Sponges belong to which one among the following phylum?

Correct — A, Porifera. The name is the answer read aloud: porus, a pore, and ferre, to bear. A sponge's body wall is drilled through with thousands of tiny inhalant pores called ostia, and water pulled in through them leaves by a single large opening at the top, the osculum. Driving that current are the choanocytes, collar cells whose flagella beat in unison and whose collars strain out the bacteria and organic particles the animal eats. Porifera sit at the very base of the animal kingdom because they never got as far as tissues. There is no nerve cell, no muscle, no mouth, no gut, no organ of any kind — cells are specialised but they are not organised into layers that act together, which is what zoologists mean by the cellular grade of organisation. Support comes not from a skeleton of bone or shell but from spicules of calcium carbonate or silica embedded in the body, or from a mesh of the protein spongin, which is what makes a bath sponge springy. Nearly all sponges are marine, all are sessile as adults, and all feed by filtering.

• (b)Platyhelminthes — The flatworms — planaria, liver fluke, tapeworm. They are bilaterally symmetrical, triploblastic and lack a body cavity, and they have a definite head end with a simple brain. Nothing like a sponge.
• (c)Cnidaria — The tempting one, because Hydra, jellyfish, sea anemones and corals include sessile marine animals that look like sponges to the eye. They are a grade above: radially symmetrical, diploblastic, with real tissues, a gastrovascular cavity with a mouth, and stinging cells called cnidocytes that no sponge possesses.
• (d)Annelida — The segmented worms — earthworm, leech, Nereis. They have a true coelom, a body divided into repeating segments, a closed blood system and a through gut. A sponge has none of these.

Porifera are the sponges, about 8,000 described species, almost all of them marine and all sessile as adults. Water enters through ostia in the body wall, passes into a central cavity called the spongocoel and leaves by the osculum, driven by the flagella of collar cells known as choanocytes. Digestion is intracellular, carried out cell by cell, because there is no gut. The body is held up by spicules of calcium carbonate or silica, or by fibres of spongin. Sponges reproduce asexually by budding and by gemmules, and sexually — most are hermaphrodite — with a free-swimming larva that later settles.

The reliable way through a phylum question is to fix one hallmark per phylum rather than trying to recall a list of examples. Pores and collar cells give Porifera; stinging cells and a single gut opening give Cnidaria; a flat, unsegmented, cavity-less body gives Platyhelminthes; repeating segments give Annelida. The examiner's trap here is Cnidaria, since corals and sponges share the same reefs and the same immobile life. The cleanest separation is the stinging cell: a cnidarian catches live prey with cnidocytes, a sponge catches nothing and simply strains what the current brings. The stem prints 'which one among the following phylum' where the sense wants the plural 'phyla' — a slip in the paper's grammar that does not touch what is being tested.

• Porifera means pore-bearing; the inhalant pores are the ostia and the single large exhalant opening is the osculum.
• Choanocytes, or collar cells, line the internal cavity and drive the water current with their flagella while trapping food particles.
• Sponges have a cellular grade of organisation — specialised cells but no true tissues, no nerves, no muscles and no digestive tract.
• The skeleton is made of calcareous or siliceous spicules, or of the protein spongin; a bath sponge is a spongin skeleton.
• Cnidarians, by contrast, reach the tissue grade, are diploblastic and radially symmetrical, and carry stinging cnidocytes.

• Reaching for Cnidaria because corals and sponges are both sessile reef animals — the stinging cell settles it.
• Assuming a sponge has a mouth because it has a large opening at the top; the osculum is where water leaves, not where food goes in.
• Calling Porifera a plant-like organism. Sponges are animals: multicellular, heterotrophic and without cell walls.

Usually as a one-line 'which phylum' recall item like this one, or in reverse as a phylum-to-example match, and sometimes as a question on which group lacks true tissues.

• practice — not a real PYQ

  The collar cells that create the water current and trap food particles in a sponge are called

  • (a)Cnidocytes
  • (b)Choanocytes
  • (c)Nephridia
  • (d)Flame cells

  Answer(b) Choanocytes — their beating flagella drive water through the sponge and their collars strain out food. Cnidocytes are the cnidarian stinging cells and flame cells are the flatworm excretory cells.

• practice — not a real PYQ

  Which one of the following animal groups shows only the cellular grade of organisation, with no true tissues?

  • (a)Cnidaria
  • (b)Platyhelminthes
  • (c)Porifera
  • (d)Echinodermata

  Answer(c) Porifera — sponges have specialised cells but no tissue layers acting together, which is why they sit at the base of the animal kingdom.