Write notes in about 150 words each for Q. Nos. 1(a) to 1(d) and answer Q. No. 1(e) :

(a) Canal system in sponges (10 marks)
(b) Nematocysts of Aurelia (10 marks)
(c) Trochophore larva and its evolutionary significance (10 marks)
(d) Origin of tetra

Question

Write notes in about 150 words each for Q. Nos. 1(a) to 1(d) and answer Q. No. 1(e) :

(a) Canal system in sponges (10 marks)
(b) Nematocysts of Aurelia (10 marks)
(c) Trochophore larva and its evolutionary significance (10 marks)
(d) Origin of tetrapods (10 marks)
(e) Draw a well-labelled diagram of avian brain. (10 marks)

UPSC Answer Check · Accepted Answer

1(a) Canal system in sponges. The canal system is the water-circulation network of sponges, formed by pores, canals and flagellated choanocytes that draw in water, filter food, and expel waste. In the ascon type, incurrent pores open directly into a central spongocoel lined by choanocytes, with a single osculum; Leucosolenia is a simple example. In the sycon type, pores lead into incurrent canals, then radial canals lined by choanocytes, and finally into a spongocoel and osculum; Sycon shows this arrangement. In the leucon type, the spongocoel is reduced or absent, choanocytes occupy numerous chambers, and incurrent and excurrent canals form a complex network; Spongilla represents this most efficient form. The flagellar current of choanocytes creates unidirectional flow, increasing contact with water and making filter-feeding more efficient as body size and complexity increase. This graded series shows how sponge body plans become more canalised and efficient, with choanocytes remaining the functional centre.

1(b) Nematocysts of Aurelia. Nematocysts are the thread-bearing cnidocysts of Aurelia, borne by cnidocytes on tentacles and oral arms for prey capture. They are classified as atrichous isorhiza, holotrichous isorhiza and heterotrichous microbasic euryteles. The atrichous isorhiza has a smooth, straight thread and a simple penetrating role in capture; the holotrichous isorhiza has barbs or spines along the thread, helping to entangle small prey; the heterotrichous microbasic euryteles has a coiled, often barbed thread with a small base, useful for gripping or immobilising captured organisms. Discharge is triggered when the cnidocil, a mechanosensitive hair, is stimulated by prey contact. This causes rapid osmotic swelling of the capsule, rupture of the operculum, and eversion of the thread, sometimes with barbs or coiled portions. The result is a fast harpoon-like strike that attaches to prey, enabling the jellyfish to bring it to the mouth. These nematocysts are thus central to Aurelia’s predatory feeding.

1(c) Trochophore larva and its evolutionary significance. The trochophore is a free-swimming, ciliated larva of many lophotrochozoan groups, especially polychaete annelids and molluscs. It has a bilaterally symmetrical body, an apical tuft, and three ciliary bands: the prototroch, a principal band around the middle of the body; the metatroch, a posterior ciliary band; and the telotroch, an apical or terminal band. In Nereis, the trochophore is a planktonic larva of a polychaete, while in Pila it appears as a molluscan trochophore-like ciliated larva. The ciliary bands provide locomotion and help in feeding by creating currents that bring particles to the mouth. It is a key larval character. Evolutionarily, the trochophore is important because it supports the trochaea theory, which proposes a common larval ancestor for lophophorates and trochophore-bearing groups. Its repeated occurrence in annelids, molluscs and related phyla is evidence for the monophyly of Lophotrochozoa, although it is a larval homology rather than proof of a direct adult ancestor.

1(d) Origin of tetrapods. Tetrapods originated from lobe-finned sarcopterygian fishes during the Devonian, when fins with internal bones, joints and weight-bearing structures changed into limbs. Tiktaalik shows a transitional skull, neck, ribs and fin with wrist-like elements; Acanthostega and Ichthyostega are early tetrapods with polydactylous limbs, having eight and seven digits respectively, not the later pentadactyl condition. The later pentadactyl limb is homologous to the sarcopterygian fin skeleton, showing continuity from fin to limb. The Devonian record includes Romer's gap, a period of low tetrapod fossil diversity, not a radiation; after this gap, tetrapods diversified. Fossil evidence from Greenland and Pennsylvania, together with Ellesmere Island material, helps bracket the transition: Greenland preserves Acanthostega and Ichthyostega, while North American Devonian sites preserve stem-tetrapod and early tetrapod forms. These fossils show the stepwise fin-to-limb transition in the fossil record.

1(e) Avian brain. In a dorsal view, the avian brain is drawn with the rostral telencephalon (cerebrum) smooth, predominantly pallial, and lacking a corpus callosum. Behind it, the diencephalon is present, with the optic nerve (CN II) arising at this level. The midbrain shows large optic lobes (tectum opticum), reflecting visual dependence; the optic lobe is a midbrain structure, not CN II. The cerebellum is well developed, with prominent lobes for flight coordination. The medulla oblongata is elongated and posterior, bearing cranial nerve roots for IX and X, while V and VII are shown at the pons. The labelled dorsal view shows cerebrum, optic lobes, cerebellum, medulla oblongata, and cranial nerve roots II, V, VII, IX and X at their correct levels. Thus, the smooth cerebrum, large optic lobes and expanded cerebellum are the key diagnostic features of the avian brain in the dorsal view.

Write notes in about 150 words each for Q. Nos. 1(a) to 1(d) and answer Q. No. 1(e) : (a) Canal system in sponges (10 marks) (b)…

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