Write short notes on/Answer the following in about 150 words each: 10×5=50 (a) Metagenesis (b) Book lungs (c)…

(a) Metagenesis. Metagenesis is the alternation of two multicellular generations in Cnidaria: an asexual polyp generation and a sexual medusa generation. In Obelia, the polyp colony reproduces asexually by budding, producing free-swimming medusae; medusae produce gametes, and fertilisation gives a planula larva that settles and develops into a new polyp. The term is used because the two phases differ in form and mode of reproduction, and the cycle is completed only when both generations are present. It is not identical with true alternation of generations in plants or some algae, where a spore phase is produced by meiosis and both generations are independent. In cnidarian metagenesis, meiosis occurs in the medusa during gamete formation, not in spore formation, and the polyp is not a spore phase. Thus metagenesis is a cnidarian-specific form of alternation, important for understanding life cycles, reproduction, and complex diploblastic life histories.

(b) Book lungs. Book lungs are respiratory organs of many arachnids, especially spiders and scorpions. They consist of a chamber filled with hemolymph in which numerous thin, leaf-like lamellae are stacked like pages of a book. Air enters through spiracles and diffuses across the thin epithelial surfaces of the lamellae, where oxygen passes into hemolymph and carbon dioxide is released. Unlike simple cutaneous diffusion, book lungs are actively ventilated by body movements: sternite movements in scorpions and abdominal muscle contractions in spiders compress the chambers and renew air. This active ventilation improves gas exchange but limits the rate of oxygen supply compared with tracheal systems of insects, so book-lunged arachnids are less suited to sustained high metabolic activity. Book lungs should be contrasted with book gills of horseshoe crabs such as Limulus, which are external, feathery, and adapted to aquatic respiration. Their structure shows how arachnids modified respiratory surfaces for terrestrial life.

(c) Paedomorphosis. Paedomorphosis is a heterochronic evolutionary pattern in which adults retain juvenile or larval characters. It arises when the timing of developmental events changes. Two mechanisms are recognised. In neoteny, somatic development is delayed, so the organism reaches sexual maturity while possessing juvenile body forms; the axolotl, Ambystoma mexicanum, is an example, retaining external gills, aquatic habits, and larval skull proportions while reproducing. In progenesis, sexual maturation is advanced while somatic growth continues later, so adults may be small but reproductively mature. Paedomorphosis is not simply arrested development; it can be adaptive, allowing retention of useful larval traits such as aquatic respiration. In humans, evolutionary biologists sometimes refer to fetalization or neotenous retention of juvenile features, such as a relatively flat face, large brain relative to body, and prolonged growth period, though this is debated. Paedomorphosis is significant because changes in developmental timing, not only new structures, can produce major evolutionary novelties.

(d) Torsion and detorsion in gastropods. Torsion is a developmental event in prosobranch gastropod larvae, in which the visceral mass, mantle cavity, and associated organs rotate 180° counter-clockwise relative to the head and foot. As a result, the originally posterior mantle cavity, gill, and anus are brought anteriorly above the head. Torsion also twists the visceral nerve cords into a figure-of-eight, producing a ligation. Functionally, it may help orient the mantle cavity for chemoreception and protection, but it destroys original bilateral symmetry, creates body-plan asymmetry, and can lead to loss or reduction of the gill and auricle in some groups. Detorsion is the partial or complete reversal of this rotation in some opisthobranchs and pulmonates. It does not restore external bilateral symmetry, because the coiled shell, asymmetric visceral mass, and other modifications remain. Detorsion is partial and is evolutionarily important as a reversal of an early gastropod innovation, allowing repositioning of the mantle cavity in lineages.

(e) Human eye diagram. A well-labelled sagittal section of the human eye should show the cornea, sclera, choroid, and retina. The cornea and sclera form the outer fibrous tunic; the choroid is vascular; the retina is neural. The iris, with its pupil, controls light entry. Behind the iris, the lens is suspended by suspensory ligaments from the ciliary body; ciliary muscle contraction relaxes the ligaments, allowing the lens to thicken for near vision. The retina contains photoreceptors, rods for dim light and cones for colour and acuity, with a fovea for sharp vision. The optic disc is the blind spot where ganglion cell axons leave as the optic nerve. In the retina, photoreceptors synapse with bipolar cells, which relay signals to ganglion cells; horizontal cells modulate the outer plexiform layer. The eye forms an inverted image on the retina, and the brain reverses it for upright vision. Its functions are refraction, accommodation, photoreception, and image formation.