Cnidaria — Anemones, Jellies and Corals

Total: 135 marks — Investigation 1: 29 · Investigation 2: 30 · Investigation 3: 31 · Investigation 4: 45.

Investigation 1 — folds and surface area

• Q1 (2): The inside of the tube is the A4 sheet itself: 29.7 cm × 21 cm (1) = 623.7 cm² (1).
• Q2 (3): r = 29.7 ÷ (2 × 3.14) = 4.73 cm (1). V = 3.14 × 4.73² × 21 (1) = about 1 475 cm³ (1). Accept 1 470–1 480 cm³ (rounding).
• Q3 (6): Each sheet adds 2 × 623.7 = 1 247.4 cm² (shown in full, 1). 2 cm: 2 sheets → 2 494.8 cm², 4 × the plain tube (2 for row). 1 cm: 3 sheets → 3 742.2 cm², 6 × (1 for row, 1 for ratio). 0.5 cm: 4 sheets → 4 989.6 cm², 8 × (1). Accept the learner's own correctly calculated results.
• Q4 (4): Bars for all four (plain, 2, 1, 0.5 cm) (1); axes labelled with units, "surface inside (cm²)" and "fold spacing" (1); a sensible, even scale (1); bars accurately plotted (1).
• Q5 (3): The finer the folds, the more surface fits inside the same tube (1). Finer pleats are shallower and lie flatter, so they take less room (1); more sheets fit side by side, and every sheet adds two faces of surface (1).
• Q6 (4): Plain: 623.7 ÷ 1 475 = 0.42 per cm (1). Lined with 0.5 cm folds: 4 989.6 ÷ 1 475 = 3.38 per cm (1) — eight times as much surface for the same volume (1). An animal that digests on its inner surface gets far more digestive cells, so it digests and absorbs food faster without becoming any bigger (1).
• Q7 (3): Tube = the body wall / column (1); corrugated sheets = the folds of the endoderm, lined with digestive cells (1); the space in the middle = the coelenteron (1).
• Q8 (2): Any two: the anemone's folds are living cells, not paper; the folds (mesenteries) run inward from the wall towards the pharynx, not around the wall; the real folds are moist and bathed in fluid; an anemone can change shape, the tube cannot; real folds also carry muscles and make gametes; the model has no mouth or pharynx.
• Q9 (2): Growing fatter adds volume faster than surface, and the cells deep in a thick body would be too far from water for diffusion to supply them (1); folding adds surface while every layer stays thin and close to water (1).

Investigation 2 — what makes an anemone sting?

• Q10 (4): ① clean twig 3 ÷ 5 = 0.6; ② juice only 4 ÷ 5 = 0.8; ③ twig with juice 13 ÷ 5 = 2.6; ④ mussel 15 ÷ 5 = 3.0 (1 each).
• Q11 (4): Four bars, correctly labelled (1); axes labelled — "mean response score (0–3)" and "stimulus" (1); a sensible scale from 0 to 3 (1); accurate plotting (1).
• Q12 (3): Touch alone (0.6) and taste alone (0.8) each gave only a weak response (1), but touch together with the chemical of food gave a strong one (2.6) (1). So the stinging cells fire most strongly when both contact and a chemical signal of prey are present — the trigger needs both (1).
• Q13 (4): Independent: the type of stimulus (1). Dependent: the response score (1). Controlled — any two: the same kind of anemone; all fully open and underwater; the same twig and juice; the same time between stimuli; the same pool, tide and time of day; the same person scoring (2).
• Q14 (2): Anemones vary — one could be fed, damaged or unusual (1); repeating with five and taking a mean makes the results more reliable (1).
• Q15 (3): Mean = 243 ÷ 5 = 48.6 s (2). Range = 61 − 38 = 23 s (1).
• Q16 (3): The muscular pharynx opens at its top to take in the food and closes behind it, then opens at its lower end to let the food into the coelenteron (1), so the mouth and pharynx are never open all the way through at once and the water is not lost (1). That water is the anemone's hydrostatic skeleton — losing it would collapse the body (1).
• Q17 (2): Any two: the anemone habituated — it learned to ignore a harmless, repeated touch with no food; the nematocysts in that patch had already fired and had not yet been replaced by the interstitial cells; the anemone was now fed and less responsive; it was tired or stressed by repeated handling.
• Q18 (3): The longitudinal muscles contracted and pulled the anemone down (1); it opened its mouth and expelled the water from its coelenteron, so its hydrostatic skeleton collapsed (1). To open again it had to slowly refill the coelenteron with water, using cilia in the pharynx, before it could expand (1).
• Q19 (2): Any two: fingers could be stung — and some people react badly; bluebottles and jellies sting painfully even when stranded; a twig gives the same light touch every time (fair test); small scraps feed the anemone without harming it or wasting animals; pulling or over-handling anemones damages them.

Investigation 3 — life cycles and classes

• Q20 (8): Six stages drawn and named in the correct order, joined into a cycle by arrows: medusa → gametes (eggs and sperm, fertilisation) → planula larva → polyp / scyphistoma → strobila (stack of discs) → ephyra → back to medusa (6: one per stage in the correct position). The medusa-to-planula half labelled sexual (1) and the polyp-to-ephyra half labelled asexual (1).
• Q21 (3): The scyphistoma is attached, feeds with tentacles and has the full polyp body plan, so it is a small polyp (1) — it even buds and lives for years (1). The true larva is the ciliated, free-swimming planula (1). (Accept the ephyra as a "larval" medusa, as the booklet calls it.)
• Q22 (12): One mark each. 1 bluebottle — Hydrozoa (a polymorphic colony). 2 hydroid — Hydrozoa. 3 compass jelly — Scyphozoa. 4 box jelly — Cubozoa. 5 sandy anemone — Anthozoa: anemones. 6 strawberry anemones — Anthozoa: colonial anemones. 7 zoanthids — Anthozoa: zoanthids. 8 soft coral — Anthozoa: soft corals. 9 hard coral — Anthozoa: hard corals. 10 sea fan — Anthozoa: sea fans. 11 comb jelly — not a cnidarian; phylum Ctenophora. 12 by-the-wind sailor (Velella) — Hydrozoa (accept any answer that recognises it as a hydrozoan colony like the bluebottle).
• Q23 (3): Number 11, the comb jelly (1). Any two: no stinging cells; prey sticks to sticky cells on its tentacles; it moves with eight rows of beating plates (combs of cilia), not by pulsing a bell (2).
• Q24 (2): Colony: what looks like one animal is made of many individuals joined together (1). Polymorphic: the individuals have different body forms for different jobs — the float, stinging tentacles, feeding parts and reproductive parts (1).
• Q25 (3): The algae in hard corals photosynthesise and supply most of the coral's food (1), so reef-building corals must live where enough sunlight reaches them — shallow, clear water (1). Sea fans have no zooxanthellae and feed by catching plankton, so they do not need light and can live at great depths (1).

Investigation 4 — the booklet's questions

• Q26 (6): (a) medusa · (b) diploblastic · (c) nematocysts · (d) sessile · (e) zooxanthellae · (f) antagonistic.
• Q27 (8): A nematocyst · B longitudinal muscle band · C interstitial cells · D amoeboid nutritive-epithelial (musculo-digestive) cell · E nerve cell · F ectoderm · G mesoglea · H endoderm.
• Q28 (4): A — to capture prey: it fires a barbed thread that pierces and injects venom, or a sticky thread that entangles (1). D — internal (intracellular) digestion: it engulfs food particles into food vacuoles and digests them (1). E — coordination: it carries signals through the nerve net between sensory cells and muscle fibres (1). F — protection and capturing food (1).
• Q29 (3): Similarity (1) — any one: both have jelly-like bodies; both have an ectoderm, a mesoglea and an endoderm-lined coelenteron; both are radially symmetrical; most have tentacles for catching food. Differences (2) — any two: ctenophores have no nematocysts (sticky cells instead); they swim with eight rows of comb-like cilia, cnidarians by muscles; most ctenophores have anal pores as well as a mouth; ctenophores have no polyp stage.
• Q30 (8): One mark per point, in a paragraph: sensory cells detect the shrimp's movement or chemicals · the tentacles may reach towards it · on contact the nematocysts fire · barbed threads inject venom while sticky threads entangle it · the shrimp is held and paralysed or killed · the tentacles push it into the mouth · the pharynx swallows it in two steps without losing the water in the coelenteron · gland cells release enzymes and digestion begins in the coelenteron (external), finishing in food vacuoles in the digestive cells (internal) · nutrients spread by diffusion · undigested remains are pushed back out of the mouth. (Any eight.)
• Q31 (12): (a) Colonial polyps (3): all three are colonies of many polyps joined together by a coenenchyme or a shared base (1); each polyp is like a small anemone with a ring of tentacles round its mouth (1); soft coral — polyps grow from one fleshy base; hard coral — each polyp in its own limestone cup; sea fan — polyps along a branching, fan-shaped frame (1). (b) Line diagrams (6): two marks each — a lobed or branching soft colony with polyps on its surface; a hard, branching or rounded colony showing polyps in cups; a flat, branching fan with a central rod and polyps along the branches. (c) Support (3): soft coral — hydrostatic: the two body layers, mesoglea and water-filled coelenterons (accept also tiny chalky spicules, sclerites, in the tissue) (1); hard coral — a skeleton of calcium carbonate cups, built by the polyps (1); sea fan — a stiff but flexible central rod of the horn-like protein gorgonin (1).
• Q32 (4): (a) Endodermal — the algae live in the inner layer (the gastrodermis) lining the coelenteron; the booklet's text and examinable definition are right and its question 1.5 is a slip (1). (b) Any one: it is next to the coelenteron, where the polyp's carbon dioxide and nitrogen-rich wastes collect, which the algae use; the algae are sheltered inside the polyp from grazers and waves (1). (c) The algae do not supply calcium carbonate — the coral builds its own skeleton (1); the algae supply sugars from photosynthesis that power the building, and remove CO₂, so corals with algae build their skeletons much faster in light (1).

Marking note. Q32 closes a loop that opened in Topic 22: two parts of this course's own sources disagreed about where a coral's algae live. A learner who can say which is right, why the layer makes sense, and what the algae really give the coral has done what scientists do with conflicting sources. In Q26(e), the booklet's original question 1.5 says "ectodermal"; the term asked for is "zooxanthellae" either way.