Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
Arrange these groups from the simplest body to the most complex: Arthropoda, Coelenterata, Porifera, Platyhelminthes. Say what the level of organisation is in each.
Porifera: pores, cells only. Coelenterata: one opening, tissues. Platyhelminthes: flat, organs. Nematoda: round. Annelida: segments. Arthropoda: jointed legs, the largest group. Mollusca: soft body, often a shell. Echinodermata: spiny skin, in the sea.
Have you tried it?
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. Cells. Porifera: cells do jobs, but there are no tissues.
Step 2. Tissues. Coelenterata: cells form tissues.
Step 3. Organs. Platyhelminthes: tissues form organs.
Step 4. Organ systems. Arthropoda: organs work together in systems.
Answer: Porifera, Coelenterata, Platyhelminthes, Arthropoda.
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