Mistake 1: Confusing Platyhelminthes (acoelomate) and Aschelminthes (pseudocoelomate)

• Wrong: "Platyhelminthes have pseudocoelom"
• Correct: Platyhelminthes = ACOELOMATE (no cavity; filled with mesenchyme). Aschelminthes = PSEUDOCOELOMATE (false cavity)
• Memory trick: Platyhelminthes are FLAT → flat like they're squeezed with no room for a cavity = ACOELOMATE

Mistake 2: Thinking ALL sponges have spicules made only of $CaCO_{3}$

• Wrong: "All sponge spicules are calcareous"
• Correct: Spicules can be calcareous ($CaCO_{3}$) OR siliceous ($SiO_{2}$). Some sponges (Euspongia) have spongin fibres instead of mineral spicules.

Mistake 3: Applying adult Echinoderm symmetry to larvae

• Wrong: "Echinoderm larvae are radially symmetrical"
• Correct: Echinoderm LARVAE (bipinnaria, pluteus) are BILATERALLY symmetrical. Only ADULTS are radially symmetrical.

Mistake 4: Placing Limulus in Crustacea

• Wrong: "Limulus (king crab) is a crustacean"
• Correct: Limulus belongs to Class MEROSTOMATA (Arthropoda) — it has chelicerae, no antennae, book gills. True crabs = Crustacea.

Mistake 5: Thinking all reptiles have 4-chambered hearts

• Wrong: "Reptiles have 4-chambered hearts"
• Correct: ONLY Crocodilia among reptiles have 4-chambered hearts. All other reptiles have 3-chambered hearts with an incompletely divided ventricle.

Mistake 6: Confusing Porifera (diploblastic) with Cnidaria

• Wrong: "Porifera are diploblastic"
• Correct: Porifera have NO true germ layers — they are at the CELLULAR level of organization. Only Cnidaria and Ctenophora are diploblastic.

Mistake 7: Saying all Mollusca have open circulation

• Wrong: "All molluscs have open circulation"
• Correct: CEPHALOPODA (Octopus, Loligo, Sepia) have CLOSED circulation — an exception among molluscs.

Mistake 8: Classifying whale as fish

• Wrong: "Blue whale is a fish because it's aquatic"
• Correct: Balaenoptera (blue whale) is a MAMMAL — has mammary glands, hair, lungs, 4-chambered heart, viviparous.