<< Chapter < Page Chapter >> Page >

Body symmetry

Animals may be asymmetrical, radial, or bilateral in form ( [link] ). Asymmetrical animals are animals with no pattern or symmetry; an example of an asymmetrical animal is a sponge ( [link] a ). An organism with radial symmetry    ( [link] b ) has a longitudinal (up-and-down) orientation: Any plane cut along this up–down axis produces roughly mirror-image halves. An example of an organism with radial symmetry is a sea anemone.

Illustration a shows an asymmetrical sponge with a tube-like body and a growth off to one side. Illustration b shows a sea anemone with a tube-like, radially symmetrical body. Tentacles grow from the top of the tube. Three vertical planes arranged 120 degrees apart dissect the body. The half of the body on one side of each plane is a mirror image of the body on the other side. Illustration c shows a goat with a bilaterally symmetrical body. A plane runs from front to back through the middle of the goat, dissecting the body into left and right halves, which are mirror images of each other. The top part of the goat is defined as dorsal, and the bottom part is defined as ventral. The front of the goat is defined as anterior, and the back is defined as posterior.
Animals exhibit different types of body symmetry. The (a) sponge is asymmetrical and has no planes of symmetry, the (b) sea anemone has radial symmetry with multiple planes of symmetry, and the (c) goat has bilateral symmetry with one plane of symmetry.

Bilateral symmetry is illustrated in [link] c using a goat. The goat also has upper and lower sides to it, but they are not symmetrical. A vertical plane cut from front to back separates the animal into roughly mirror-image right and left sides. Animals with bilateral symmetry also have a “head” and “tail” (anterior versus posterior) and a back and underside (dorsal versus ventral).

Concept in action

Watch this video to see a quick sketch of the different types of body symmetry.

Layers of tissues

Most animal species undergo a layering of early tissues during embryonic development. These layers are called germ layers . Each layer develops into a specific set of tissues and organs. Animals develop either two or three embryonic germs layers ( [link] ). The animals that display radial symmetry develop two germ layers, an inner layer (endoderm) and an outer layer (ectoderm). These animals are called diploblasts . Animals with bilateral symmetry develop three germ layers: an inner layer (endoderm), an outer layer (ectoderm), and a middle layer (mesoderm). Animals with three germ layers are called triploblasts .

The left illustration shows the two embryonic germ layers of a diploblast. The inner layer is the endoderm, and the outer layer is the ectoderm. Sandwiched between the endoderm and the ectoderm is a non-living layer. The right illustration shows the three embryonic germ layers of a triploblast. Like the diploblast, the triploblast has an inner endoderm and an outer ectoderm. Sandwiched between these two layers is a living mesoderm.
During embryogenesis, diploblasts develop two embryonic germ layers: an ectoderm and an endoderm. Triploblasts develop a third layer—the mesoderm—between the endoderm and ectoderm.

Presence or absence of a coelom

Triploblasts may develop an internal body cavity derived from mesoderm, called a coelom    (pr. see-LŌM). This epithelial-lined cavity is a space, usually filled with fluid, which lies between the digestive system and the body wall. It houses organs such as the kidneys and spleen, and contains the circulatory system. Triploblasts that do not develop a coelom are called acoelomates , and their mesoderm region is completely filled with tissue, although they have a gut cavity. Examples of acoelomates include the flatworms. Animals with a true coelom are called eucoelomates (or coelomates) ( [link] ). A true coelom arises entirely within the mesoderm germ layer. Animals such as earthworms, snails, insects, starfish, and vertebrates are all eucoelomates. A third group of triploblasts has a body cavity that is derived partly from mesoderm and partly from endoderm tissue. These animals are called pseudocoelomates . Roundworms are examples of pseudocoelomates. New data on the relationships of pseudocoelomates suggest that these phyla are not closely related and so the evolution of the pseudocoelom must have occurred more than once ( [link] ). True coelomates can be further characterized based on features of their early embryological development.

Part a shows the body plan of acoelomates, including flatworms. Acoelomates have a central digestive cavity. Outside this digestive cavity are three tissue layers: an inner endoderm, a central mesoderm, and an outer ectoderm. The photo shows a swimming flatworm, which has the appearance of a frilly black and pink ribbon. Part b shows the body plan of eucoelomates, which include annelids, mollusks, arthropods, echinoderms, and chordates. Eucoelomates have the same tissue layers as acoelomates, but a cavity called a coelom exists within the mesoderm. The coelom is divided into two symmetrical parts that are separated by two spokes of mesoderm. The photo shows a swimming annelid known as a bloodworm. The bloodworm has a tubular body that is tapered at each end. Numerous appendages radiate from either side. Part c shows the body plan of pseudocoelomates, which include roundworms. Like the acoelomates and eucoelomates, the pseudocoelomates have an endoderm, a mesoderm, and an ectoderm. However, in pseudocoelomates, a pseudocoelom separates the endoderm from the mesoderm. The photo shows a roundworm, or nematode, which has a tubular body.
Triploblasts may be acoelomates, eucoelomates, or pseudocoelomates. Eucoelomates have a body cavity within the mesoderm, called a coelom, which is lined with mesoderm tissue. Pseudocoelomates have a similar body cavity, but it is lined with mesoderm and endoderm tissue. (credit a: modification of work by Jan Derk; credit b: modification of work by NOAA; credit c: modification of work by USDA, ARS)

Protostomes and deuterostomes

Bilaterally symmetrical, triploblastic eucoelomates can be divided into two groups based on differences in their early embryonic development. Protostomes include phyla such as arthropods, mollusks, and annelids. Deuterostomes include the chordates and echinoderms. These two groups are named from which opening of the digestive cavity develops first: mouth or anus. The word protostome comes from Greek words meaning “mouth first,” and deuterostome originates from words meaning “mouth second” (in this case, the anus develops first). This difference reflects the fate of a structure called the blastopore ( [link] ), which becomes the mouth in protostomes and the anus in deuterostomes. Other developmental characteristics differ between protostomes and deuterostomes, including the mode of formation of the coelom and the early cell division of the embryo.

The illustration compares the development of protostomes and deuterostomes. In both protostomes and deuterostomes, the gastrula, which resembles a hollow ball of cells, contains an indentation called a blastopore. In protostomes, two circular layers of mesoderm form inside the gastrula, containing the coelom. As the protostome develops, the mesoderm grows and fuses with the gastrula cell layer. The blastopore becomes the mouth, and a second opening forms opposite the mouth, which becomes the anus. In deuterostomes, two groups of gastrula cells in the blastopore grow inward to form the mesoderm. As the deuterostome develops, the mesoderm pinches off and fuses, forming a second body cavity. The body plan of the deuterostome at this stage looks very similar to that of the protostome, but the blastopore becomes the anus, and the second opening becomes the mouth.
Eucoelomates can be divided into two groups, protostomes and deuterostomes, based on their early embryonic development. Two of these differences include the origin of the mouth opening and the way in which the coelom is formed.

Section summary

Animals constitute a diverse kingdom of organisms. Although animals range in complexity from simple sea sponges to human beings, most members share certain features. Animals are eukaryotic, multicellular, heterotrophic organisms that ingest their food and usually develop into motile creatures with a fixed body plan. Most members of the animal kingdom have differentiated tissues of four main classes—nervous, muscular, connective, and epithelial—that are specialized to perform different functions. Most animals reproduce sexually, leading to a developmental sequence that is relatively similar across the animal kingdom.

Organisms in the animal kingdom are classified based on their body morphology and development. True animals are divided into those with radial versus bilateral symmetry. Animals with three germ layers, called triploblasts, are further characterized by the presence or absence of an internal body cavity called a coelom. Animals with a body cavity may be either coelomates or pseudocoelomates, depending on which tissue gives rise to the coelom. Coelomates are further divided into two groups called protostomes and deuterostomes, based on a number of developmental characteristics.

Art connection

[link] Which of the following statements is false?

  1. Eumetazoa have specialized tissues and Parazoa do not.
  2. Both acoelomates and pseudocoelomates have a body cavity.
  3. Chordates are more closely related to echinoderms than to rotifers according to the figure.
  4. Some animals have radial symmetry, and some animals have bilateral symmetry.

[link] B

Got questions? Get instant answers now!

Questions & Answers

what makes golgi body in plants
Abdulkareem Reply
name the membrane of the plants
Abdulkareem
how can turners syndrome be corrected before birth
Balinda
which animal survive from being preyed just because of being humble, slow, and not aggressive
Balinda
During organs transplantation, the organs cannot be taken from just anybody since the graft would be rejected sooner or later due to
Liter Reply
Non-MHC compatibility on the organ and an attack from the patient's immune system.
Eric
what makes golgi body in plants
Abdulkareem
what is integument system
Joy Reply
Cellular respiration
Lucy Reply
what are the characteristics of living things
Ruth Reply
Movement Respiration Nutrition/Feeding Irritability/Sensitivity Growth Excretion Reproduction Deat/Life span
Hashim
What makes children from the same father and mother sometimes don't look alike?
Hashim
identification of problems
Nana Reply
what happens in the process of raising the human arms
Nana
what is biology
Brandi Reply
first step in scientific method
Brandi
In an investigation the pancreatic duct of a mammal was blocked.It was found that the blood sugar regulation remained normal while food digestion was impaired.Explain
Mac Reply
To begin with, obstruction of pancreatic duct will alter the blood sugar level as the juices responsible for glucose regulation will be rendered inconsequential. This will in turn affect the rate of digestion and absorbtion of digested food substances by the Villus .
Muktar
characteristics of algae
OMIME Reply
Algae are eukaryotic organisms. Algae do not have roots and stems. Algae have chlorophyll and helps in carrying out photosynthesis.
Aditi
Cell wall is the rigid layer enclosed by membranes of plants and prokayortic cell, it maintains the shape of the cell and serve as a protective barrier.
chizoba Reply
ECOLOGY: is a branch of biology that studies the interactions among organisms and their biophysical environment, which includes both biotic and abiotic components. 
chizoba
via nutrient cycles and energy flows. For instance, the energy from the sun is captured by plants through photosynthesis. Photosynthesis is a biological process through which plants manufacture their own food with the aid of light from the sun and frc sources (e.g. cabon dioxide and water)
chizoba
What is cell wall
Taiwo Reply
cell wall is the outemost rigid covering of the plants ,that provides protection to the plants.
Aditi
what is integument system
Joy
The integument system is your skin and the largest system in the body.
Eric
what is protoplast?
Liter
The cytoplasm is inside of the cell, minus the nucleus. The protoplasm is the inside of the cell and the nucleus.
Eric
Out line of cell
Salma
what is ecology, ecosystem?
Nkeng Reply
what is digestive system
Lucky Reply
digestive system is the human syman system that icludes esopuges stomach o braking down of food in to useful substance to our body
samrawit
definition of biology basics
Ritu Reply
the potential energy of a molecule can be inquired by their number of?
Jesus Reply
note on the structure of DNA
bernadette
The chemical potential of a species in a mixture is defined as the rate of change of free energy of a thermodynamic system with respect to the change in the number of atoms or molecules of the species that are added to the system. 
Eric

Get the best Concepts of biology course in your pocket!





Source:  OpenStax, Concepts of biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11487/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Concepts of biology' conversation and receive update notifications?

Ask