Lesson 4 of 12 · 8 min
Sponges, cnidarians and comb jellies
NCERT §4.2.1–§4.2.3
At low tide Ammu finds a yellow sponge on a rock, a jellyfish stranded in a pool and, at night from the jetty, a faint glowing blob drifting in the water. The fisherman beside her calls it a comb jelly.
The lesson in notes
In short
Porifera (sponges) are mostly marine, mostly asymmetrical, primitive multicellular animals at the cellular level. Water enters through tiny pores called ostia, reaches the central spongocoel and leaves through the osculum; this canal system serves food gathering, gas exchange and waste removal.
The spongocoel and the canals are lined by choanocytes (collar cells), and food is digested inside cells. Spicules or spongin fibres form the skeleton. Sponges are hermaphrodite, reproduce asexually by fragmentation and sexually by gametes, and have internal fertilisation with a distinct larval stage. Examples: Sycon (also called Scypha), Spongilla (a freshwater sponge), Euspongia (the bath sponge).
Coelenterata (Cnidaria) are aquatic, mostly marine, sessile or free-swimming and radially symmetrical. They are named after cnidoblasts (cnidocytes), cells on the tentacles and body that hold stinging nematocysts used for anchorage, defence and catching prey.
Cnidarians are diploblastic with tissue-level organisation. They have a gastro-vascular cavity with a single opening, the mouth, on the hypostome. Digestion is both extracellular and intracellular. Corals have a calcium carbonate skeleton.
Two body forms occur: the polyp, sessile and cylindrical (Hydra, Adamsia), and the medusa, umbrella-shaped and free-swimming (Aurelia). Where both occur there is metagenesis: polyps give rise to medusae asexually and medusae give rise to polyps sexually, as in Obelia.
Cnidarian examples: the sea anemone Adamsia, the Portuguese man-of-war Physalia, the brain coral Meandrina, the sea-pen Pennatula and the sea-fan Gorgonia.
Ctenophora (sea walnuts, comb jellies) are exclusively marine, radially symmetrical, diploblastic and at the tissue level. Eight external rows of ciliated comb plates drive locomotion. Digestion is extracellular and intracellular, and bioluminescence is well marked.
Ctenophores are hermaphrodite and reproduce only sexually, with external fertilisation and indirect development. Examples: Pleurobrachia, Ctenoplana.