Sponges, cnidarians and comb jellies · Biology Class 11–12 | StemPy Lab
Course 4.2·Lesson 1 of 3
Sponges, cnidarians and comb jellies
Sponges (Porifera) have a cellular level of organisation. Water enters through ostia, passes the collar cells in the spongocoel and leaves by the osculum. Cnidarians have stinging cnidoblasts, a gastro-vascular cavity with one opening, and two body forms, the polyp and the medusa, which alternate in Obelia. Comb jellies (Ctenophora) swim with eight rows of comb plates and glow by bioluminescence.
about 30 min1 simulation1 practice question
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Simple animals of the water
A bath sponge, a jellyfish and a coral reef look nothing like a cat or a frog. Yet all of them are animals. They have no backbone, so they are non-chordates.
In this lesson you meet the three simplest phyla: the sponges, the cnidarians and the comb jellies. All of them live in water, and most live in the sea.
Phylum Porifera: the sponges
The word Porifera means "bearing pores". Sponges are the most primitive multicellular animals.
They are mostly marine and mostly asymmetrical.
They have a cellular level of organisation. Their cells do not form true tissues.
They live fixed to rocks and do not move about.
The key feature of a sponge is its water transport or canal system.
Water enters through tiny pores in the body wall, the ostia.
It reaches a central cavity, the spongocoel.
It leaves through a large opening at the top, the osculum.
The spongocoel and the canals are lined by choanocytes, or collar cells. Their flagella keep the water moving. This flow of water brings food and oxygen, and carries away wastes. Digestion is intracellular: food is digested inside the cells.
A simple sponge shaped like a vase, cut open lengthways and fixed to a rock at its base. Its thick body wall has small round pores, the ostia, on both sides. Blue arrows show water entering through the ostia, passing through the wall and reaching a large central cavity, the spongocoel. Small orange dots, the collar cells or choanocytes, line the cavity. Short grey needles, the spicules, lie in the body wall. Three blue arrows leave through the wide opening at the top, the osculum, where the water goes out.
The body is held up by a skeleton of spicules or of spongin fibres.
Reproduction. Sponges are hermaphrodite: the same animal makes both eggs and sperms. They reproduce asexually by fragmentation, and sexually by gametes. Fertilisation is internal. Development is indirect, with a larval stage that looks quite unlike the adult.
Examples:Sycon (Scypha), Spongilla (a freshwater sponge) and Euspongia (the bath sponge).
Phylum Cnidaria: animals that sting
Cnidarians are aquatic and mostly marine. Some stay fixed (sessile); others swim freely. They are radially symmetrical.
The phylum takes its name from the cnidoblasts, or cnidocytes. These are stinging cells on the tentacles and the body. Each holds a nematocyst, a stinging capsule. Cnidoblasts are used for anchorage, defence and catching prey.
They have a tissue level of organisation, and are diploblastic: the body is built from two layers of cells.
A central gastro-vascular cavity has a single opening, the mouth, on a raised part called the hypostome.
Digestion is both extracellular and intracellular.
Corals build a skeleton of calcium carbonate.
Two body forms. A polyp is a fixed, cylindrical form, such as Hydra and Adamsia. A medusa is umbrella-shaped and swims freely, such as Aurelia, a jellyfish.
The two body forms of cnidarians, side by side. On the left, a polyp: a pink tube fixed to a rock at its base, with a hollow gastro-vascular cavity inside, and a mouth at the top ringed by wavy tentacles that point upwards. It is labelled as fixed, like a tube, with Hydra and Adamsia as examples. On the right, a medusa: a pink umbrella, free-swimming, with its mouth on a short stalk facing down and wavy tentacles hanging from its rim. It is labelled with Aurelia as the example. Between them, an arrow from the polyp to the medusa says it buds off asexually, and an arrow back says that the medusa forms gametes and a zygote sexually. The title says that in Obelia the two forms alternate.
In some cnidarians, such as Obelia, both forms appear in one life cycle. Polyps make medusae asexually. Medusae make polyps sexually. This alternation of generations is called metagenesis.
Examples:Physalia (the Portuguese man-of-war), Adamsia (the sea anemone), Pennatula (the sea pen), Gorgonia (the sea fan) and Meandrina (the brain coral).
Phylum Ctenophora: the comb jellies
Ctenophores are called sea walnuts or comb jellies.
They are exclusively marine, radially symmetrical and diploblastic, with a tissue level of organisation.
The body bears eight external rows of ciliated comb plates. The beating cilia help it to swim.
Digestion is both extracellular and intracellular.
Bioluminescence is well marked. This is the power of a living thing to give out light.
The sexes are not separate. They reproduce only sexually.
Fertilisation is external, and development is indirect.
Examples:Pleurobrachia and Ctenoplana.
You should find that a sponge pumping 40 mL a minute moves 2.4 litres in an hour. In a whole day that is 57.6 litres, through an animal smaller than your fist.
Try it in Python
The program keeps the three phyla in a dictionary, with their level and one telling feature each.
You should see Cnidaria and Ctenophora at the tissue level, and Porifera alone at the cellular level. A dictionary keeps one value for each key, so the new Physalia line replaces the Aurelia one.
Practise with new numbers
Common mistakes
Calling a jellyfish a fish.Aurelia is a cnidarian. It has no backbone, no fins and no gills.
Saying that water enters a sponge through the osculum. Water comes in through the many small ostia, and goes out through the one osculum.
Thinking that sponges have tissues. They have a cellular level of organisation only.
Mixing up cnidarians and comb jellies. Both are diploblastic and radially symmetrical. Only cnidarians have stinging cells; only comb jellies have comb plates.