Dichotomous Key (SCI201-Learning Event 5)
The dichotomous key consists of a series of pairs of statements that require you to place the organism
into one of two categories. For each of the organisms, begin at step 1 of the key and proceed through the
key until you have identified the organism.
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b
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Irregular-shaped body; structure with many pores
Regular-shaped body (with right and left halves or a cylindrical shape
Radial symmetry (disk-shaped or barrel-shaped)
Bilateral symmetry (similar right and left body halves)
Animal has internal skeleton
Animal has external skeleton or no apparent skeleton
Body hard, arms extend from a central disk, or spines present
Soft body; little or no color
Saucer-shaped transparent body with small tentacles
Barrel-shaped body, tentacles at one end
Hard outer covering
No hard outer covering
Body flattened
Body not flattened
Smooth, nonsegmented body
Apparently segmented, flattened body
Nonsegmented
Segemented body
Body has jointed legs
Body inside shell is soft, has no jointed legs
Tentacles or other appendages present
Body long and tubular, no appendages
Appears as a snail without a shell
Tentacles and eyes present
Bivalved shell (two halves)
Univalved shell (single unit)
Jointed appendages on most body sections
Jointed appendages on certain body segments; not all appendages
are legs
One pair of legs per body segment
Two pairs of legs per body segment
Two pairs of antennae, large claws often present
One pair of antennae or none, no large claws
Four pairs of legs, no antennae or wings
Three pairs of legs, wings present
Arms present, body surface knobby
Many-spined animal, resembles a pincushion
Fishlike, flattened body, appendages finlike not jointed
Not fishlike, body not flattened, appendages jointed or absent
Scales on body to not overlap, skeleton of cartilage
Scales on body overlap, skeleton bony
Body covered by scales, zero or four legs
Body not covered by scales
Claws absent
Claws or nails present on toes, skin covered with feathers or hair
Feathered, claws present
Hair present
Phylum Porifera
2
4
3
19
6
18
5
Class Scyphozoa
Class Anthozoa
10
7
8
9
Class Trematoda
Class Cestoda
11
Phylum Annelida
14
13
12
Phylum Nematoda
Class Gastropoda
Class Cephalopoda
Class Bivalvia
Class Gastropoda
15
16
Class Chilopoda
Class Diplopoda
Class Crustacea
17
Class Arachnida
Class Insecta
Class Asteroidea
Class Echinoidea
20
21
Class Chondrichthyes
Class Osteichthyes
Class Reptilia
22
Class Amphibia
23
Class Aves
Class Mammals