Introduction to Animal Classification

Animals are the living organisms that are eukaryotic, multicellular, and heterotrophic (they obtain nutrition and energy by consuming other organisms). Thus, we are talking about everything from sponges to complex animals like mammals. More than one million animal species have been identified and named.

How are the many species of animals (for that matter, all living things) arranged, categorized, and classified? How does the classification scheme reflect form and function, ancestral traits, and derived characteristics? It can be a bewildering yet extremely interesting problem.

Taxonomy is the practice and science of classification. The word finds its roots in the Greek τάξις, taxis (meaning 'order', 'arrangement') and νόμος, nomos ('law' or 'science'). The basis of our modern taxonomic system was set forth by a Swedish botanist/zoologist/physician, Carolus Linnaeus (a Latinized version of his legal name Carl von Linné). Linnaeus is considered the father of modern taxonomy and one of the fathers of modern ecology. He devised practical techniques for the naming of groups of organisms and their ranking and ordering. His hierarchical system of ranks was first published in Systema Naturae (1758). The strength of Linnaean taxonomy is that it can be used to organize the different kinds of living organisms, simply and practically. Prior to Linnaean taxonomy, animals were classified according to their mode of movement.

The greatest innovation of Linnaeus, and still the most important aspect of this system, is the general use of binomial nomenclature (the scientific name). Every species can be given a unique (and hopefully stable) name in Latin that consists of a collective genus name and a specific or species name. For example, the human species is uniquely identified by the name Homo sapiens.

No other species of organism can have this same binomen (the technical term for a binomial in the case of animals).
We note that this is the scientific name by using Italics when typing.

When handwritten, a scientific name is underlined.

We capitalize the first letter of the genus name (e.g. Homo).
The species name is never capitalized (e.g. sapiens).

Common names for organisms often are neither unique nor consistent from place to place or language to language. Consider a spider species that is commonly called the brown recluse, fiddle-back, or violin spider. This spider’s scientific name is Loxosceles recluse. Whether one speaks Korean, English, or Italian, the scientific name remains constant.

The taxonomic rankings, from most inclusive to most exclusive, are as follows:

· Domain

· Kingdom

· Phylum

· Class

· Order

· Family

· Genus

· Species

Sometimes, a group of animals is so large that it necessitates subdivision of the taxonomic rankings. Consider the classification of the fishes.

Phylum Chordata

Subphylum Hyperotreti – Hagfishes

Subphylum Vertebrata

Class Cephalospidomorphi – Lampreys

Class Chondrichthyes

Subclass Elasmobranchii – Sharks, skates, and rays

Subclass Holocephali – Ratfishes

Class Osteichthyes

Subclass Sarcopterygii – Lungfishes and coelacanths

Sublcass Actinopterygii – Ray-finned fishes

There are three domains, divided into six kingdoms. Kingdom Animalia is divided into 36 extant phyla. Groups of organisms at any of these ranks are called taxa (singular: taxon) or taxonomic groups.

Domain	Kingdom	Cell type	Organization	Nutrition	Organisms
Archaea	Archaebacteria	Prokaryotic	Unicellular-small; Lacking peptidoglycan	Absorb, Photsyn., Chemosyn.	Archaeacteria
Bacteria	Eubacteria	Prokaryotic	Unicellular-small; Peptidoglycan in cell wall	Absorb, Photsyn., Chemosyn.	Bacteria, Cyanobacteria
Eukarya	Protista	Eukaryotic	Unicellular or colonial	Ingestion, Photosynthesis	Protozoa, Algae
	Fungi	Eukaryotic	Multicellular	Absorption	Fungi, yeast, molds
	Plantae	Eukaryotic	Multicellular	Photosynthesis	Plants
	Animalia	Eukaryotic	Multicellular	Ingestion	Animals

Consider the following group of animals: human, domestic fruit fly, house cat, chimpanzee, lion, and tiger. The animal that is very different from the others is the domestic fruit fly (an insect). The remaining animals are mammals. Of the remaining animals, house cats, lions, and tigers are most closely related, while the chimpanzee and human share characteristics that distinguish them from the other groups. These shared characteristics (which reflect phylogeny) are evident in the animals’ taxonomy.

	Kingdom	Phylum	Class	Order	Family	Genus	Species
Fruit Fly	Animalia	Arthropoda	Insecta	Diptera	Drosophilidae	Drosophila	melanogaster
House cat	Animalia	Chordata	Mammalia	Carnivora	Felidae	Felis	domestica
Lion	Animalia	Chordata	Mammalia	Carnivora	Felidae	Panthera	leo
Tiger	Animalia	Chordata	Mammalia	Carnivora	Felidae	Panthera	tigris
Human	Animalia	Chordata	Mammalia	Primates	Hominidae	Homo	sapiens
Chimpanzee	Animalia	Chordata	Mammalia	Primates	Hominidae	Pan	troglodytes

The uniqueness and stability of taxonomy result from the acceptance by working systematists (biologists specializing in taxonomy). Without a set of international rules to follow, the results of taxonomy would be confusing at best. The rules of zoological nomenclature are contained in a document known as the International Code of Zoological Nomenclature (ICZN). The object of the code is to promote stability and universality in the scientific names of animals.

Much of the task of the taxonomist consists of assigning taxa to the appropriate categorical rank. The hierarchy of categories that the classifying taxonomist recognizes is an attempt to express similarity ("characters in common") and recency of common descent. The most closely related species (occasionally subject to intense debate) are combined into genera, groups of related genera into subfamilies and families, these into orders, classes, and phyla. In this procedure there is a drastic difference between the species taxon and the higher taxa (genus on up to phylum). Higher taxa are defined by intrinsic characters. For instance, the Class Aves (birds) is the class of "feathered vertebrates". Any and all species that satisfy the definition of "feathered vertebrates" belong to the Class Aves.

Again, with more than one million described species, memorizing the taxonomy of every species would be a daunting challenge. Fortunately, memorization is not required. Scientists have developed aids, known as keys that assist people in assigning animals to their correct taxonomic rankings. One common form of aid is the dichotomous key. A dichotomous key is a reference tool where a series of choices between alternative characters leads progressively to the identification of the species. The dichotomous key consists of a series of paired 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.

Assignment 1:

Let us practice with some example organisms. Identify the phylum and class of each using the following dichotomous key.


Common name	Red-knobbed starfish	Garden spider	Dyeing (poison) dart frog
Scientific name	Protoreaster linckii	Argiope aurantia	Dendrobates tinctorius
Phylum
Class

Common name	Northern mockingbird	Great white shark	Pacific sea nettle
Scientific name	Mimus polyglottos	Carcharodon carcharias	Chrysaora fuscescens
Phylum
Class

Common name	Yellow lined plated lizard	Common earthworm	Monarch butterfly
Scientific name	Gerrhosaurus flavigularis	Lumbricoides terrestris	Danaus plexippus
Phylum
Class

Dichotomous Key of Common Animal Phyla and Classes

1	a	Body asymmetrical (Irregular-shaped); structure with many pores………	Phylum Porifera
	b	Body symmetrical (Regular-shaped; with right and left halves or a cylindrical shape)…………………………………………………………...….	2
2	a	Bilateral symmetry (similar right and left body halves)…………………….	3
	b	Radial symmetry (disk-shaped or barrel-shaped)…………………………..	4
3	a	Animal has external skeleton or no apparent skeleton…………………….	6
	b	Animal has internal skeleton… Phylum Chordata…………….……………………………………………….	22
4	a	Soft body; little or no color... Phylum Cnidaria………………………………………………………………	5
	b	Body hard, arms extend from a central disk, or spines present... Phylum Echinodermata……………………………………………………...	21
5	a	Free-swimming, medusa-shaped (saucer-shaped) transparent body with tentacles dangling down………………………………...…………………….	Class Scyphozoa
	b	Sessile, polyp-shaped (hollow, cylindrical) body, attached at one end; mouth surrounded by tentacles at free end; tentacles pointing upward…	Class Anthozoa
6	a	No hard outer covering………………………………………………………..	7
	b	Hard outer covering……………………………………………………………	12
7	a	Body flattened… Phylum Platyhelminthes...……………………………………………….	8
	b	Body not flattened…………………………………………………………..	9
8	a	Smooth, nonsegmented body……………………………………………..	Class Trematoda
	b	Apparently segmented, flattened body……………………………………	Class Cestoda
9	a	Body wormlike and segmented throughout; setae, parapodia, or both often present; no jointed appendages… Phylum Annelida…………………………………………………………...	10
	b	Nonsegmented body………………………………………………………..	13
10	a	Setae present on each segment; segments distinct; suckers absent……	11
	b	Setae absent; no parapodia; segments indistinct and with many annuli; clitellum present; suckers present anteriorly and posteriorly or posteriorly only………………………………………………………………	Class Hirudinea
11	a	Many setae on each segment; parapodia or fleshy lateral appendages present; clitellum absent……………………………………	Class Polychaeta
	b	Few setae per segment; no parapodia; clitellum present…………….	Class Oligochaeta
12	a	Body is soft, often with a shell; no jointed legs…………………………….	13
	b	Body has jointed legs… Phylum Arthropoda….……………………………………………………	16
13	a	Ventral muscular foot or fleshy arms or tentacles present… Phylum Mollusca………………………………………………………..	14
	b	Body long and tubular, no appendages…………………………………	Phylum Nematoda
14	a	No prehensile arms with suckers; eyes small or absent………………..	15
	b	Head large and well developed with two large eyes; foot modified into 8 or 10 prehensile arms with suckers…………………………………….	Class Cephalopda
15	a	Shell of two lateral valves with ligamentous hinge; muscular foot present; head reduced; no tentacles or radula………………….……..	Class Bivalvia
	b	Shell (if present) usually coiled or spiraled; head with radula; one or two pairs of tentacles and one pair of eyes…………………………………….	Class Gastropoda
16	a	Paired antennae present……………………………………………………	17
	b	Antennae absent; body of cephalothorax and abdomen; segmentation often obscured… Subphylum Chelicerata….……………………………………….…….	20
17	a	One pair of antennae… Subphylum Uniramia……………………………………………………...	18
	b	Two pairs of antennae; appendages mostly biramous and specialized for different functions; many with gills………………………	Subphylum Crustacea
18	a	Body elongate, 15 or more pairs of jointed legs……………………….	19
	b	Head, thorax, and abdomen distinct; three pairs of legs on thorax; one or two pairs of wings often present…………………………………..	Class Insecta
19	a	Each segment with one pair of legs; body dorsoventrally flattened…	Class Chilopoda
	b	Each segment with two pairs of legs; body subcylindrical……………	Class Diplopoda
20	a	Four pairs of walking legs; no antennae or wings; head completely fused with thorax…………………………………………………………….	Class Arachnida
	b	Five pairs of walking legs; lateral compound eyes present…………….	Class Merostomata
21	a	Arms present, body surface knobby………………………………………	Class Astroidea
	b	Many-spined animal, resembles a pincushion………………………….	Class Echinoidea
22	a	Body fishlike; appendages finlike not jointed……...……......	23
	b	Skeleton bony; not fishlike, body not flattened, appendages jointed or absent………….........................................................................................	24
23	a	Skeleton cartilaginous; body fishlike; five to seven pairs of gills …….	Class Chondrichthyes
	b	Scales on body overlap, skeleton bony…………………………………..	Class Osteichthyes
24	a	Body not covered by scales………………………………………………..	25
	b	Body covered by scales, zero or four legs………………………………..	Class Reptilia
25	a	Claws or nails present on toes, skin covered with feathers or hair….	26
	b	Claws absent………………………………………………………………..	Class Amphibia
26	a	Feathered, claws present…………………………………………………..	Class Aves
	b	Hair present………………………………………………………………….	Class Mammalia

Assignment 2:

Students, working in groups, will construct a dichotomous key for items provided by the instructor.

Resources:

Hickman, Cleveland P. Jr. et al. Integrated Principles of Zoology, 13^th ed. McGraw Hill

Higher Education. 2006.

Hickman, Cleveland P. Jr. and Lee B. Kats. Laboratory Studies in Animal Diversity, 4^th

ed. McGraw Hill Higher Education. 2007.

Miller, Stephen A. and John P. Harley. Zoology, 7^th ed. McGraw Hill Higher Education.

2007.

Schuh, Randall T. and Andrew V. Z. Brower. Biological Systematics: Principles and

Applications, 2^nd ed. Cornell University Press. 2009.

http://www.ucmp.berkeley.edu/history/linnaeus.html

http://jrscience.wcp.muohio.edu/lab/TaxonomyLab.html

http://en.wikipedia.org/wiki/Starfish (photo source)

http://en.wikipedia.org/wiki/Argiope_aurantia (photo source)

http://en.wikipedia.org/wiki/Poison_arrow_frog (photo source)

http://en.wikipedia.org/wiki/Mocking_bird (photo source)

http://en.wikipedia.org/wiki/Great_white_shark (photo source)

http://en.wikipedia.org/wiki/Chrysaora_fuscescens (photo source)

www.faunaimportuk.com/photolizards.htm (photo source)

http://en.wikipedia.org/wiki/Earthworm (photo source)

http://en.wikipedia.org/wiki/Monarch_butterfly (photo source)