Zoology 2021 Paper I 50 marks Describe

Q2

(a) What is metamorphosis? Describe the hormonal regulation of metamorphosis in insects. (20 marks) (b) What is alternation of generations? Illustrate this phenomenon with life history of Obelia. (15 marks) (c) Describe the structure and functions of internal ear in mammals. (15 marks)

हिंदी में प्रश्न पढ़ें

(a) कायांतरण क्या है? कीटों में कायांतरण के हार्मोनल विनियमन का वर्णन कीजिए। (20 अंक) (b) पीढ़ियों का प्रत्यावर्तन क्या है? ओबेलिया के जीवन-इतिहास के साथ इस परिघटना को स्पष्ट कीजिए। (15 अंक) (c) स्तनधारियों में आंतरिक कान की संरचना और कार्यों का वर्णन कीजिए। (15 अंक)

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Directive word: Describe

This question asks you to describe. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.

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How this answer will be evaluated

Approach

The directive 'describe' demands comprehensive, structured exposition with precise anatomical and physiological detail. Allocate approximately 40% of time/words to part (a) given its 20 marks, with 30% each to parts (b) and (c). Structure as: brief definitional introduction for each sub-part, followed by detailed body covering hormonal axes for insect metamorphosis, life cycle stages of Obelia with alternation clarity, and mammalian ear architecture with functional correlations. Conclude with integrative remarks on developmental/evolutionary significance across all three systems.

Key points expected

Part (a): Definition of metamorphosis (complete vs incomplete); neuroendocrine axis involving brain (PTTH), corpora allata (JH), and prothoracic glands (ecdysone); hormonal interplay triggering moulting and metamorphic transitions
Part (a): Specific hormonal mechanisms—JH as 'status quo' hormone maintaining larval features, ecdysone pulses driving moulting; decline of JH permitting adult differentiation; role of 20-hydroxyecdysone
Part (b): Definition of alternation of generations (metagenesis) distinguishing between asexual polypoid generation and sexual medusoid generation; haplodiplontic life cycle characteristic of Cnidaria
Part (b): Detailed Obelia life history—hydrorhiza, hydrocaulus, feeding hydranths, reproductive gonangia; planula larva formation; medusa liberation and gamete production; fertilization and settlement
Part (c): Internal ear structure—bony labyrinth vs membranous labyrinth; cochlea (scala vestibuli, scala media, scala tympani), organ of Corti with hair cells and tectorial membrane; vestibular apparatus (utricle, saccule, semicircular canals)
Part (c): Functional mechanisms—mechanotransduction by hair cells; frequency discrimination via basilar membrane tonotopy; vestibular functions in static/dynamic equilibrium; role of endolymph and perilymph

Evaluation rubric

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	20%	10	Precise definitions across all parts: metamorphosis types correctly distinguished; PTTH-JH-ecdysone axis accurately described with glandular sources; alternation of generations correctly identified as haplodiplontic with proper ploidy levels; Obelia life cycle stages in correct sequence; mammalian ear compartments and fluid spaces accurately delineated with no anatomical errors	Generally correct definitions with minor errors—confusion between complete/incomplete metamorphosis examples, imprecise gland-hormone associations, simplified or slightly out-of-sequence Obelia stages, basic ear structure present but missing key compartments like scala media details	Fundamental conceptual errors—metamorphosis conflated with growth, major hormone-source mismatches (e.g., JH from prothoracic glands), alternation of generations confused with metamerism, Obelia described as purely polypoid or medusoid, ear anatomy conflating external/middle/internal structures
Diagram / labelling	20%	10	Three high-quality diagrams: (a) neuroendocrine axis of insect brain-prothoracic gland-corpora allata with hormone flow arrows; (b) Obelia colony structure with gonangium detail AND liberated medusa with gametogenic tissue; (c) cross-section of cochlea showing three scalae and organ of Corti detail, plus semicircular canal orientation; all fully labelled with anatomical precision	Two adequate diagrams with partial labelling—simplified hormonal pathway without feedback loops, Obelia colony only without medusa stage, cochlea cross-section missing tectorial membrane or specific scalae identification; labels present but incomplete	Single diagram or none; crude sketches without proper proportions; missing critical structures (e.g., no corpora allata in insect diagram, no medusa in Obelia, no organ of Corti in ear); labels absent or erroneous
Examples & nomenclature	20%	10	Specific exemplification: for (a) Lepidoptera (Bombyx mori) or Diptera (Drosophila) with Indian sericulture relevance; precise hormone names (prothoracicotropic hormone, juvenile hormone, α-ecdysone/20E); for (b) correct taxonomic placement of Obelia in Hydrozoa, Cnidaria; for (c) specific mammal (human preferred) with Indian research context (e.g., hearing studies at AIIMS); anatomical terms fully accurate	Generic examples without specificity—'butterfly' instead of species, 'caterpillar' without lepidopteran identification; hormone abbreviations without full names; Obelia correctly named but without phylum/class; mammalian ear described without species reference; minor terminological lapses	Incorrect or absent examples—non-insect arthropods for metamorphosis, jellyfish (Scyphozoa) confused with Obelia, external ear structures mislabelled as internal; hormone names garbled; taxonomic errors (e.g., Obelia as Scyphozoan)
Process explanation	20%	10	Clear mechanistic exposition: for (a) sequential hormone release—brain PTTH → prothoracic gland ecdysone secretion → JH threshold determining moult type; for (b) developmental transitions—planula settlement, colony establishment, gonangial budding, medusa liberation, gametogenesis, fertilization, larval release; for (c) sound transduction pathway—tympanic vibration → ossicular chain → oval window → perilymph wave → basilar membrane displacement → hair cell shearing → neural encoding; vestibular signal transduction similarly detailed	Process described in broad sequence without mechanistic detail—hormones 'control' metamorphosis without explaining how JH titre determines outcome; Obelia stages listed without developmental transitions; sound pathway present but missing hair cell mechanotransduction or frequency coding explanation	No process explanation—static descriptions only; cause-effect relationships absent; developmental sequences jumbled; sound conduction confused with conduction through air; no mention of how stimuli become neural signals
Evolutionary / applied context	20%	10	Integrative evolutionary insights: for (a) adaptive significance of holometaboly vs hemimetaboly, JH as evolutionary innovation enabling dramatic morphological reorganization; applied aspect—insect growth regulators (IGRs) for pest management in Indian agriculture (e.g., against Helicoverpa armigera); for (b) evolutionary interpretation of polyp-medusa transition reflecting ancestral cnidarian body plan modification; for (c) evolutionary origin of cochlea from lateral line; applied relevance—hearing impairment in Indian context, cochlear implant technology, noise pollution studies in urban India	Brief mention of applied or evolutionary significance without development—pest control mentioned without IGR mechanism, evolution of alternation not explained, hearing loss mentioned without technological or epidemiological context	No evolutionary or applied context; purely descriptive answer; missed opportunity to connect endocrine disruption to agriculture, life cycle complexity to cnidarian radiation, or hearing mechanisms to clinical audiology

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