<< Chapter < Page Chapter >> Page >

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

Questions & Answers

Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Which of the following is best at showing the life expandency of an individual within a a population
Daniel Reply
perianth is present in which gymnosperms ?
DebaXish Reply
perianth is present in which gymnos4perms ?
DebaXish Reply

Get the best Algebra and trigonometry course in your pocket!

Source:  OpenStax, Bi 101 for lbcc ilearn campus. OpenStax CNX. Nov 28, 2013 Download for free at http://legacy.cnx.org/content/col11593/1.1
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Bi 101 for lbcc ilearn campus' conversation and receive update notifications?