Download Animals may be characterized by the presence of a coelom

Animals may be characterized by the presence of a coelom, formation
of the mouth, and type of cell cleavage during embryonic development.
LEARNING OBJECTIVE [ edit ]
Detail the ways in which animals can be characterized by features of embryological development
KEY POINTS [ edit ]
Diploblasts contain two germ layers (inner endoderm and outer ectoderm),
while triploblasts contain three germ layers (endoderm, mesoderm, and ectoderm).
The endoderm becomes the digestive and respiratory tracts; the ectoderm becomes the outer
epithelial covering of the body surface and the central nervous system; and the mesoderm
becomes all muscle tissues, connective tissues, and most other organs.
Triploblasts can be further categorized into those without acoelom (acoelomates), those with a
true coelom (eucoelomates), and those with "false" coeloms (pseudocoelomates).
Bilaterally symmetrical, tribloblastic eucoelomates can be divided into protostomes, those
animals that develop a mouth first, and deuterstomes, those animals that develop an anus first
and a mouth second.
In protostomes, the coelom forms when the mesoderm splits through the process of schizocoely,
while in deuterostomes, the coelom forms when the mesoderm pinches off through the process
of enterocoely.
Protostomes undergo spiral cleavage, while deuterostomes undergo radial cleavage.
TERMS [ edit ]
enterocoely
the process by which deuterostome animal embryos develop; the coelom forms from pouches
"pinched" off of the digestive tract
schizocoely
the process by which protostome animal embryos develop; it occurs when a coelom (body cavity)
is formed by splitting the mesodermal embryonic tissue
protostome
any animal in which the mouth is derived first from the embryonic blastopore ("mouth first")
coelomate
any animal possessing a fluid­filled cavity within which the digestive system is suspended.
triploblast
a blastula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm;
formed duringgastrulation of the blastula
acoelomate
any animal without a coelom, or body cavity
diploblast
a blastula in which there are two primary germ layers: the ectoderm and endoderm
deuterostome
Any animal in which the initial pore formed during gastrulation becomes the anus, and the second
pore becomes the mouth
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Animal Characterization Based
on Features of Embryological
Development
Most animal species undergo a separation
of tissues into germ layers during
embryonic development. These germ
layers are formed during gastrulation,
developing into the animal's specialized
tissues and organs. Animals develop
either two or three embryonic germs
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layers . Radially­symmetrical animals are diploblasts, developing two germ layers: an inner
layer (endoderm) and an outer layer (ectoderm). Diploblasts have a non­living layer between
the endoderm and ectoderm. Bilaterally­symmetrical animals are called triploblasts,
developing three tissue layers: an inner layer (endoderm), an outer layer (ectoderm), and a
middle layer (mesoderm).
Germ development in embryogenesis
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
Germ Layers
Each of the three germ layers in a blastula, or developing ball of cells, becomes particular
body tissues and organs. The endoderm gives rise to the stomach, intestines, liver, pancreas,
and the lining of the digestive tract, as well as to the lining of the trachea, bronchi, and lungs
of the respiratory tract. The ectoderm develops into the outer epithelial covering of the body
surface and the central nervous system. The mesoderm, the third germ layer forming
between the endoderm and ectoderm in triploblasts, gives rise to all muscle tissues
(including the cardiac tissues and muscles of the intestines), connective tissues such as the
skeleton and blood cells, and most other visceral organs such as the kidneys and the spleen.
Presence or Absence of a Coelom
Triploblasts can be differentiated into three categories: those that do not develop an internal
body cavity called a coelom (acoelomates), those with a true coelom (eucoelomates), and
those with "false" coeloms (pseudocoelomates) .
Differentiation in triploblasts
Triploblasts may be (a) acoelomates, (b) eucoelomates, or (c) pseudocoelomates. Acoelomates have no
body cavity. Eucoelomates have a body cavity within the mesoderm, called a coelom, which is lined with
mesoderm. Pseudocoelomates also have a body cavity, but it is sandwiched between the endoderm and
mesoderm.
Acoelomates
Triploblasts that do not develop a coelom are called acoelomates: their mesoderm region is
completely filled with tissue. Flatworms in the phylum Platyhelminthes are acoelomates.
Eucoelomates
Eucoelomates (or coelomates) have a true coelom that arises entirely within the mesoderm
germ layer and is lined by an epithelial membrane. This coelomic cavity represents a fluid­
filled space that lies between the visceral organs and the body wall. It houses the digestive
system, kidneys, reproductive organs, and heart, and it contains the circulatory system. The
epithelial membrane also lines the organs within the coelom, connecting and holding them
in position while allowing them some free motion. Annelids, mollusks, arthropods,
echinoderms, and chordates are all eucoelomates. The coelom also provides space for
the diffusion of gases andnutrients, as well as body flexibility and improved animal motility.
The coelom also provides cushioning and shock absorption for the major organ systems,
while allowing organs to move freely for optimal development and placement.
Pseudocoelomates
The pseudocoelomates have a coelom derived partly from mesoderm and partly from
endoderm. Although still functional, these are considered false coeloms. The phylum
Nematoda (roundworms) is an example of a pseudocoelomate.
Embryonic Development of the Mouth
Bilaterally symmetrical, tribloblastic eucoelomates can be further divided into two groups
based on differences in their early embryonic development. These two groups are separated
based on which opening of the digestive cavity develops first: mouth (protostomes) or anus
(deuterostomes) . The word protostome comes from the Greek word meaning "mouth first. "
The protostomes include arthropods, mollusks, and annelids. Deuterostome originates from
the word meaning "mouth second. " Deuterostomes include morecomplex animals such as
chordates, but also some simple animals such as echinoderms.
Early embryonic development in eucoelomates
Eucoelomates can be divided into two groups based on their early embryonic development. In
protostomes, part of the mesoderm separates to form the coelom in a process called schizocoely. In
deuterostomes, the mesoderm pinches off to form the coelom in a process called enterocoely.
Development of the Coelom
The coelom of most protostomes is formed through a process called schizocoely, when a solid
mass of the mesoderm splits apart and forms the hollow opening of the coelom.
Deuterostomes differ in that their coelom forms through a process called enterocoely, when
the mesoderm develops as pouches that are pinched off from the endoderm tissue. These
pouches eventually fuse to form the mesoderm, which then gives rise to the coelom.
Embryonic Cleavage
Protostomes undergo spiral cleavage: the cells of one pole of the embryo are rotated and,
thus, misaligned with respect to the cells of the opposite pole. This spiral cleavage is due to
the oblique angle of the cleavage. Protostomes also undergo determinate cleavage: the
developmental fate of each embryonic cell is pre­determined. Deuterostomes undergo radial
cleavage where the cleavage axes are either parallel or perpendicular to the polar axis,
resulting in the alignment of the cells between the two poles. Unlike protostomes,
deuterostomes undergo indeterminate cleavage: cells remainundifferentiated until a later
developmental stage. This characteristic of deuterostomes is reflected in the existence of
familiar embryonic stem cells, which have the ability to develop into any cell type.