(a) Erythrocytic schizogony (10 marks) (b) Statocyst of medusa (10 marks) (c) Evolutionary status of Onychophora (10 marks) (d)…

(a) Erythrocytic schizogony

Erythrocytic schizogony is the asexual multiplication of Plasmodium inside human red blood cells, which follows the silent hepatic (exo-erythrocytic) phase. Merozoites released from the liver invade erythrocytes and develop as follows: Ring stage: a young trophozoite with a central vacuole pushes the nucleus to one side. Amoeboid trophozoite: it feeds on haemoglobin, leaving the toxic pigment haemozoin. Schizont: repeated nuclear division forms a multinucleate schizont, which cleaves into 12 to 24 merozoites arranged like a rosette. The red cell bursts and releases merozoites, haemozoin and parasite antigens. The merozoites infect fresh red cells, or some become gametocytes that continue the cycle in the female Anopheles. The synchronous bursting causes the typical paroxysm of chill, fever and sweating, recurring every 48 hours in P. vivax and P. falciparum and every 72 hours in P. malariae.

(b) Statocyst of medusa

The statocyst is the organ of balance of medusae, which senses the direction of gravity and tilt. Structure: a small fluid-filled pit or vesicle of the umbrella margin lined by sensory epithelium bearing sensory hairs. It contains a statolith, a dense concretion of calcium salts such as calcium sulphate secreted by lithocyte cells. Location: in hydromedusae the statocysts lie at the margin near the bases of the tentacles. In scyphomedusae such as Aurelia they are enclosed in the rhopalia, the sensory clubs lying in marginal notches between lappets. Working: when the animal tilts, the statolith presses on the hairs on the lower side. The stimulated sensory cells send impulses through the marginal nerve ring to the subumbrellar muscles, which contract unequally and restore the upright posture.

(c) Evolutionary status of Onychophora

Onychophora, represented by Peripatus, was long regarded as a connecting link between Annelida and Arthropoda. Annelidan features: a thin flexible cuticle without segmented plates, a muscular body wall of circular and longitudinal layers, segmentally arranged nephridia, ciliated reproductive ducts, and unjointed, soft lobopod legs. Arthropodan features: a chitinous cuticle that is moulted, tracheae opening by spiracles, a coelom reduced to a haemocoel with an open circulation and a tubular dorsal heart with ostia, claws on the legs, and jaws formed from modified appendages. Own features: slime glands that squirt an adhesive defensive jet, and antennae on the unsegmented head. Present view: molecular studies place Onychophora in the Panarthropoda with Tardigrada and Arthropoda, as the sister group of the other two, so it is a relict lineage showing the early stage in the origin of arthropods rather than a direct ancestor.

(d) Neoteny

Neoteny is the retention of larval or juvenile features in a sexually mature adult, because somatic development is slowed while the gonads mature on time. It is one way of producing paedomorphosis, the other being progenesis, in which sexual maturity is hastened while the body develops at the normal rate. Example: the axolotl, Ambystoma mexicanum, remains aquatic and keeps its external gills, gill slits and tail fin all its life and breeds in this larval form. Its tissues fail to respond to thyroid hormone because of a deficiency in the thyroid axis, so natural metamorphosis does not occur. Treatment with thyroxine or iodine induces metamorphosis into the land form. Significance: Bolk's foetalisation hypothesis suggests that humans show neoteny in a large brain, rounded skull, flat face and delayed maturity. Neoteny is also suggested as the route by which chordates arose from a larva-like ancestor.

(e) Migration in birds

Bird migration is the regular seasonal movement of birds between breeding grounds and wintering areas, usually in a fixed route called a flyway. Examples in India: the Siberian crane once wintered at Keoladeo Ghana National Park, Bharatpur, but has not been recorded there since 2002. The Amur falcon passes in thousands through the Doyang reservoir area of Nagaland on its journey from Siberia and north-east Asia to southern Africa. Causes: the ultimate causes are food shortage, cold and the need for good breeding sites. The immediate stimulus is the changing day length acting through the pineal gland and hypothalamus, which raises hormone levels, induces restlessness (Zugunruhe) and fat storage. Navigation: birds use the sun and stars, the earth's magnetic field sensed by cryptochromes in the eye, landmarks and smell.