27.2 Features Used to Classify Animals

• The anterior-posterior axis of the animal's body is reflected by the order of the genes.
• Increased animal body complexity is due to the fact that Hox genes have undergone at least two and perhaps as many as four duplication events during animal evolution, with the additional genes allowing for more complex body types to evolve.
• All animals have at least one set of Hox genes.

• The course of embryonic development in animals is determined by Hox genes.
• The genes have been duplicated into four clusters on different chromosomes.
• At certain stages of development, certain genes are expressed in certain body segments.
• The Hox genes are found in mice and humans.
• The orange, pink, blue, and green shading shows how Hox gene expression occurs in the same body segments in both the mouse and the human.
• Some Hox genes are missing in some chromosomal sets, while at least one copy of each Hox gene is present in humans.

• Two of the five clades within the animal kingdom do not have Hox genes.
• The Cnidaria have a number of Hox genes, but the Ctenophora do not.
• The omission of Hox genes from the ctenophores has led to the idea that they might bebasal animals.
• The Placozoa has at least one Hox gene.
• The inclusion of the three groups in a "Parahoxozoa" clade is due to the presence of a Hox gene in the Placozoa.
• The reclassification of the Animal Kingdom is still tentative and requires more study.

• There are exceptions to most "rules" governing animal classification, but scientists have developed a classification scheme that categorizes all members of the animal kingdom.
• The body plan and developmental pathway are the two main characteristics of animals.
• The major feature of the body plan is how the body parts are distributed.
• Symmetrical animals can be divided into equal parts.
• The number of germ tissue layers formed during development, the origin of the mouth and anus, and the presence or absence of an internal body cavity are some of the features of embryological development.

• The tree of animals is based on evidence.
• The "Parahoxozoa" (Placozoa + Eumetazoa) is related to the "Ctenophora" because of the absence of Hox genes.

• True animals can be divided into three groups based on the type of symmetry of their body plan.
• The Parazoa and Placozoa are modern clades.
• The Cnidaria has radial or biradial symmetry.
• Bilateral symmetry can be seen in the Bilateria, the largest of the clades.

• Animals have top and bottom surfaces, but no left and right sides or front or back.
• The side with a mouth and the side without a mouth are both mirror images of the same animal.
• There is a form of symmetry that marks the body plans of many animals.
• These sea creatures are able to experience the environment equally from all directions because of radial symmetry.
• Bilaterally symmetrical animals, like butterflies, have only a single plane along which the body can be divided into equal parts.
• The Ctenophora are considered to have rotational symmetry because they are divided into two copies of the same half by the oral axis.

• The sponge is asymmetrical.
• The (b) jellyfish and (c) anemone are both symmetrical.
• The ctenophore Beroe is shown swimming open-mouthed.

• Animals with bilateral symmetry have a head and tail.
• All Eumetazoa are symmetrical.
• The evolution of bilateral symmetry that allowed for the formation of anterior and posterior 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, bilateral symmetry allows for streamlined and directed motion.
• This form of symmetry promoted active and controlled mobility and increased sophistication of resource-seeking and predator-prey relationships.

• The human body can be divided into several planes.

• As adults, the sea stars, sand dollars, and sea urchins display modified radial symmetry, but as they mature, they exhibit bilateral symmetry.
• They are classified as bilaterally symmetrical because they evolved from symmetrical animals.

• There are different types of body symmetry shown in the video.

• Animals separation of tissues into germ layers is done during embryo development.
• Each germ layer typically gives rise to specific types of embryonic tissues and organs.
• Animals have either two or three germ layers.
• The animals that display symmetry develop two germ layers, an inner layer and an outer layer.
• There is little information about development in Placozoa, although the four clades considered to be diploblastic have different levels of complexity.
• Animals with bilateral symmetry develop three tissue layers: an inner layer, an outer layer, and a middle layer.

• The embryo has two germ layers: an ectoderm and an mesendoderm.
• The third layer of triploblasts is the mesendoderm.

• Animals with only radial symmetry are diploblasts.

• Animals with bilateral symmetry are triploblasts.

• The three germ layers are programmed to give rise to specific body tissues and organs.
• The lining of the digestive tract is one of the structures that the endoderm gives rise to.
• The central nervous system is one of the structures that develops into the ectoderm.
• The third germ layer is the mesoderm.
• The germ layer gives rise to all specialized muscle tissues, including the cardiac tissues and muscles of the intestines, as well as the skeleton and blood cells.
• Epitheliomuscular cells, which serve as a covering as well as contractile cells, may be found in diploblastic animals.

• The fluid lies between the body wall and the visceral organs.
• It contains many organs such as the heart, lungs, and reproductive systems, as well as the major arteries and veins of the circulatory system.
• In mammals, the body is divided into two parts, the thoracic and abdominal.
• The pericardial and pleural cavity are located in the thoracic cavity and provide space for the lungs to expand during breathing.
• There are many functional advantages associated with the evolution of the coelom.
• The coelom provides shock absorption for the major organ systems.
• Organs housed within the coelom can grow and move freely, which promotes optimal organ development and placement.
• The coelom gives space for the movement of gases and fluids, as well as body flexibility.

• Flatworms are examples of acoelomates.
• In such cases, a true coelom arises entirely within the germ layer and is lined by the epithelium.
• The organs within the coelom can be connected and held in position with the help of this membrane.
• Eucoelomates include Annelids, mollusks, arthropods, and echinoderms.

• The roundworms are an example of a pseudocoelomate.
• The early embryological development of true coelomates can be further characterized.

• Triploblasts can be acoelomates, eucoelomates, or pseudocoelomates.
• Acoelomates don't have a body part.
• Both the gut and the body wall are lined with the same substance, called the coelom.
• The body wall of pseudocoelomates is lined with mesoderm.
• Bilaterally symmetrical, tribloblastic eucoelomates can be further divided into two groups based on the origin of the mouth.
• The blastopore is the opening that connects the gut to the embryo.
• The mouth at one end of the gut and anus at the other are openings.
• There are arthropods, mollusks, and annelids.
• More complex animals such as echinoderms are included in the Deuterostomes.
• The theory of a relationship between the location of the blastopore and the formation of the mouth has been challenged by recent evidence.
• These details of mouth and anus formation are indicative of differences in the organization of deuterostome embryos.

• The method of coelom formation is one of the differences between deuterostomes and Protostomes.
• The formation of the mesoderm differs from the formation of the body cavity.
• These organisms have specific blastomeres that migrate into the embryo and form two clumps of mesodermal tissue.
• The hollow opening of the coelom is formed within each clump.
• The mesoderm is pinched off from the endoderm tissue.
• The space between the gut and the body wall is eventually filled by these pouches.

• During cleavage, there is a difference in the organization of embryos.

• The angle of cleavage is related to the two poles of the embryo.

• Eucoelomates can be divided into two groups based on their early development.

• The mouth and body are formed by splitting the body mass during the process of schizocoely.
• The mouth forms at a site opposite the blastopore end of the embryo and the mesoderm pinches off to form the coelom.

• The fate of the resulting blastomeres is related to the types of cleavage.
• At this early stage, the fate of each cell is already determined.
• A cell can't develop into any other cell type than its original destination.
• The removal of a blastomere from an embryo can result in missing structures.
• The loss of embryonic structures does not result from the removal of individual blastomeres.
• Twins can be produced from blastomeres that have been separated from the original mass of blastomere cells.
• If some blastomeres are damaged during embryogenesis, adjacent cells are able to compensate for the missing cells, and the embryo is not damaged.
• Unanswered cells are referred to.
• The ability to develop into any cell type until their fate is programmed at a later stage is reflected in the existence of familiar embryonic stem cells.

• The first step in the classification of animals is to examine the animal's body.
• The coelom is a structure used in classification of animals.
• Only triploblastic animals can have body cavities.
• The Bilateria contains the body cavities.
• The gut is separated from the body wall in other animal clades.
• The body is important for two reasons.
• The organs are protected from shock and compression.
• The lining of the body's internal organs can be strengthened by the development of muscle, blood vessels, and other tissues in triploblastic embryos.

• Animals that do not have a coelom are called acoelomates.
• The flatworms are the major acoelomate group in the Bilateria.