(a) Metagenesis (b) Book lungs (c) Paedomorphosis (d) Torsion and detorsion in gastropods (e) Draw a well-labelled diagram of…

(a) Metagenesis

Metagenesis is the regular alternation of an asexual polyp generation with a sexual medusa generation in the life cycle of many cnidarians (especially Hydrozoa and Scyphozoa).

• In Obelia, the fixed, colonial polyp generation has feeding polyps (gastrozooids) and reproductive polyps (blastostyles, within gonothecae). The blastostyles bud off small, free-swimming medusae asexually.
• The medusae are dioecious; they produce eggs or sperm by meiosis in gonads on the radial canals, and fertilisation gives a zygote.
• The zygote develops into a ciliated planula larva, which settles on a substrate and grows into a new polyp that buds to form a colony.
• In Aurelia (Scyphozoa) the medusa is the dominant stage, and the small scyphistoma polyp produces young medusae (ephyrae) by transverse fission (strobilation).

Metagenesis differs from alternation of generations in plants: both cnidarian generations are diploid, and meiosis occurs only in gamete formation, not in spore formation. It combines the advantages of rapid asexual multiplication and a fixed feeding stage (polyp) with sexual recombination and dispersal by the free-swimming medusa. It is reduced or lost in some groups, such as Hydra (no medusa) and Anthozoa (no medusa).

(b) Book lungs

Book lungs are the respiratory organs of terrestrial arachnids, derived from the book gills of aquatic ancestors.

• Occurrence: scorpions have four pairs on the ventral side of mesosomal segments 3–6; primitive spiders (mygalomorphs) have two pairs, and most araneomorph spiders have one pair together with tubular tracheae.
• Structure: each book lung is an air-filled pocket (atrium) opening by a slit-like spiracle (stigma). From its front wall about 15–150 thin, hollow, leaf-like lamellae project, arranged like the pages of a book. Each lamella is a flattened pouch filled with haemolymph; the air spaces between lamellae are kept open by cuticular struts.
• Gas exchange: oxygen diffuses from the air spaces across the very thin lamellar wall into the haemolymph, where it is carried bound to haemocyanin, and CO₂ diffuses outward. Exchange is mainly by diffusion; the spiracle can be opened or closed by muscles to control water loss, and in scorpions some movement of the atrium aids air renewal.
• Comparison: book lungs are internal, which reduces water loss on land, whereas the book gills of the horseshoe crab Limulus are external appendicular plates bathed by water. Compared with insect tracheae, which deliver air directly to tissues, book lungs depend on circulating haemolymph, so arachnids generally have lower sustained activity and rely on bursts of activity.

(c) Paedomorphosis

Paedomorphosis is the retention of juvenile or larval characters of the ancestor in the sexually mature adult of the descendant. It is a form of heterochrony, an evolutionary change in the timing or rate of developmental events (Gould, 1977), and the opposite of peramorphosis, in which descendants develop beyond the ancestral adult form.

• Neoteny: somatic development is slowed while sexual maturation proceeds normally, so a sexually mature adult keeps larval features. The Mexican axolotl (Ambystoma mexicanum) breeds while retaining external gills, a tail fin and aquatic habits; it fails to metamorphose because of low thyroid hormone activity, and injection of thyroxine makes it metamorphose into a terrestrial salamander. Necturus and Proteus are permanently neotenic.
• Progenesis: sexual maturation is accelerated, so the animal reproduces while still small and juvenile in form, as in some small salamanders and insects with larval reproduction (paedogenesis in gall midges).
• Evolutionary significance: paedomorphosis allows rapid, major changes in body form with few genetic changes, especially in regulatory genes controlling timing. It has been proposed (Garstang) for the origin of chordates from a tunicate-like larva retaining the notochord and tail, and for the origin of some insect groups; in humans, a flat face, large brain relative to body and reduced body hair have been interpreted as neotenic traits, though this is debated.

(d) Torsion and detorsion in gastropods

Torsion is a 180° anticlockwise rotation (viewed from above) of the visceral mass, mantle and mantle cavity relative to the head and foot, which takes place during the veliger larval stage of gastropods. In primitive forms it occurs in two phases: a rapid first 90° driven by the asymmetrical larval retractor muscle, and a slower second 90° by differential growth.

Results of torsion:

• The mantle cavity, with the gills, anus, kidney and genital openings, moves from the posterior end to lie anteriorly above the head.
• The originally right-side organs come to lie on the left and vice versa, and the visceral nerve cords cross into a figure-of-eight (streptoneury or chiastoneury).
• The gut becomes U-shaped.

Possible advantages: the head can be withdrawn first into the protective shell and mantle cavity (Garstang's hypothesis of larval protection), the osphradium and gills face forward into clean water, and the shell balances better over the body. Disadvantages: waste from the anus and kidneys is discharged over the head and gills (fouling). Gastropods solved this by a slit or holes in the shell (Haliotis, Fissurella), by loss of the right gill and right auricle (Mesogastropoda, Neogastropoda), and by directing water flow from left to right.

Detorsion is a partial or complete secondary reversal of torsion, seen in opisthobranchs (e.g., Aplysia, Cephalaspidea such as Bulla) and pulmonates. The mantle cavity shifts back to the right side or posteriorly, the nerve cords become uncrossed (euthyneury), and the shell is reduced or lost, as in sea slugs (nudibranchs). Detorsion does not restore the original bilateral symmetry, because the visceral mass remains asymmetrical.

(e) Human eye: structure and functions

The human eye is a nearly spherical organ about 24 mm in diameter, with three layers:

• Outer fibrous layer: the tough white sclera maintains shape and gives attachment to the extrinsic muscles; in front it becomes the transparent cornea, which does about two-thirds of the refraction of light.
• Middle vascular layer (uvea): the choroid supplies blood and its pigment absorbs stray light; in front it forms the ciliary body (ciliary muscle and processes secreting aqueous humour) and the iris, whose sphincter and dilator muscles change the size of the pupil to control the amount of light entering.
• Inner nervous layer: the retina contains about 120 million rods (dim-light, black-and-white vision, containing rhodopsin) and about 6 million cones (colour and fine detail in bright light), connected through bipolar cells to ganglion cells whose axons form the optic nerve. The fovea centralis in the macula contains only cones and gives the sharpest vision; the optic disc, where the optic nerve leaves, has no receptors (blind spot).

The biconvex, elastic lens is held by suspensory ligaments (zonules) from the ciliary body. The aqueous humour fills the space in front of the lens and the jelly-like vitreous humour the space behind it, maintaining shape and intraocular pressure.

Functions:

• Refraction and image formation: the cornea and lens focus light to form a real, inverted, diminished image on the retina; the brain interprets it as upright.
• Accommodation: for near vision the ciliary muscle contracts, the zonules slacken and the lens becomes more convex; for distant vision the muscle relaxes and the lens flattens.
• Control of light: the pupil constricts in bright light and dilates in dim light (pupillary reflex).
• Photoreception and transduction: light converts 11-cis-retinal to all-trans-retinal in the visual pigments, causing hyperpolarisation of the photoreceptors and nerve impulses in the optic nerve.
• Colour vision (three types of cones), dark and light adaptation, and binocular vision with depth perception.