Snail (class Gastropoda, clams (class Bivalvia) and cephalopods (class Cephalopoda) - all compositions.
• What are the main differences you see between classes?
• Explain how homologous structures that define molluscs have been modified in the different classes?

Earthworm (Clitellata class, subclass Oligochata), leeches (Clitellata class, subclass Hirudinea) and many polychaetes (class Polychaeta) - all compositions.
• What are the main characters that distinguish the two classes and the two sub-classes (as you can see by studying the compositions - use loupes)?

Daphnia sp. (class Branchiopoda and large crayfish
• Describe the differences and how these may be related to lifestyle and food uptake. Note in particular body shape, carapace and appendages.

Expert Answers

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Snails, clams and cephalopods are all class under the phylum Mollusca, a group of invertebrate animals. These three classes are distinctively different from each other based on certain classifications. However, genetic mapping concludes that these three are close relatives generating from a single root. 

Snails (Class Gastropoda) - are freshwater and seawater dwelling creatures that have coil-shaped shells. Not all gastropods have external shells; some have them inside (internal shells) such the slugs. They are univalves, which mean that they only have one shell. 

Bivalves (Class Bivalvia) - from their name, they have two sets of shells, hinged together and enclosing the creature inside. Examples of species under this class are oysters, clams and scallops. 

Cephalopod (Class Cephalopoda) - are marine dwelling creatures that have bilateral body symmetry. Examples of which are octopus, squid and the nautilus. They also have several arms or tentacles which is different from snails that only have two. Compared with the two classes, cephalopods can only be found in oceans and not in freshwater. 

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