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Taxonomic Classification
I. Classification
One common method used in science is developing generalizations – drawing inferences
about a large group from observations of a relatively few members of that group. Thus,
although it is impossible to examine every clam or every porpoise in the world ocean, we
can still state with some assurance that, on the basis of previous observations of a limited
number of clams and porpoises, all clams have two pieced hinged shells and all porpoises
have two eyes and lack rear legs.
The classification system described here draws on such generalizations to define a
group, or taxon, of related organisms sharing a number of common features and a
common evolutionary history. The enormous variety of existing organism can be
reduced to fewer but more manageable categories by grouping organism in this manner.
The taxonomic system of classification is based on a formal hierarchy or
organization for the following defined groupings beginning with the largest group, the
domain, and subdividing that again and again to the smallest group, the species:
Domain
Kingdom
Phylum
Class
Order
Family
Genus
Species
Prefixes such as sub-, super-, or infra- are often used to subdivided or recombine these
basic taxonomic categories.
II. Identification
Identification of marine organism that you may encounter in labs and on field trips is
really a process of deciphering the organism’s previously established placement in the
taxonomic classification system. This process requires the use of some type of
identification guide to direct you through the maze of marine plant and animal taxa.
These identification guides are generally written for specific geographical regions and for
restricted groups of organisms.
Identification guides generally employ a key, a series of paired statements or
questions referring to the organism to be identified. Does it have a particular
characteristic or not? If the organism has that characteristic, you proceed in one direction
in the key; if it does not, you are directed to another part of the key. This format is known
as a dichotomous key; it consists of a series of choices that progressively narrow the
taxonomic possibilities for the organism being identified until the appropriate
identification is found.
To make the correct choices, you must know something about the animal you are
attempting to identify. Most identification keys emphasize external features that are quite
obvious, even if the organism is preserved. Other features, such as the type of digestive
or nervous system present, may be taxonomically more important but are often difficult
to observe without dissection.
A simple dichotomous identification key to the common phyla of marine benthic
invertebrates is given. Common features such as size and body shape are indicated to
make phylum determinations of animals using this key. But, so that more precise phylum
determinations of benthic animals can be made with this key, addition characteristics are
also emphasized. Some of the most important differentiating features are described
below.
A. Skeleton
Several types of skeletal systems can be observed in benthic invertebrates. The skeleton
may consist of a series of internal rigid parts covered by soft tissue, or the skeleton may
be external, such as the skeleton of a crab or a clam, with the soft tissues inside. Other
invertebrates lack rigid skeletal units entirely and, instead, utilize their body’s enclosed
fluids as a hydrostatic skeleton against which the muscles of the body wall can work.
B. Segmentation
Several animal phyla feature body plans that are subdivided along the animal’s length to
form a linear series of body segments, each one similar to those adjacent to it. Internal
segmentation is usually evident at the body surface as a repetitive pattern of body
divisions, each one frequently equipped with a pair of appendages.
C. Body Symmetry
Multicellular animals exist in two basic patterns of body symmetry: radial or bilateral.
Animals with radial symmetry are circular. Several different planes of symmetry can be
drawn to divide the animal into mirror-image halves. If a mouth is present, it is located at
the center of the circular body. Without a head or tail, the animal has neither a right nor a
left side. The side with the mouth is defined as the oral side; the opposite side is the
aboral side.
Tentacles or spines often radiate outward from the axis of the body. Sensory cells
or organs, such as light receptors, balance organs, and tough receptors, are also
distributed radially around the body. Radially symmetrical animals are characterized by
a simply organized nerve net without a true brain.
Most radially symmetrical animals are sessile. Radial symmetry appears in some
sponges, all cnidarians, and ctenophores, and most adult echinoderms. The radial
symmetry of adult echinoderms is a secondary condition because they develop from
larval forms that are not radially symmetrical.
The remaining multicellular animals are characterized by bilateral symmetry.
Only one plane of symmetry can be drawn to divide the animal into mirror-image halves.
Thus, all bilaterally symmetrical animals possess left and right sides, distinct head
(anterior) and rear (posterior) ends, and a top (dorsal) and bottom (ventral) side.
Associated with the head region are numerous specialized sensory receptors (ears, eyes,
etc.) and an enlarged complex portion of the nervous system, the brain.
In contrast to the few phyla that included in this key, local areas that you study on
field trips may be occupied by several hundred species of benthic invertebrates. Keys for
identifying these species have only regional applicability. After you have become
familiar with the way your key works, you may wish to key out one or two labeled
specimens. Then key out 10 unknown specimens. Specimens can be found on the lab
report sheet. Key out the specimens and record the complete classification next to each of
the pictures on the lab report sheet.
Lab activity taken from Dr. Gary Kovnat, Biology 22, Los Angeles Valley College
Taxonomy Lab Report
Name
Specimens of organisms
Organism
Phylum
Arthropoda
Mollusca
2 millimeters, bilaterial symmetry