(a) Write peculiar characters of phylum Annelida. Classify this phylum into different classes and mention features of each class…

(a) Annelida: peculiar characters and classification (20 marks)

Peculiar characters.

• Bilaterally symmetrical, triploblastic and coelomate (a true coelom of schizocoelic origin) with organ-system organisation.
• Metameric segmentation, both external (rings) and internal, in which most organs repeat in each segment.
• Body wall with a thin cuticle, epidermis, and circular and longitudinal muscles; locomotor chaetae (setae) in most.
• Closed blood-vascular system with respiratory pigment (haemoglobin) dissolved in the plasma.
• Excretion by paired segmental nephridia.
• Nervous system with a dorsal cerebral (supra-pharyngeal) ganglion, a nerve ring and a double ventral nerve cord.
• Complete alimentary canal; sexes separate or hermaphrodite; development direct or through a trochophore larva.

Classification and features of the classes.

• Class Polychaeta: mostly marine, with a distinct head bearing eyes, tentacles and palps, parapodia bearing many setae, no clitellum, sexes separate, external fertilisation and a trochophore larva. Examples: Nereis, Aphrodite, Arenicola, Chaetopterus.
• Class Oligochaeta: mostly terrestrial or freshwater, with no distinct head, no parapodia, few setae, a clitellum in the reproductive season, hermaphrodite with cross-fertilisation, cocoons and direct development. Examples: Pheretima, Lumbricus, Tubifex.
• Class Hirudinea: leeches, with a fixed number of segments (34), no setae or parapodia, anterior and posterior suckers, coelom reduced to sinuses, hermaphrodite, mostly blood-sucking. Examples: Hirudinaria, Hirudo.
• Class Archiannelida: small, simple marine worms with no parapodia or setae, and little external segmentation. Examples: Polygordius, Protodrilus.

(b) Respiratory organs of Lamellidens and mechanism of respiration (15 marks)

Structure. Lamellidens, the freshwater mussel, breathes with a pair of gills (ctenidia), one on each side of the foot in the mantle cavity, and with the mantle.

• Each ctenidium is of the eulamellibranch type, made of two demibranchs (outer and inner). Each demibranch has a descending lamella and an ascending lamella, joined at the lower margin to form a V or W.
• The lamellae are made of vertical gill filaments, united by tissue junctions, and the two lamellae are connected by interlamellar junctions, which divide the space between them into vertical water tubes.
• The lamellae are perforated by small openings, the ostia, that lead into the water tubes, which open above into the suprabranchial chamber.
• The filaments carry frontal, latero-frontal and lateral cilia, and are rich in blood vessels.

Mechanism.

• The lateral cilia beat and create a current in which water enters through the ventral inhalant siphon into the mantle cavity.
• The water passes through the ostia into the water tubes, moves up into the suprabranchial chamber and leaves by the dorsal exhalant siphon.
• Blood in the filaments, brought by the afferent branchial vessel, takes up oxygen from the water and gives off carbon dioxide, and it leaves by the efferent vessel to the heart. The vascular mantle also exchanges gases.
• The same current brings food particles, which are trapped in mucus on the gills and carried by cilia to the mouth.

(c) Flight adaptations in birds (15 marks)

• External form: a spindle-shaped, streamlined body; forelimbs modified into wings; feathers, with light contour and flight feathers (remiges, rectrices) that give lift and control, and down for insulation; no teeth, a light horny beak and no tail bones (a short pygostyle).
• Skeleton: light, pneumatic bones, with air sacs and hollow cavities; fusion of the vertebrae (synsacrum), pelvic bones and the wrist and hand bones (carpometacarpus), which gives rigidity; a keeled sternum (carina) for flight muscles; a furcula (wishbone) that acts as a spring; and uncinate processes that stiffen the ribs.
• Muscles: very large pectoralis (downstroke) and supracoracoideus (upstroke) muscles, the latter working through a pulley, both attached to the keel and held low to keep the centre of gravity low.
• Respiration: compact lungs with air sacs and one-way airflow through the parabronchi, so that oxygen is taken up in both inspiration and expiration (double respiration), meeting the high demand.
• Circulation: a four-chambered heart, with complete separation of oxygenated and deoxygenated blood, a fast heart rate and high blood pressure.
• Metabolism: warm-blooded, with a high metabolic rate.
• Excretion and reproduction: uric acid excreted with no urinary bladder, a single functional ovary, and eggs laid outside, all of which reduce weight.
• Nervous and sense organs: a large cerebellum for balance and coordination, large optic lobes and keen eyesight.