What Features Of Animals Separate Them From Their Nearest Relative The Choanoflagellates?

Introduction to Animal Variety

138 Features Used to Allocate Animals

Learning Objectives

By the end of this section, you volition exist able to practice the following:

• Explicate the differences in animate being trunk plans that support basic beast classification
• Compare and contrast the embryonic evolution of protostomes and deuterostomes

<!–<anchor id="ch27_module_2″/>–>Scientists accept adult a classification scheme that categorizes all members of the animal kingdom, although at that place are exceptions to most "rules" governing animal classification ((Figure)). Animals accept been traditionally classified according to two characteristics: torso programme and developmental pathway. The major feature of the torso plan is its symmetry: how the body parts are distributed along the major body axis. Symmetrical animals tin can be divided into roughly equivalent halves along at to the lowest degree i axis. Developmental characteristics include the number of germ tissue layers formed during development, the origin of the mouth and anus, the presence or absence of an internal body cavity, and other features of embryological evolution, such as larval types or whether or not periods of growth are interspersed with molting.

Visual Connectedness

Animal phylogeny. The phylogenetic tree of animals is based on morphological, fossil, and genetic evidence. The Ctenophora and Porifera are both considered to exist basal because of the absence of Hox genes in this group, but how they are related to the "Parahoxozoa" (Placozoa + Eumetazoa) or to each other, continues to be a matter of argue.

Which of the following statements is simulated?

1. Eumetazoans have specialized tissues and parazoans don't.
2. Lophotrochozoa and Ecdysozoa are both Bilataria.
3. Acoela and Cnidaria both possess radial symmetry.
4. Arthropods are more closely related to nematodes than they are to annelids.

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Animal Label Based on Body Symmetry

At a very bones level of classification, true animals can be largely divided into 3 groups based on the type of symmetry of their body plan: radially symmetrical, bilaterally symmetrical, and asymmetrical. Asymmetry is seen in two modern clades, the Parazoa ((Figure)a) and Placozoa. (Although we should annotation that the ancestral fossils of the Parazoa apparently exhibited bilateral symmetry.) 1 clade, the Cnidaria ((Figure)b,c), exhibits radial or biradial symmetry: Ctenophores have rotational symmetry ((Figure)e). Bilateral symmetry is seen in the largest of the clades, the Bilateria ((Figure)d); still the Echinodermata are bilateral every bit larvae and metamorphose secondarily into radial adults. All types of symmetry are well suited to encounter the unique demands of a particular animate being's lifestyle.

Radial symmetry is the arrangement of body parts around a central centrality, as is seen in a bicycle wheel or pie. It results in animals having top and bottom surfaces simply no left and correct sides, nor front or dorsum. If a radially symmetrical brute is divided in any direction along the oral/aboral centrality (the side with a mouth is "oral side," and the side without a oral cavity is the "aboral side"), the two halves volition be mirror images. This form of symmetry marks the body plans of many animals in the phyla Cnidaria, including jellyfish and developed sea anemones ((Effigy)b, c). Radial symmetry equips these sea creatures (which may be sedentary or only capable of boring movement or floating) to feel the surround every bit from all directions. Bilaterally symmetrical animals, like butterflies ((Figure)d) have only a single aeroplane along which the body tin can be divided into equivalent halves. The Ctenophora ((Figure)e), although they look similar to jellyfish, are considered to take rotational symmetry rather than radial or biradial symmetry because division of the body into two halves along the oral/aboral centrality divides them into two copies of the same half, with ane copy rotated 180^o, rather than ii mirror images.

Symmetry in animals. The (a) sponge is asymmetrical. The (b) jellyfish and (c) anemone are radially symmetrical, the (d) butterfly is bilaterally symmetrical. Rotational symmetry (e) is seen in the ctenophore Beroe, shown swimming open-mouthed. (credit a: modification of work by Andrew Turner; credit b: modification of work past Robert Freiburger; credit c: modification of work past Samuel Chow; credit d: modification of work by Cory Zanker; credit e: modification of piece of work by NOAA)

Bilateral symmetry involves the division of the animal through a midsagittal plane, resulting in two superficially mirror images, right and left halves, such equally those of a butterfly ((Figure)d), crab, or human torso. Animals with bilateral symmetry have a "head" and "tail" (anterior vs. posterior), front and back (dorsal vs. ventral), and right and left sides ((Figure)). All Eumetazoa except those with secondary radial symmetry are bilaterally symmetrical. The evolution of bilateral symmetry that allowed for the formation of inductive and posterior (head and tail) ends promoted a phenomenon chosen cephalization, which refers to the collection of an organized nervous system at the fauna's anterior finish. In contrast to radial symmetry, which is all-time suited for stationary or limited-motion lifestyles, bilateral symmetry allows for streamlined and directional motion. In evolutionary terms, this uncomplicated form of symmetry promoted agile and controlled directional mobility and increased sophistication of resources-seeking and predator-casualty relationships.

Bilateral symmetry. The bilaterally symmetrical human trunk tin be divided by several planes.

Animals in the phylum Echinodermata (such every bit sea stars, sand dollars, and sea urchins) display modified radial symmetry as adults, but every bit nosotros have noted, their larval stages (such as the bipinnaria) initially exhibit bilateral symmetry until they metamorphose in animals with radial symmetry (this is termed secondary radial symmetry). Echinoderms evolved from bilaterally symmetrical animals; thus, they are classified every bit bilaterally symmetrical.

Link to Learning

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

Animal Characterization Based on Features of Embryological Development

Most animal species undergo a separation of tissues into germ layers during embryonic development. Recall that these germ layers are formed during gastrulation, and that each germ layer typically gives ascension to specific types of embryonic tissues and organs. Animals develop either two or three embryonic germ layers ((Figure)). The animals that display radial, biradial, or rotational symmetry develop two germ layers, an inner layer (endoderm or mesendoderm) and an outer layer (ectoderm). These animals are called diploblasts, and accept a nonliving centre layer betwixt the endoderm and ectoderm (although individual cells may be distributed through this middle layer, at that place is no coherent third layer of tissue). The four clades considered to be diploblastic have different levels of complexity and dissimilar developmental pathways, although there is picayune data most development in Placozoa. More complex animals (usually those with bilateral symmetry) develop iii tissue layers: an inner layer (endoderm), an outer layer (ectoderm), and a middle layer (mesoderm). Animals with three tissue layers are called triploblasts.

Visual Connection

Diploblastic and triploblastic embryos. During embryogenesis, diploblasts develop ii embryonic germ layers: an ectoderm and an endoderm or mesendoderm. Triploblasts develop a third layer—the mesoderm—which arises from mesendoderm and resides between the endoderm and ectoderm.

Which of the following statements about diploblasts and triploblasts is fake?

1. Animals that display just radial symmetry during their lifespans are diploblasts.
2. Animals that display bilateral symmetry are triploblasts.
3. The endoderm gives ascension to the lining of the digestive tract and the respiratory tract.
4. The mesoderm gives rise to the central nervous organisation.

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Each of the three germ layers is programmed to give rise to specific body tissues and organs, although there are variations on these themes. Generally speaking, the endoderm gives rise to the lining of the digestive tract (including the tum, intestines, liver, and pancreas), every bit well as to the lining of the trachea, bronchi, and lungs of the respiratory tract, along with a few other structures. The ectoderm develops into the outer epithelial covering of the trunk surface, the central nervous system, and a few other structures. The mesoderm is the third germ layer; it forms betwixt the endoderm and ectoderm in triploblasts. This germ layer gives rise to all specialized muscle tissues (including the cardiac tissues and muscles of the intestines), connective tissues such as the skeleton and claret cells, and nigh other visceral organs such equally the kidneys and the spleen. Diploblastic animals may accept jail cell types that serve multiple functions, such equally epitheliomuscular cells, which serve as a covering too equally contractile cells.

Presence or Absenteeism of a Coelom

Further subdivision of animals with three germ layers (triploblasts) results in the separation of animals that may develop an internal body cavity derived from mesoderm, called a coelom, and those that do not. This epithelial jail cell-lined coelomic crenel, usually filled with fluid, lies between the visceral organs and the body wall. It houses many organs such every bit the digestive, urinary, and reproductive systems, the heart and lungs, and also contains the major arteries and veins of the circulatory arrangement. In mammals, the torso cavity is divided into the thoracic cavity, which houses the heart and lungs, and the abdominal cavity, which houses the digestive organs. In the thoracic cavity further subdivision produces the pleural cavity, which provides infinite for the lungs to expand during breathing, and the pericardial cavity, which provides room for movements of the heart. The evolution of the coelom is associated with many functional advantages. For instance, the coelom provides cushioning and shock absorption for the major organ systems that it encloses. In addition, organs housed within the coelom tin abound and motility freely, which promotes optimal organ development and placement. The coelom likewise provides space for the improvidence of gases and nutrients, as well as body flexibility, promoting improved fauna move.

Triploblasts that exercise not develop a coelom are chosen acoelomates, and their mesoderm region is completely filled with tissue, although they do still have a gut crenel. Examples of acoelomates include animals in the phylum Platyhelminthes, also known as flatworms. Animals with a true coelom are called eucoelomates (or coelomates) ((Effigy)). In such cases, a truthful coelom arises entirely within the mesoderm germ layer and is lined by an epithelial membrane. This membrane too lines the organs within the coelom, connecting and holding them in position while allowing them some liberty of movement. Annelids, mollusks, arthropods, echinoderms, and chordates are all eucoelomates. A third group of triploblasts has a slightly different coelom lined partly by mesoderm and partly by endoderm. Although all the same functionally a coelom, these are considered "imitation" coeloms, and and so we call these animals pseudocoelomates. The phylum Nematoda (roundworms) is an example of a pseudocoelomate. Truthful coelomates can be farther characterized based on other features of their early embryological development.

Body cavities. Triploblasts may exist (a) acoelomates, (b) eucoelomates, or (c) pseudocoelomates. Acoelomates have no body cavity. Eucoelomates have a body crenel within the mesoderm, chosen a coelom, in which both the gut and the body wall are lined with mesoderm. Pseudocoelomates also take a body cavity, just only the body wall is lined with mesoderm. (credit a: modification of work by Jan Derk; credit b: modification of work past NOAA; credit c: modification of work by USDA, ARS)

Embryonic Development of the Mouth

Bilaterally symmetrical, tribloblastic eucoelomates tin exist farther divided into ii groups based on differences in the origin of the mouth. When the archaic gut forms, the opening that beginning connects the gut cavity to the outside of the embryo is called the blastopore. Most animals accept openings at both ends of the gut: mouth at one end and anus at the other. 1 of these openings volition develop at or almost the site of the blastopore. In Protostomes ("mouth commencement"), the mouth develops at the blastopore ((Figure)). In Deuterostomes ("oral cavity second"), the mouth develops at the other end of the gut ((Figure)) and the anus develops at the site of the blastopore. Protostomes include arthropods, mollusks, and annelids. Deuterostomes include more than complex animals such as chordates but also some "simple" animals such as echinoderms. Recent prove has challenged this simple view of the relationship between the location of the blastopore and the formation of the oral fissure, all the same, and the theory remains under contend. Nevertheless, these details of mouth and anus formation reflect general differences in the organisation of protostome and deuterostome embryos, which are also expressed in other developmental features.

Ane of these differences between protostomes and deuterostomes is the method of coelom germination, beginning from the gastrula stage. Since trunk cavity formation tends to accompany the germination of the mesoderm, the mesoderm of protostomes and deuterostomes forms differently. The coelom of most protostomes is formed through a process chosen schizocoely. The mesoderm in these organisms is unremarkably the product of specific blastomeres, which migrate into the interior of the embryo and form ii clumps of mesodermal tissue. Inside each clump, cavities develop and merge to form the hollow opening of the coelom. Deuterostomes differ in that their coelom forms through a process called enterocoely. Here, the mesoderm develops as pouches that are pinched off from the endoderm tissue. These pouches eventually fuse and aggrandize to make full the infinite between the gut and the trunk wall, giving rise to the coelom.

Another difference in organization of protostome and deuterostome embryos is expressed during cleavage. Protostomes undergo spiral cleavage, significant that the cells of one pole of the embryo are rotated, and thus misaligned, with respect to the cells of the opposite pole. This is due to the oblique bending of cleavage relative to the two poles of the embryo. Deuterostomes undergo radial cleavage, where the cleavage axes are either parallel or perpendicular to the polar axis, resulting in the parallel (upwardly-and-down) alignment of the cells between the two poles.

Protostomes and deuterostomes. Eucoelomates can be divided into 2 groups based on their early embryonic evolution. In protostomes, the mouth forms at or near the site of the blastopore and the trunk cavity forms past splitting the mesodermal mass during the procedure of schizocoely. In deuterostomes, the oral cavity forms at a site opposite the blastopore end of the embryo and the mesoderm pinches off to form the coelom during the process of enterocoely.

A 2d distinction between the types of cleavage in protostomes and deuterostomes relates to the fate of the resultant blastomeres (cells produced by cleavage). In add-on to spiral cleavage, protostomes besides undergo determinate cleavage. This means that even at this early stage, the developmental fate of each embryonic cell is already determined. A given cell does not have the ability to develop into whatever prison cell type other than its original destination. Removal of a blastomere from an embryo with determinate cleavage tin can outcome in missing structures, and embryos that fail to develop. In contrast, deuterostomes undergo indeterminate cleavage, in which cells are non notwithstanding fully committed at this early on stage to develop into specific prison cell types. Removal of private blastomeres from these embryos does not result in the loss of embryonic structures. In fact, twins (clones) tin exist produced as a outcome from blastomeres that have been separated from the original mass of blastomere cells. Unlike protostomes, even so, if some blastomeres are damaged during embryogenesis, side by side cells are able to recoup for the missing cells, and the embryo is not damaged. These cells are referred to as undetermined cells. This characteristic of deuterostomes is reflected in the existence of familiar embryonic stem cells, which have the power to develop into whatsoever prison cell type until their fate is programmed at a later developmental phase.

Evolution Connection

The Evolution of the CoelomOne of the offset steps in the classification of animals is to examine the animate being'south torso. One construction that is used in classification of animals is the trunk crenel or coelom. The body cavity develops within the mesoderm, and then only triploblastic animals tin take torso cavities. Therefore body cavities are institute merely within the Bilateria. In other animal clades, the gut is either close to the body wall or separated from it by a jelly-similar textile. The trunk crenel is important for two reasons. Fluid within the torso cavity protects the organs from daze and compression. In addition, since in triploblastic embryos, virtually muscle, connective tissue, and blood vessels develop from mesoderm, these tissues developing within the lining of the body cavity tin reinforce the gut and trunk wall, aid in motion, and efficiently broadcast nutrients.

To recap what we have discussed above, animals that do not have a coelom are called acoelomates. The major acoelomate group in the Bilateria is the flatworms, including both gratis-living and parasitic forms such every bit tapeworms. In these animals, mesenchyme fills the infinite betwixt the gut and the body wall. Although two layers of muscle are found just under the epidermis, at that place is no muscle or other mesodermal tissue around the gut. Flatworms rely on passive diffusion for food transport across their body.

In pseudocoelomates, there is a torso cavity between the gut and the body wall, but merely the body wall has mesodermal tissue. In these animals, the mesoderm forms, but does not develop cavities within it. Major pseudocoelomate phyla are the rotifers and nematodes. Animals that have a true coelom are called eucoelomates; all vertebrates, as well as molluscs, annelids, arthropods, and echinoderms, are eucoelomates. The coelom develops inside the mesoderm during embryogenesis. Of the major bilaterian phyla, the molluscs, annelids, and arthropods are schizocoels, in which the mesoderm splits to form the body cavity, while the echinoderms and chordates are enterocoels, in which the mesoderm forms as 2 or more buds off of the gut. These buds separate from the gut and coalesce to form the body cavity. In the vertebrates, mammals accept a subdivided torso cavity, with the thoracic crenel separated from the abdominal cavity. The pseudocoelomates may accept had eucoelomate ancestors and may have lost their power to grade a complete coelom through genetic mutations. Thus, this step in early embryogenesis—the formation of the coelom—has had a large evolutionary affect on the various species of the animal kingdom.

Section Summary

Organisms in the animate being kingdom are classified based on their torso morphology, their developmental pathways, and their genetic affinities. The relationships between the Eumetazoa and more basal clades (Ctenophora, Porifera, and Placozoa) are still existence debated. The Eumetazoa ("true animals") are divided into those with radial versus bilateral symmetry. More often than not, the simpler and ofttimes nonmotile animals display radial symmetry, which allows them to explore their environment in all directions. Animals with radial symmetry are also generally characterized by the development of ii embryological germ layers, the endoderm and ectoderm, whereas animals with bilateral symmetry are by and large characterized by the development of a third embryologic germ layer, the mesoderm. Animals with three germ layers, called triploblasts, are further characterized by the presence or absenteeism of an internal body crenel called a coelom. The presence of a coelom affords many advantages, and animals with a coelom may exist termed true coelomates or pseudocoelomates, depending the extent to which mesoderm lines the torso cavity. Coelomates are farther divided into one of two groups called protostomes and deuterostomes, based on a number of developmental characteristics, including differences in zygote cleavage, the method of coelom germination, and the rigidity of the developmental fate of blastomeres.

Visual Connection Questions

(Figure) Which of the following statements is false?

1. Eumetazoans accept specialized tissues and parazoans don't.
2. Lophotrochozoa and Ecdysozoa are both Bilataria.
3. Acoela and Cnidaria both possess radial symmetry.
4. Arthropods are more than closely related to nematodes than they are to annelids.

(Figure) C

(Figure) Which of the following statements about diploblasts and triploblasts is false?

1. Animals that display radial symmetry are diploblasts.
2. Animals that display bilateral symmetry are triploblasts.
3. The endoderm gives rise to the lining of the digestive tract and the respiratory tract.
4. The mesoderm gives rise to the primal nervous organization.

(Figure) D

Review Questions

Which of the following organisms is almost probable to be a diploblast?

1. sea star
2. shrimp
3. jellyfish
4. insect

C

Which of the following is not possible?

1. radially symmetrical diploblast
2. diploblastic eucoelomate
3. protostomic coelomate
4. bilaterally symmetrical deuterostome

B

An animal whose development is marked by radial cleavage and enterocoely is ________.

1. a deuterostome
2. an annelid or mollusk
3. either an acoelomate or eucoelomate
4. none of the above

A

Critical Thinking Questions

Using the following terms, explain what classifications and groups humans fall into, from the almost general to the most specific: symmetry, germ layers, coelom, cleavage, embryological evolution.

Humans have body plans that are bilaterally symmetrical and are characterized by the development of three germ layers, making them triploblasts. Humans have true coeloms and are thus eucoelomates. Every bit deuterostomes, humans are characterized by radial and indeterminate cleavage.

Explain some of the advantages brought most through the development of bilateral symmetry and coelom formation.

The evolution of bilateral symmetry led to designated head and tail trunk regions, and promoted more than efficient mobility for animals. This improved mobility allowed for more practiced seeking of resources and casualty escaping from predators. The advent of the coelom in coelomates provides many internal organs with daze assimilation, making them less prone to concrete damage from bodily assail. A coelom likewise gives the body greater flexibility, which promotes more than efficient movement. The relatively loose placement of organs within the coelom allows them to develop and abound with some spatial liberty, which promoted the development of optimal organ organisation. The coelom also provides space for a circulatory system, which is an advantageous manner to distribute trunk fluids and gases.

Glossary

acoelomate

beast without a trunk cavity

bilateral symmetry

type of symmetry in which in that location is only one aeroplane of symmetry, so the left and right halves of an animal are mirror images

blastopore

opening into the archenteron that forms during gastrulation

coelom

lined torso cavity

determinate cleavage

cleavage pattern in which developmental fate of each blastomere is tightly defined

deuterostome

blastopore develops into the anus, with the second opening developing into the mouth

diploblast

animal that develops from two germ layers

enterocoely

mesoderm of deuterostomes develops every bit pouches that are pinched off from endodermal tissue, cavity independent within the pouches becomes coelom

eucoelomate

animal with a torso cavity completely lined with mesodermal tissue

indeterminate cleavage

cleavage pattern in which individual blastomeres have the grapheme of "stem cells," and are not yet predetermined to develop into specific jail cell types

protostome

blastopore develops into the rima oris of protostomes, with the second opening developing into the anus

pseudocoelomate

animal with a body cavity located between the mesoderm and endoderm

radial cleavage

cleavage axes are parallel or perpendicular to the polar axis, resulting in the alignment of cells between the two poles

radial symmetry

blazon of symmetry with multiple planes of symmetry, with body parts (rays) arranged effectually a central deejay

schizocoely

during development of protostomes, a solid mass of mesoderm splits apart and forms the hollow opening of the coelom

spiral cleavage

cells of i pole of the embryo are rotated or misaligned with respect to the cells of the opposite pole

triploblast

animal that develops from three germ layers

Source: https://opentextbc.ca/biology2eopenstax/chapter/features-used-to-classify-animals/

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