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By the end of this section, you will be able to:
  • Explain the differences in animal body plans that support basic animal classification
  • Compare and contrast the embryonic development of protostomes and deuterostomes

Scientists have developed a classification scheme that categorizes all members of the animal kingdom, although there are exceptions to most “rules” governing animal classification ( [link] ). Animals are primarily classified according to morphological and developmental characteristics, such as a body plan. One of the most prominent features of the body plan of true animals is that they are morphologically symmetrical. This means that their distribution of body parts is balanced along an axis. Additional characteristics include the number of tissue layers formed during development, the presence or absence of an internal body cavity, and other features of embryological development, such as the origin of the mouth and anus.

Art connection

The phylogenetic tree of metazoans, or animals, branches into parazoans with no tissues and eumetazoans with specialized tissues. Parazoans include Porifera, or sponges. Eumetazoans branch into Radiata, diploblastic animals with radial symmetry, and Bilateria, triploblastic animals with bilateral symmetry. Radiata includes cnidarians and ctenophores (comb jellies). Bilateria branches into Acoela, which have no body cavity, and Protostomia and Deuterostomia, which possess a body cavity. Deuterostomes include chordates and echinoderms. Protostomia branches into Lophotrochozoa and Ecdysozoa. Ecdysozoa includes arthropods and nematodes, or roundworms. Lophotrochozoa includes Mollusca, Annelida, Brachopoda, Ectoprocta, Rotifera, and Platyhelminthes.
The phylogenetic tree of animals is based on morphological, fossil, and genetic evidence.

Which of the following statements is false?

  1. Eumetazoans have specialized tissues and parazoans don’t.
  2. Lophotrochozoa and Ecdysozoa are both Bilataria.
  3. Acoela and Cnidaria both possess radial symmetry.
  4. Arthropods are more closely related to nematodes than they are to annelids.

Animal characterization based on body symmetry

At a very basic level of classification, true animals can be largely divided into three groups based on the type of symmetry of their body plan: radially symmetrical, bilaterally symmetrical, and asymmetrical. Asymmetry is a unique feature of Parazoa ( [link] a ). Only a few animal groups display radial symmetry. All types of symmetry are well suited to meet the unique demands of a particular animal’s lifestyle.

Radial symmetry is the arrangement of body parts around a central axis, as is seen in a drinking glass or pie. It results in animals having top and bottom surfaces but no left and right sides, or front or back. The two halves of a radially symmetrical animal may be described as the side with a mouth or “oral side,” and the side without a mouth (the “aboral side”). This form of symmetry marks the body plans of animals in the phyla Ctenophora and Cnidaria, including jellyfish and adult sea anemones ( [link] bc ). Radial symmetry equips these sea creatures (which may be sedentary or only capable of slow movement or floating) to experience the environment equally from all directions.

Part a shows several sponges, which form irregular, bumpy blobs on the sea floor. Part b shows a jellyfish with long, slender tentacles, radiating from a flexible, disc-shaped body. Part c shows an anemone sitting on the sea floor with thick tentacles, radiating up from a cup-shaped body. Part d shows a black butterfly with two symmetrical wings.
The (a) sponge is asymmetrical. The (b) jellyfish and (c) anemone are radially symmetrical, and the (d) butterfly is bilaterally symmetrical. (credit a: modification of work by Andrew Turner; credit b: modification of work by Robert Freiburger; credit c: modification of work by Samuel Chow; credit d: modification of work by Cory Zanker)

Bilateral symmetry involves the division of the animal through a sagittal plane, resulting in two mirror image, right and left halves, such as those of a butterfly ( [link] d ), crab, or human body. Animals with bilateral symmetry have a “head” and “tail” (anterior vs. posterior), front and back (dorsal vs. ventral), and right and left sides ( [link] ). All true animals except those with radial symmetry are bilaterally symmetrical. The evolution of bilateral symmetry that allowed for the formation of anterior and posterior (head and tail) ends promoted a phenomenon called cephalization, which refers to the collection of an organized nervous system at the animal’s anterior end. In contrast to radial symmetry, which is best suited for stationary or limited-motion lifestyles, bilateral symmetry allows for streamlined and directional motion. In evolutionary terms, this simple form of symmetry promoted active mobility and increased sophistication of resource-seeking and predator-prey relationships.

Questions & Answers

name the resources to be conserved
Oreva Reply
define natural resources
Oreva
name the agencies responsible for the conservation of natural resources
Oreva
land,water,forests,
Kiiza
nema,uwa
Kiiza
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Peace
land ,water
Erika
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garnhial
food, fish, livelihoods,forest, land and water
garnhial
explain the importance of carbon dioxide in the body
Kiiza Reply
how does that works
SAMUEL
diagram of Prokaryotic cells
Magreth Reply
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Yazi
ʜɪ
Malikie
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Malikie
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Yazi
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Yazi
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Malikie
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Joelia
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Hannalyn
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Paul
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Joelia
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Cosmo
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Kisa
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Ellie
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Ken
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Sulaiman
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Peace
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Ellie
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Ken
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James
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Reine
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James
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Peace
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Peace
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Tafadzwa
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James
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Rons
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Israel
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Reine
what is a cell of a bacteria called
Sahfe Reply
what is genotype
James
genotype: this is the combination of alleles an organism has for a given characteristic..
Cosmo
what is maiotic
Gabriel Reply
what is sexual reproduction
Gabriel
what are the effects of concentration gradient in the uptake of water by plant's
Harleen Reply
ls it ok if you have sex during pregnancy
Kags Reply
no
Bernard
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Yazi
Yes
Babie
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Cabdi
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kf
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REAL
Who are you?
Babie
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Lamin
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Lamin
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Lamin
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Lamin
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Tony
an assignment for you pls
Lamin
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Nyakato
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george
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Lamin
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george
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Cosmo
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Cosmo
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Joelia
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Lamin
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Joelia
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Lamin
yes
James
What is the meaning of organ
Ronald Reply
tissue combined to form organ
zameer
join
Elishs
orga- these are tissues join together to perform a specific functions.
Elishs
then what are tissues
Kags
group of cells working together to perform a particular function
Harleen
tissue are group of cells put together to perform a certain goal
Bernard
What will happen when read blood cell placed in 0.5 % Nacl solution
zameer Reply
what is biology
Tariro Reply
biology is the study of living organisms
Bernard
examply is the study of.
Nantamu
biology is the study of life.
Nantamu
what is biology
Francis Reply
kidny with large and and many glomeruli are characcteristic feathure of ?
zameer
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Jesus Reply
I can't
Francis
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Francis
A cell is the basic functional unit of life.
Jael
what is a cell ?
Rabecca Reply
A cell is the basic functional unit of all organisms
Glandwell
A cell is a basic functional and structural unit of life.
Mavis
please, emphasize more on the coronary circuit
Toyyiba Reply
define the photosynthesis
Sabrinah Reply
list the type of protein
Sabrinah
sugar.glucose
Tamirat
renin,pepsin
Nyakato
peptides
Nyakato
what is photosynthesis
Francis

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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