foregut and midgut, which secrete digestive juice (B -> II). At the junction of midgut and hindgut is present another ring of 100-150 yellow coloured thin filamentous Malpighian tubules. They help in removal of excretory products from haemolymph (C -> III).

A-IV, B-II, C-III, D-I corresponds to Option (3).

Connective tissues are divided into three major categories based on ground matrix and fiber density:

Ligaments attach bone to bone and consist of parallel bundles of collagen and elastic fibers arranged in orderly regular rows. Hence, ligaments are dense regular connective tissue. Statement I calls them "dense irregular", which is FALSE.

Cartilage has a solid, pliable, chondroitin-rich intercellular material with chondrocytes enclosed in lacunae. NCERT explicitly categorizes cartilage as a specialised connective tissue, alongside bones and blood. Statement II calls it "dense regular", which is FALSE.

In cockroach, nitrogenous waste is extracted from haemolymph as potassium urate and converted to uric acid. While the primary excretory organs are Malpighian tubules, accessory excretory structures include:

NCERT states under Cockroach Excretion: "In addition, the fat body, nephrocytes and urecose glands also help in excretion."

Phallic gland (conglobate gland) is an accessory gland of the male reproductive system located near the ejaculatory duct. Collaterial glands are branched glands of the female reproductive system that secrete the scleroprotein hardening the ootheca.

Analyzing the specific tissue classifications across human body locations:

NCERT states under Epithelial and Connective Tissues: - "Ciliated epithelium... mainly present in the inner surface of hollow organs like bronchioles and fallopian tubes." - "Cuboidal epithelium is commonly found in ducts of glands and tubular parts of nephrons in kidneys." - "Areolar tissue... contains fibroblasts, macrophages and mast cells." - "Blood is a fluid connective tissue (specialised connective tissue)."

A-II, B-I, C-IV, D-III unambiguously selects Option (4).

Male and female cockroaches ($\textit{Periplaneta americana}$) exhibit distinct external abdominal appendages:

NCERT states under Cockroach Morphology: "In both sexes, the 10th segment bears a pair of jointed filamentous structures called anal cerci. In males, genital pouch contains a pair of short, thread-like anal styles which are absent in females."

Body color (dark brown/reddish brown) and exoskeletal chitinous plates (sclerites: tergites and sternites) are general morphological features shared identically across both sexes.

Smooth muscle fibres are spindle-shaped (fusiform), unbranched, tapering at both ends, and strictly non-striated (unstriped) because their contractile myofilaments (actin and myosin) are not arranged in regular repeating sarcomeres. - They are involuntary and regulated by the autonomic nervous system. - They form the muscularis coat in the walls of internal visceral organs, including the stomach, intestine, urinary bladder, uterus, and blood vessels (tunica media).

- In smooth muscle, adjacent fusiform myocytes are bound together by loose connective tissue and communicate via simple gap junctions (nexus) across cell membranes. - In cardiac muscle, adjacent branched fibres are joined end-to-end by specialized zig-zag transverse thickenings called intercalated discs. These structures contain adherens junctions, desmosomes, and gap junctions that synchronize rapid electrical depolarization so the myocardium contracts as a functional syncytium. - Therefore, claiming that smooth muscle cells communicate via intercalated discs is factually wrong.

The digestive tract of *Periplaneta americana* is divided into three distinct morphological regions: - Foregut (Stomodaeum): Comprises mouth, pharynx, oesophagus, crop (food storage), and gizzard / proventriculus (cuticular teeth for grinding). - Junction of Foregut and Midgut: Possesses a ring of 6–8 blind tubules termed hepatic or gastric caeca, which secrete digestive juices into the mesenteron. - Midgut (Mesenteron): True secretory and absorptive epithelial stomach. - Junction of Midgut and Hindgut: Possesses a ring of 100–150 yellow-coloured, thin filamentous Malpighian tubules, which perform excretion by extracting uric acid and potassium urate from the haemolymph. - Hindgut (Proctodaeum): Differentiated into ileum, colon, and rectum.

- Option (2): The hypopharynx is a median flexible lobe acting as the tongue, situated within the preoral cavity enclosed by labrum, mandibles, maxillae, and labium (True). - Option (3): In females, the 7th sternum is boat-shaped and, together with the 8th and 9th sterna, forms a female genital pouch (gynatrium) enclosing the gonopore, spermathecal pores, and collateral glands (True). - Option (4): The 10th abdominal segment in both males and females bears a pair of jointed filamentous sensory structures called anal cerci (True; while anal styles are present only on the 9th sternum of males).

Epithelial tissues line body surfaces, cavities, and tubules, providing protective and barrier functions. Adjacent epithelial cells are interlinked by three specialized junctional complexes: - 1. Tight Junctions (Zonula Occludens): Transmembrane claudin and occludin proteins fuse adjacent plasma membranes into a continuous impermeable belt, preventing the paracellular leakage of luminal fluids, ions, and solutes across the tissue. - 2. Adhering Junctions (Zonula Adherens / Desmosomes): Cadherin adhesion molecules anchored to actin microfilaments or intermediate keratin filaments cement adjacent cells together, providing mechanical tensile strength. - 3. Gap Junctions (Nexus): Hexameric rings of connexin proteins form hollow cylindrical channels (connexons) spanning the intercellular gap, directly coupling the cytoplasms of adjacent cells to permit the rapid, unhindered transfer of inorganic ions, second messengers (cAMP, IP₃), metabolites, and small molecules.

The question specifically requires the respective pairing for (1) stopping leakage and (2) communicating via ion transfer. This corresponds strictly to "Tight junctions and Gap junctions, respectively" (Option 2).

The anterior cephalic end of the earthworm features specialized adaptations for subterranean burrowing in soil: - Prostomium: A fleshy, muscular dorsal lobe projecting above and overhanging the ventral mouth opening. It is pre-segmental (not a true metamere / body segment): - (a) Mouth Covering: It acts as a protective dorsal hood over the mouth. - (b) Soil Wedging: It functions as a powerful mechanical wedge to force open crevices and fissures in compact soil, allowing the worm to crawl forward. - (c) Sensory Function: It is heavily innervated with photoreceptors and chemoreceptive tactile cells to sample light, chemical gradients, and soil moisture.

- Peristomium (Buccal Segment): The first true body segment of the earthworm is called the peristomium, which completely encircles the crescentic mouth opening. - Because the prostomium is merely a pre-segmental fleshy lobe overhanging segment 1, stating that the prostomium is the first body segment ((d)) is a direct anatomical error. - Therefore, statements (a), (b), and (c) are correct, while statement (d) is incorrect.

The proximal convoluted tubule (PCT) of the renal nephron performs the bulk of tubular reabsorption: - Approximately 70–80% of filtered water, electrolytes ($Na^+$, $K^+$, $Cl^-$, $HCO_3^-$), and virtually 100% of essential nutrients (glucose, amino acids) are actively and passively reabsorbed here. - To support this massive transport load, the PCT is lined by simple cuboidal epithelial cells featuring a densely packed apical array of finger-like projections called microvilli (brush border). - This brush border increases the luminal absorptive surface area more than 20-fold. The basolateral membranes are deeply invaginated and packed with abundant mitochondria to power $Na^+/K^+$ ATPase pumps.

- Lining of intestine: Lined by simple columnar epithelium with microvilli and goblet cells, not cuboidal. - Ducts of salivary glands: Lined by simple cuboidal/columnar or compound stratified epithelium without an apical brush border. - Eustachian tube: Lined by ciliated pseudostratified columnar epithelium.

The nervous system of *Periplaneta americana* is segmental and decentralized: - Head Region: The cephalic region contains the supra-oesophageal ganglion (cerebral ganglion / "brain"), which supplies sensory nerves strictly to the antennae and compound eyes. It represents only a tiny fraction of the total central nervous system. - Body Trunk: The rest of the central nervous system consists of a double, solid ventral nerve cord carrying 9 segmental ganglia: - 3 large ganglia in the thorax (pro-, meso-, and meta-thoracic ganglia controlling legs and wings). - 6 ganglia in the abdomen coordinating autonomic segmental and reproductive reflexes.

Because respiration occurs passively via segmental lateral spiracles and trachea (independent of head or mouth breathing), and because locomotion and visceral tone are controlled by thoracic and abdominal ganglia along the ventral nerve cord, a decapitated cockroach can survive for up to a week until it ultimately dies of dehydration or starvation.

Some of the columnar or cuboidal cells get specialized for secretion and are called glandular epithelium. They are mainly of two types: unicellular, consisting of isolated glandular cells (goblet cells of the alimentary canal), and multicellular, consisting of cluster of cells (salivary glands).

In the mucosal lining of the gastrointestinal tract (stomach, small intestine, colon), goblet cells are interspersed among tall absorptive enterocytes and are direct modifications of columnar epithelial cells.

Goblet cells synthesize and secrete mucin glycoproteins that form a protective lubricating mucus barrier over the mucosal surface, shielding epithelial cells from digestive enzymes and low pH.

Squamous epithelium lines blood vessels and air sacs; chondrocytes are cartilage cells; compound epithelium forms multi-layered protective skin. Only columnar epithelial cells differentiate into intestinal goblet cells.