(a) Describe the mechanism of parental care in amphibians with suitable examples. 20 (b) Describe the different stages of life…

(a) Parental care in amphibians

Parental care is any post-fertilisation investment by a parent that raises the survival of eggs or young at some cost to the parent. Amphibian eggs are shell-less and exposed to desiccation, predators, fungi and unstable water bodies, so many species trade high fecundity for fewer, better-protected offspring. Care occurs in roughly a tenth to a fifth of anurans and is widespread in caecilians; it may be given by the male, the female or both.

• Nest building and egg guarding: the male African bullfrog (Pyxicephalus adspersus) guards eggs and tadpoles in shallow pools and even digs channels to rescue tadpoles from drying pools. The gladiator frog (Boana faber, formerly Hyla faber) builds mud basins at the water's edge. Leaf frogs (Phyllomedusa) fold leaves around eggs laid over water, and many rhacophorid tree frogs (Rhacophorus, Polypedates) whip secretions into foam nests that keep eggs moist. In the Western Ghats, males of the night frogs (Nyctibatrachus) attend eggs laid on leaves and rocks above streams.
• Egg transport: the male midwife toad (Alytes obstetricans) winds the egg strings around his hind legs, keeps them moist and releases the hatching tadpoles into water.
• Brooding in body structures: in the Surinam toad (Pipa pipa) the eggs sink into pockets of the female's back skin and emerge as froglets. Female marsupial ("backpack") frogs (Gastrotheca) carry eggs in a dorsal pouch. Male Darwin's frogs (Rhinoderma darwinii) take the developing young into the vocal sac, where they complete metamorphosis. The now-extinct gastric-brooding frogs (Rheobatrachus) raised young in the female's stomach, where acid secretion was suppressed.
• Tadpole transport and feeding: in poison frogs (Dendrobatidae) a parent carries hatched tadpoles on its back to water in bromeliad axils or tree holes; in the strawberry poison frog (Oophaga pumilio, formerly Dendrobates pumilio) the female returns to lay unfertilised trophic eggs on which the tadpoles feed.
• Caecilian care: the female Ichthyophis (for example I. glutinosus in Sri Lanka and related species in the Western Ghats) coils around her egg clutch in a moist burrow. In Boulengerula taitana and Siphonops annulatus the young practise maternal dermatophagy, peeling and eating the lipid-rich outer skin that the mother grows for them; viviparous caecilians such as Typhlonectes nourish foetuses in the oviduct.

Mechanisms: care is triggered by the sight or contact of the clutch and by hormonal changes after breeding. Prolactin is linked with brooding and water balance, and the nonapeptides arginine vasotocin and mesotocin, together with corticosterone, influence clutch attendance, aggression and the decision to transport tadpoles; these pathways are less studied than in birds and mammals.

Significance: care improves hatching success and tadpole survival, allows breeding away from open water, and makes direct development and terrestrial reproduction possible.

**(b) Life cycle of *Pheretima posthuma***

Pheretima posthuma is a protandrous hermaphrodite with direct development and no larval stage. Its life cycle has the following stages.

• Copulation: during the rains two worms come together with their ventral surfaces opposed and heads pointing in opposite directions. The male genital apertures (18th segment) of one are placed against the spermathecal openings (four pairs, in the grooves between segments 5/6 to 8/9) of the other, and sperm are exchanged reciprocally. The worms then separate.
• Cocoon formation: some days later the glandular clitellum (segments 14–16) secretes a tough membranous girdle and nutritive albumin. As the worm wriggles backward, the girdle slips forward, receiving ova from the single female genital aperture (14th segment) and then the stored sperm of the partner from the spermathecae. It slips off the head, and its ends close to form a lemon-shaped cocoon, laid in moist soil.
• Fertilisation: the ova are fertilised inside the cocoon, so fertilisation is external to the body but within the cocoon.
• Development: cleavage is holoblastic and unequal, giving a blastula; gastrulation occurs by invagination and epiboly; mesodermal bands then form the segmented coelom and organ systems, the embryo feeding on the albumin.
• Hatching and growth: after about three weeks young worms emerge; standard Indian texts give two to twenty young per cocoon, about four on average, although many embryos fail. The juveniles lack a clitellum; they feed on humus, grow by adding segments posteriorly and become sexually mature, with a clitellum, in a few months.

(c) Modes of nutrition in protozoa

Protozoa are unicellular eukaryotes whose nutrition ranges from photosynthesis to ingestion, absorption and parasitism.

• Holozoic (phagotrophic): solid food is ingested. Amoeba proteus engulfs prey by pseudopodia (circumvallation, circumfluence, invagination or import), while Paramecium caudatum drives food down the oral groove and buccal cavity into the cytostome by ciliary currents. The food vacuole fuses with lysosomes; digestion passes through an acid then an alkaline phase as the vacuole circulates (cyclosis). Undigested residue is egested at the body surface in Amoeba or through a fixed cytopyge in Paramecium. Contractile vacuoles expel the excess water.
• Holophytic (autotrophic): Euglena viridis photosynthesises with chloroplasts containing chlorophylls a and b and carotenoids, storing paramylon; Volvox and Chlamydomonas are similar phytoflagellates.
• Saprozoic (osmotrophic): dissolved organic matter is absorbed across the body surface by diffusion, active transport or pinocytosis. Examples include Monocystis in the earthworm's seminal vesicles, Opalina in the frog's rectum, and Trypanosoma in the blood plasma, which also takes up material through its flagellar pocket.
• Parasitic: Plasmodium vivax ingests red-cell cytoplasm through a cytostome (micropore), digesting haemoglobin and leaving haemozoin pigment; Entamoeba histolytica secretes lytic enzymes, invades the colonic mucosa and phagocytoses red blood cells; Leishmania donovani lives inside macrophages.
• Mixotrophic: Euglena photosynthesises in light but can absorb organic nutrients saprozoically in prolonged darkness.
• Symbiotic: flagellates such as Trichonympha in the termite hindgut digest cellulose for the host and themselves.