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27.3 Animal Phylogeny
There are two layers of muscle under the skin, but there is no muscle around the gut.
Flatworms rely on passive diffusion for their nutrition.
There is a body wall between the gut and the body wall in pseudocoelomates.
The mesoderm in these animals doesn't develop cavities.
The rotifers and nematodes are pseudocoelomates.
Eucoelomates are animals that have a true coelom, and include all mammals, birds, arthropods, and echinoderms.
During embryogenesis, the coelom develops.
The molluscs, annelids, and arthropods are schizocoels, in which the mesoderm splits to form the body cavity.
The body cavity is formed by the buds that separate from the gut.
mammals have a body that is divided into two parts, the abdominal and the thoracic.
The pseudocoelomates may have lost their ability to form a complete coelom because of genetic changes.
The formation of the coelom has had a large evolutionary impact on the various species of the animal kingdom.
Biologists try to understand the evolutionary history of members of the animal kingdom and all of life.
The branching sequence of evolution is the focus of the study.
The animal kingdom is divided into 35 to 40 phyla.
Pequeas are hypotheses about which species have evolved from which ancestors.
Until recently, only the fossil record was used to determine relationships among animals.
Scientific understanding of the distinctions between the different parts of the body gave much of the knowledge.
This information can be misleading if used alone.
Animals may have similar characteristics, but their evolution may be different.
Modern phylogenetics is informed by genetic and molecular analyses, in addition to traditional fossil data.
With a growing understanding of genetics, the animal evolutionary tree has changed substantially and continues to change as new DNA and RNA analyses are performed on additional animal species.
There are similarities between the feeding cells of sponges and choanoflagellate protists.
The choanocyte flagella draws water through the sponge so that it can be removed.
Bilateria and Radiata are the clades of Eumetazoa, which are radially symmetrical animals and bilaterally symmetrical animals.
The cnidarians and ctenophores are animals with true radial, biradial, or rotational symmetry.
The Bilateria clade has all the other Eumetazoa.
There are two distinct clades of Protostomes, including ecdysozoans and lophotrochozoans, in Figure 27.13a,b.
Some lophotrochozoan phyla are characterized by a feeding structure called a lophophore, which is the shorter term, "lopho-trochozoa".
The lophophore is a feeding structure.
Click to learn more about the organisms and their evolutionary relationships.
Evolutionary biologists are debating and refining the groupings.
New evidence changes the relationships described in a tree diagram.
The following video shows how biologists use genetic data to determine relationships among organisms.
The modern phylogenetic animal tree has been greatly modified by the use of genes.
Some of the data come from a variety of sources.
The modern tree has many evolutionary relationships that have only recently been determined.
A group of animals called lophophorates, which included brachiopods and bryozoans, were long thought to be primitive.
These animals are related to annelids and mollusks in many ways.
The discovery allowed for the distinction of the clade.
The placement of the Platyhelminthes is problematic because of some differences within the lophotrochozoan group.
Some scientists think that the phyla Platyhelminthes should be part of their own clade.
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