Dichotomous Key

Dichotomous Key

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Dichotomous Key

Prior to evolutionary theory, scientists relied on similarity of
physical characteristics to classify organisms. An 18th century
naturalist, Carolus Linneas, developed a taxonomic classification
system. This hierarchical system divides organisms into 5 kingdoms.
Organisms visible to the unaided human eye fall into the plant,
animal, or fungi kingdom. Kingdoms are then divided into phylum.
This division is based body type and skeletal organization. Phylum
are then divided into subphylum. Subphylum are divided into classes,
orders, genus, and then species. This system of classification is
still used by scientists today[1].

In this assignment, seven organisms are presented which are to be
classified into phylum or class based on a dichotomous key. The first
organism presented in this assignment is a starfish. Starfish fall
into the class Asteroidea. Starfish have a regular body shape with
radial symmetry. It has arms which extend from a central disk and has
a knobby body surface. Using these physical characteristics, we would
classify the starfish into the Kingdom Animalia, phylum Echinodermata,
and class Asteroidea.

The second organism is a species of bird. Again using the dichotomous
key, we find that this organism has a regular body shape. The bird
has bilateral symmetry with an internal skeleton. The bird’s
appendages are jointed, not fin-like. The body of the bird is not
covered in scales, the skin is covered with feathers and claws are
present. Using these physical characteristics we find that this
organism falls into the class Aves

The third organism appears to be a type of shark. The shark has a
regular body shape with bilateral symmetry and an internal skeleton.
The body of this animal is not fish-like and appendages are jointed.
The body is not covered by scales and claws are absent. Therefore,
this organism falls into the class Amphibian.

The fourth organism has a regular body shape with bilateral symmetry
and an internal skeleton. The organism has a fish-like body with

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fin-like appendages. The scales on the body of this creature do not
overlap and its skeleton is made of cartilage. This organism therefore
falls into the class Chondrichthyes.

The fifth organism has a regular body shape with bilateral symmetry
and an internal skeleton. This animal’s appendages are jointed, not
fin-like. There are no scales on the body of this animal and the skin
is covered with hair. Using this information and the dichotomous key
we determine that this animal should be classified in the class

The sixth organism has a regular body shape with bilateral symmetry
and has no apparent skeleton. It has a hard outer covering and has
jointed legs which are only on certain body sections, and not all
appendages are legs. This organism does not have claws. It has four
pairs of legs and no wings or antennae. Given these physical
characteristics, we can use the dichotomous key to determine that this
organism falls into the class Arachnida.

The seventh organism has a regular body shape with bilateral symmetry
and an internal skeleton. The animal’s appendages are jointed, not
fin-like. The body is covered by scales and the animal has four legs.
The organism therefore falls into the class Reptilia.


Organism 1

1.b, 2.a, 4.a, 18.a Class Asteroidea

Organism 2-

1.b, 2.b., 3.a, 19. b, 21. b, 22.b, 23. a Class Aves

Organism 3

1.b, 2.b., 3.a, 19. b, 21.b, 22.a Class Amphibia

Organism 4

1.b, 2.b, 3.a, 19. a, 20.a Class Chondrichthyes

Organism 5

1.b, 2b., 3a., 19.b, 21. b, 22.b, 23.b Class Mammals

Organism 6

1.b, 2.b, 3b., 6.a, 10a, 14.b, 16.b, 17.a Class Arachnida

Organism 7

1.b, 2.b, 3.a, 19. b, 21.a Class Reptilia


Audesirk, T., Audesirk, G. & Byers, B. Biology: Life on Earth. New
York: Prentice Hall,



[1] Audesirk et al., Biology: Life on Earth (New York, Prentice Hall,
2001) 146-151.
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