(a) Explain habitat, habits, general features and life cycle of Wuchereria. (20 marks) (b) Describe different types of canal…

Wuchereria: Habitat, Habits, Features, and Life Cycle

Wuchereria bancrofti is an endoparasitic, pseudocoelomate nematode inhabiting human lymphatic vessels and lymph nodes, heavily distributed across tropical and subtropical regions including endemic belts in India. The microscopic embryos, termed microfilariae, exhibit nocturnal periodicity, migrating from deep pulmonary vessels to peripheral blood vessels between 10 PM and 4 AM to synchronize with the nocturnal biting activity of mosquito vectors (Culex quinquefasciatus, Anopheles, Aedes).

`` [Human Host] ──(Microfilariae in blood)──> [Mosquito Vector: Gut → Thoracic Muscles (L1 Sausage → L2 → L3 Filariform)] ▲ │ └───────────(Inoculation of L3 Larvae → Lymphatics → Adults)────────────────┘ ``

Morphologically, adults are dioecious, creamy-white, and filiform. Females measure 80–100 mm in length with a vulva located anteriorly, while males are smaller (40 mm) with a ventrally curved tail carrying two unequal copulatory spicules. The life cycle is digenetic:

1. Mosquito Phase: A mosquito ingests sheathed microfilariae during a blood meal. Within 1–2 hours, microfilariae exsheath in the midgut, bore through the stomach wall, and migrate to thoracic muscles. They transform into short, thick first-stage (L1) "sausage" larvae, moult into second-stage (L2) larvae, and finally moult into infective third-stage (L3) filariform larvae (10–14 days), which migrate to the mosquito's proboscis.
2. Human Phase: During feeding, L3 larvae penetrate the bite wound, travel into lymphatic channels, and settle in lymph nodes. Over 6–12 months, they undergo two moults into sexually mature adults. Gravid females ovoviviparously release sheathed microfilariae into the bloodstream, sustaining the transmission cycle.

Canal Systems in Porifera and Functions

The canal system represents an evolutionary progression increasing surface area for efficient water transport:

`` Ascon: Ostia ─────────────> Flagellated Spongocoel ───────────────────────> Osculum Sycon: Ostia ─> Incurrent Canal ─(Prosopyle)─> Flagellated Radial Canal ─(Apopyle)─> Spongocoel ─> Osculum Leucon: Ostia ─> Incurrent Canal ────────────> Flagellated Chambers ─────> Excurrent Canals ───────> Osculum ``

Asconoid System: Simplest, vase-shaped body with thin walls. Water enters through microscopic dermal ostia formed by porocytes into a spacious central spongocoel lined with choanocytes, exiting directly via the osculum (e.g., Leucosolenia). Syconoid System: Formed by horizontal folding of body walls, creating alternating incurrent canals (lined by pinacocytes) and radial canals (lined by choanocytes). Water moves from incurrent to radial canals via prosopyles, then through internal ostia (apopyles) into a reduced, non-flagellated spongocoel before exiting through the osculum (e.g., Scypha). Leuconoid System: Formed by extensive folding of mesohyl and branching of canals. Choanocytes are restricted to distinct, sub-spherical flagellated chambers. Spongocoel is virtually absent, replaced by a complex network of excurrent canals leading to one or more oscula (e.g., Spongilla, Euspongia found in the Gulf of Mannar and Palk Bay).

Functions: Choanocyte flagella generate a continuous unidirectional current that brings suspended micro-food particles for intracellular phagocytosis (filter feeding), drives gas exchange across cell membranes, removes nitrogenous wastes (ammonia), and carries sperm and fertilized ova during reproduction.

Structure of Human Eye

`` [CORNEA] ──┐ [ANTERIOR │ [CILIARY BODY & SUSPENSORY LIGAMENTS] CHAMBER] │ │ [PUPIL/IRIS] ├──[LENS] │ │ [POSTERIOR │ └── [VITREOUS BODY] ──> [RETINA] ──> [FOVEA] CHAMBER] ───┘ │ │ [CHOROID] ──┼─────────────┤ [SCLERA] ──┘ └── [OPTIC NERVE] ``

The human eyeball is a spherical photoreceptor organ organized into three concentric coats:

1. Fibrous Tunic: Consists of the anterior transparent cornea (composed of stratified non-keratinized epithelium, stroma, and endothelium, providing major optical refraction) and the posterior opaque sclera that maintains intraocular shape.
2. Vascular Tunic (Uvea): Comprises the nutrient-rich choroid, the ciliary body containing smooth ciliary muscles that adjust lens curvature via suspensory ligaments (zonules of Zinn) during accommodation, and the pigmented iris with pupillary sphincter and dilator muscles controlling light entry.
3. Neural Tunic (Retina): Contains an outer pigmented epithelium and an inner neurosensory layer with rods (scotopic vision, rhodopsin) and cones (photopic and trichromatic color vision, iodopsin). Visual acuity peaks at the fovea centralis of the macula lutea (packed solely with cones), while the optic disc lacks photoreceptors, forming the blind spot where the optic nerve exits alongside the central retinal artery (branch of the ophthalmic artery).

Refractive media include the cornea, aqueous humor, biconvex crystalline lens, and gel-like vitreous humor. Impairments such as lens opacification (cataract) and elevated intraocular pressure from obstructed aqueous outflow (glaucoma) represent major clinical conditions managed nationally through the National Programme for Control of Blindness and Visual Impairment (NPCBVI).

These structural adaptations across phyla illustrate how tissue organization evolves from cellular canal networks in Porifera to specialized organ systems in metazoans to maintain physiological homeostasis.