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NCERT Solutions for Class 9th Science Chapter 12 Porifera and Cnidaria — In-text Questions

Book page 241 Updated on2026-09-08

Q1.
Sponges remain fixed in one place and are found in aquatic environments. Do you think they would be able to survive on land? Why or why not?
Answer

No. A sponge cannot survive on land, because every one of its life processes depends on water flowing through its body.

Why — the mechanism: a sponge is multicellular but lacks tissues and organs. It has no mouth, no gut, no gills, no heart, no muscles and no nervous system. Instead, numerous pores allow water to flow continuously through it, and that current does all four jobs at once:
  • it brings food particles directly to individual cells;
  • it brings oxygen directly to individual cells;
  • it carries the waste away;
  • it also carries the sponge's reproductive cells out and in.
Take the water away and all four stop at once. There is no organ that could take over, because there are no organs. A land animal needs lungs or tracheae to breathe, a gut to digest, and a waterproof covering to stop drying — a sponge has none of these, and its body is porous, so it would dry out in minutes.
Did you know? Research studies show that one kilogram of sponge can filter up to 24,000 litres of sea water per day. That single number tells you how completely the animal depends on the current: it is not living in water so much as living on the water passing through it.
Q2.
However, cnidarians possess a single opening that functions for both food intake and waste removal. Can you hypothesise what limitations this body structure might have?
Answer

A single opening means feeding and waste removal cannot happen at the same time, and digestion cannot be divided into stages. Three limitations follow.

  1. Feeding must stop while waste is expelled. The same door is used both ways, so the animal cannot eat continuously. A gut with a mouth at one end and an anus at the other lets food move in one direction and keeps the two flows separate.
  2. No specialisation along the gut. In a one-opening body cavity, all of digestion happens in a single chamber. With a tube, different regions can do different jobs in sequence — grinding, then enzyme digestion, then absorption, then water reabsorption. That is far more efficient.
  3. Undigested waste mixes with incoming food. Nutrients and waste share one space, which limits how much can be absorbed and raises the risk of re-ingesting waste.
Why it still works for a cnidarian: a hydra or jellyfish has a thin body only two cell layers thick, so every cell of the inner layer touches the digestive cavity directly and can absorb food from it — no blood supply needed. The design is a limitation only once the animal gets larger and thicker. That is exactly what the next steps in the chapter do about it: nematodes have two openings, a mouth and an anus, which is the first true one-way gut in this sequence.
See the pattern: Porifera (no tissues, water current does everything) → Cnidaria (true tissues, tentacles catch prey actively, but one opening) → Platyhelminthes (bilateral symmetry, still one opening) → Nematoda (two openings). Each stage keeps what worked and fixes one thing that did not.
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