Is there any problems ? -1a with stalked compound eyes? -1bwithout stalked compound eyes? problems: how about compound eyes without stalk? -1a. with hard shell? -1b. without hard shell? problems: Does crab has a hard shell? - 1a. with many segments? - 1b. Without many segments? - Problems: many = ?

Misconceptions in key 1a. With hair 1b. Without hair Problem: only mammals have hair, spider has sensory hair but not hair? Some crabs also have sensory hair 1a. With 2 pairs of antennae 1b. With 1 pair of antennae? Problem: it is not dichotomous, starfish has 5 antennae! It cannot be classified into 1a or 1b.

Principle for writing dichotomous key 1. Write down the unique features of each organisms to be classified e.g. tube feet, cheliped, fins, suctorial foot … that cannot be found in other animals. 2. Wipe out one organism at one time (easier to do this) 3. Revise the unique features of each phylum / class. 4. Dichotomous should be : 1a + 1b should include all possibility.

Unique features Fish – backbone, fins, skin with wet scales Starfish – tube feet, radial symmetrical, bilateral symmetrical Chiton – suctorial foot Crab – cheliped, body into head and cephalothorax Spider – 8 jointed appendages on thorax, body divided head, thorax, abdomin Centiped – one pair of jointed appendanges per segment, more than 3 segments