In a sea aquarium, Meera sees tiny branched colonies fixed to a rock…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
In a sea aquarium, Meera sees tiny branched colonies fixed to a rock. A week later, small umbrella-shaped animals swim in the same tank. The guide says that both belong to one species, Obelia. (a) Name the two body forms she saw. (b) How does each form give rise to the other? (c) What is this pattern called?
Need a reminder? The key ideaShow
Porifera: cellular level; water enters by ostia, passes the collar cells (choanocytes) in the spongocoel and leaves by the osculum; spicules or spongin; hermaphrodite. Cnidaria: cnidoblasts with nematocysts; gastro-vascular cavity with one opening; polyp (Hydra, Adamsia) and medusa (Aurelia); metagenesis in Obelia; Physalia, Pennatula, Gorgonia, Meandrina. Ctenophora: eight rows of comb plates, bioluminescence, only sexual reproduction; Pleurobrachia, Ctenoplana.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The forms. The fixed colony is made of polyps. The swimming umbrellas are medusae.
Step 2. How they arise. The polyps make medusae asexually, by budding. The medusae make gametes; after fertilisation, the zygote grows into a new polyp, so polyps arise sexually.
Step 3. The name. The alternation of the two forms is metagenesis.
Answer: (a) polyps and medusae; (b) polyps bud off medusae asexually, and medusae give rise to polyps sexually; (c) metagenesis.
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