11: Deuterostomes - Biology

11: Deuterostomes - Biology

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11: Deuterostomes

Characteristics of Protostomes and Deuterostomes

Spiral cleavage pattern the spiral cleavage is masked at the 6th cleavages (64-cell stage), e.g., Sipuncula, Echiura, Annelida, Pogonophora, Mollusca and some groups of Arthropoda (e.g., Myriapoda, Crustacea and Insecta except Chelicerata).

2. Fate of embryonic blastomeres:

Determinate or mosaic pattern development.

The blastopore is either becomes mouth (e.g., Mollusca) or gives rise to both mouth and anus (e.g., some molluscs, polychaetes and onychophores) in adult.

4. Formation of mesoderm:

From the 4th cell which is also called mesentoblast and increases its number by pro­liferation.

5. Formation of coelom:

Coelom originates by the splitting of a solid mass of mesodermal band (schizocoely).

6. Arrangent of coelomic cavities:

The coelomic cavities are variable in number in different groups. In sipunculans and echiurans the body cavities are two in number, but in echiurans a septum separates the body cavity into two. In annelids the single coelom is di­vided by many septa.

The most characteristic larval type is trochophore.

8. Larval ciliary bands:

Compound cilia from multi-ciliated cells.

Coelomate protostomes (e.g., Sipuncula, Echiura, Annelida, Pogonophora, Mollusca, Onychophora, Tardigrada, Pentastomida and some groups of arthropods).


Developmental Characteristics:

1. Pattern of embryonic cleavage:

Radial pattern of embryonic cleavage (e.g., phoronids, echinoderms, chaetognaths, hemichordates and chordates excepting sea squirts (bilateral cleavage).

2. Fate of embryonic blastomeres:

Indeterminate and regulative development.

Blastopore becomes the adult anus and then the formation of mouth takes place from a second opening on the dorsal surface of the embryo.

4. Formation of mesoderm:

From the sides of archenteron as a hollow coe­lomic pouches.

5. Formation of coelom:

Evagination of pouches from the wall of arch­enteron and each diverticulum becomes sepa­rated from the archenteron and develops inde­pendent coelomic pouch (enterocoely).

6. Arrangement of coelomic cavities:

8. Larval ciliary bands:

Simple cilia single cilium in each cell.

Echinodermata, Hemichordata and Chordata. Some developmental pattern is seen among lophophorates (e.g., Phoronida, Brachiopoda and Ectoprocta) and their inclusions within deuterostomes are in controversy.

Phylogeny of Coelomate Protostomes:

The coelomate protostomes include two lineages—one group includes echiurans, annelids, pogonophorans, onychophorans and arthropods which are characterised by segmentation and spiral cleavage, and another group which is non-segmented includes sipunculans and molluscs which have evolved from non-segmented ancestor.

The absence of segmentation in echiurans, sipunculans and molluscs is a secondary loss (Ruppert and Barnes, 1994).

Based on current molecular data, it has been suggested that the coelomate protostome animals can be divided into 2 groups:

1. Ecdysozoa, e.g., Arthropods and Nematodes (pseudocoelomates)

2. Lophotrochozoa, e.g., Molluscs and Annelids.

Phylogeny of Deuterostomes:

The deuterostomes include Echinodermata, Hemichordata and Chordata. With these groups, the other few minor groups such as lophophorates and chaetognaths are included under deuterostomes but in considerable controversy.

The echinoderms, hemichordates and chordates share some common features such as gill slits (absent in living echinoderms but are found in fossil carpoids), protocoelic nephridium (present in echinoderms and hemichordates but in chordates it has secondarily lost).

All the five mentioned groups are included under deuterostomes on the basis of embryonic development features. Recent molecular data such as 18SrRNA and mitochondrial DNA gene sequences of lophophorate phyla indicate that they (e.g., Phoronids, Brachiopods and Ectoprocts) place in the protostomes.

Barnes (1987) has suggested that chordates evolved from non-chordata group and hypothetical echinoderm larva (dipleurula) and other echinoderm larvae hold the key position. According to him, the evidences of phylogenetic relationship between echinoderms hemichordates and chordates are very convincing on the basis of larval stages.

Hyman (1940) has stated that all multicellular animals have evolved from some single-called protest, probably from a colonial flagellate and some advanced phyla which are grouped as protostomes and deuterostomes, originate separately through the trochophore and dipleurula larva.

Characteristics of Superphylum Deuterostomia

The phyla Echinodermata and Chordata (the phylum in which humans are placed) both belong to the superphylum Deuterostomia. Recall that protostome and deuterostomes differ in certain aspects of their embryonic development, and they are named based on which opening of the digestive cavity develops first. The word deuterostome comes from the Greek word meaning “mouth second,” indicating that the anus is the first to develop. There are a series of other developmental characteristics that differ between protostomes and deuterostomes, including the mode of formation of the coelom and the early cell division of the embryo. In deuterostomes, internal pockets of the endodermal lining called the archenteron fuse to form the coelom. The endodermal lining of the archenteron (or the primitive gut) forms membrane protrusions that bud off and become the mesodermal layer. These buds, known as coelomic pouches, fuse to form the coelomic cavity, as they eventually separate from the endodermal layer. The resultant coelom is termed an enterocoelom. The archenteron develops into the alimentary canal, and a mouth opening is formed by invagination of ectoderm at the pole opposite the blastopore of the gastrula. The blastopore forms the anus of the alimentary system in the juvenile and adult forms. The fates of embryonic cells in deuterostomes can be altered if they are experimentally moved to a different location in the embryo due to indeterminant cleavage in early embryogenesis.

Embryonic Development of the Mouth

Figure 3. Eucoelomates can be divided into two groups based on their early embryonic development. In protostomes, the mouth forms at or near the site of the blastopore and the body cavity forms by splitting the mesodermal mass during the process of schizocoely. In deuterostomes, the mouth 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.

Bilaterally symmetrical, tribloblastic eucoelomates can be further divided into two groups based on differences in the origin of the mouth. When the primitive gut forms, the opening that first connects the gut cavity to the outside of the embryo is called the blastopore. Most animals have openings at both ends of the gut: mouth at one end and anus at the other. One of these openings will develop at or near the site of the blastopore . In Protostomes (“mouth first”), the mouth develops at the blastopore (Figure 3).

In Deuterostomes (“mouth second”), the mouth develops at the other end of the gut (Figure 3) and the anus develops at the site of the blastopore. Protostomes include arthropods, mollusks, and annelids. Deuterostomes include more complex animals such as chordates but also some “simple” animals such as echinoderms. Recent evidence has challenged this simple view of the relationship between the location of the blastopore and the formation of the mouth, however, and the theory remains under debate. Nevertheless, these details of mouth and anus formation reflect general differences in the organization of protostome and deuterostome embryos, which are also expressed in other developmental features.

One of these differences between protostomes and deuterostomes is the method of coelom formation, beginning from the gastrula stage. Since body cavity formation tends to accompany the formation of the mesoderm, the mesoderm of protostomes and deuterostomes forms differently. The coelom of most protostomes is formed through a process called schizocoely . The mesoderm in these organisms is usually the product of specific blastomeres, which migrate into the interior of the embryo and form two clumps of mesodermal tissue. Within 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 expand to fill the space between the gut and the body wall, giving rise to the coelom.

Another difference in organization of protostome and deuterostome embryos is expressed during cleavage. Protostomes undergo spiral cleavage , meaning 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 angle 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 (up-and-down) alignment of the cells between the two poles.

A second distinction between the types of cleavage in protostomes and deuterostomes relates to the fate of the resultant blastomeres (cells produced by cleavage). In addition to spiral cleavage, protostomes also 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 any cell type other than its original destination. Removal of a blastomere from an embryo with determinate cleavage can result in missing structures, and embryos that fail to develop. In contrast, deuterostomes undergo indeterminate cleavage , in which cells are not yet fully committed at this early stage to develop into specific cell types. Removal of individual blastomeres from these embryos does not result in the loss of embryonic structures. In fact, twins (clones) can be produced as a result from blastomeres that have been separated from the original mass of blastomere cells. Unlike protostomes, however, if some blastomeres are damaged during embryogenesis, adjacent cells are able to compensate 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 ability to develop into any cell type until their fate is programmed at a later developmental stage.

The Evolution of the Coelom

One of the first steps in the classification of animals is to examine the animal’s body. One structure that is used in classification of animals is the body cavity or coelom. The body cavity develops within the mesoderm, so only triploblastic animals can have body cavities. Therefore body cavities are found only within the Bilateria. In other animal clades, the gut is either close to the body wall or separated from it by a jelly-like material. The body cavity is important for two reasons. Fluid within the body cavity protects the organs from shock and compression. In addition, since in triploblastic embryos, most muscle, connective tissue, and blood vessels develop from mesoderm, these tissues developing within the lining of the body cavity can reinforce the gut and body wall, aid in motility, and efficiently circulate 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 free-living and parasitic forms such as tapeworms. In these animals, mesenchyme fills the space between the gut and the body wall. Although two layers of muscle are found just under the epidermis, there is no muscle or other mesodermal tissue around the gut. Flatworms rely on passive diffusion for nutrient transport across their body.

In pseudocoelomates, there is a body cavity between the gut and the body wall, but only 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 within 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 two or more buds off of the gut. These buds separate from the gut and coalesce to form the body cavity. In the vertebrates, mammals have a subdivided body cavity, with the thoracic cavity separated from the abdominal cavity. The pseudocoelomates may have had eucoelomate ancestors and may have lost their ability to form a complete coelom through genetic mutations. Thus, this step in early embryogenesis—the formation of the coelom—has had a large evolutionary impact on the various species of the animal kingdom.

Origins and evolution

The majority of animals more complex than jellyfish and other Cnidarians are split into two groups, the protostomes and deuterostomes. Chordates (which include all the vertebrates) are deuterostomes. ⎜] It seems likely that the 555 million year old Kimberella was a member of the protostomes. ⎝] ⎞] That implies that the protostome and deuterostome lineages split some time before Kimberella appeared — at least 558 million years ago , and hence well before the start of the Cambrian 541 million years ago , ⎜] i.e. during the later part of the Ediacaran Period (circa 635-542 Mya, around the end of global Marinoan glaciation in the late Neoproterozoic). The oldest discovered proposed deuterostome is Saccorhytus coronarius, which lived approximately 540 million years ago. ΐ] ⎟] The researchers that made the discovery believe that the Saccorhytus is a common ancestor to all previously-known deuterostomes. ⎟]

Fossils of one major deuterostome group, the echinoderms (whose modern members include sea stars, sea urchins and crinoids), are quite common from the start of Series 2 of the Cambrian, 521 million years ago . ⎠] The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate. ⎡] Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate. ⎢] ⎣] Another Chengjiang fossil, Haikouella lanceolata, also from the Chengjiang fauna, is interpreted as a chordate and possibly a craniate, as it shows signs of a heart, arteries, gill filaments, a tail, a neural chord with a brain at the front end, and possibly eyes — although it also had short tentacles round its mouth. ⎣] Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as fish. ⎤] ⎥] Pikaia, discovered much earlier but from the Mid Cambrian Burgess Shale, is also regarded as a primitive chordate. ⎦]

On the other hand, fossils of early chordates are very rare, as non-vertebrate chordates have no bone tissue or teeth, and fossils of no Post-Cambrian non-vertebrate chordates are known aside from the Permian-aged Paleobranchiostoma, trace fossils of the Ordovician colonial tunicate Catellocaula, and various Jurassic-aged and Tertiary-aged spicules tentatively attributed to ascidians.


Below is a phylogenetic tree showing consensus relationships among deuterostome taxa. Phylogenomic evidence suggests the enteropneust family, Torquaratoridae, fall within the Ptychoderidae. The tree is based on 16S +18S rRNA sequence data and phylogenomic studies from multiple sources. ⎧] The approximate dates for each radiation into a new clade are given in millions of years ago (Mya). Not all dates are consistent, as of date ranges only the center is given. ⎨]

A deuterostome pharyngeal gene cluster

One conserved deuterostome-specific micro-syntenic cluster with functional implications for deuterostome biology is a cluster of genes expressed in the pharyngeal slits and surrounding pharyngeal endoderm (Fig. 4 Supplementary Note 9). This six-gene cluster contains four transcription factor genes in the order nkx2.1, nkx2.2, pax1/9 and foxA, along with two non-transcription-factor genes slc25A21 and mipol1, whose introns harbour regulatory elements for pax1/9 and foxA, respectively 34,35,36 . The cluster was first found conserved across vertebrates including humans (see chromosome 14 1.1 Mb length from nkx2.1 to foxA1) 34,37 . In S. kowalevskii, it is intact with the same gene order as in vertebrates (0.5 Mb length from nkx2.1 to foxA), implying that it was present in the deuterostome and ambulacrarian ancestors. The full ordered gene cluster also exists on a single scaffold in the crown-of-thorns sea star Acanthaster planci. Since these genes are not clustered in available protostome genomes, there is no evidence for deeper bilaterian ancestry. Two non-coding elements that are conserved across vertebrates and amphioxus 38 are found in the hemichordate and A. planci clusters at similar locations (A2 and A4, in Fig. 4a).

a, Linkage and order of six genes including the four genes encoding transcription factors Nkx2.1, Nkx2.2, Pax1/9 and FoxA, and two genes encoding non-transcription factors Slc25A21 (solute transporter) and Mipol1 (mirror-image polydactyly 1 protein), which are putative ‘bystander’ genes containing regulatory elements of pax1/9 and foxA, respectively. The pairings of slc25A21 with pax1/9 and of mipol1 with foxA occur also in protostomes, indicating bilaterian ancestry. The cluster is not present in protostomes such as Lottia (Lophotrochozoa), Drosophila melanogaster, Caenorhabditis elegans (Ecdysozoa), or in the cnidarian, Nematostella. SLC25A6 (the slc25A21 paralogue on human chromosome 20) is a potential pseudogene. The dots marking A2 and A4 indicate two conserved non-coding sequences first recognized in vertebrates and amphioxus 36 , also present in S. kowalevskii and, partially, in P. flava and A. planci. b, The four transcription factor genes of the cluster are expressed in the pharyngeal/foregut endoderm of the Saccoglossus juvenile: nkx2.1 is expressed in a band of endoderm at the level of the forming gill pore, especially ventral and posterior to it (arrow), and in a separate ectodermal domain in the proboscis. It is also known as thyroid transcription factor 1 due to its expression in the pharyngeal thyroid rudiment in vertebrates. The nkx2.2 gene is expressed in pharyngeal endoderm just ventral to the forming gill pore, shown in side view (arrow indicates gill pore) and ventral view and pax1/9 is expressed in the gill pore rudiment itself. In S. kowalevskii, this is its only expression domain, whereas in vertebrates it is also expressed in axial mesoderm. The foxA gene is expressed widely in endoderm but is repressed at the site of gill pore formation (arrow). An external view of gill pores is shown up to 100 bilateral pairs are present in adults, indicative of the large size of the pharynx.

The pax1/9 gene, at the centre of the cluster, is expressed in the pharyngeal endodermal primordium of the gill slit in hemichordates, tunicates, amphioxus, fish, and amphibians 8,9 , and in the branchial pouch endoderm of amniotes (which do not complete the last steps of gill slit formation), as well as other locations in vertebrates. The nkx2.1 (thyroid transcription factor 1) gene is also expressed in the hemichordate pharyngeal endoderm in a band passing through the gill slit, but not localized to a thyroid-like organ 39 . Here we also examined the expression of nkx2.2 and foxA in S. kowalevskii. We find that nkx2.2, which is expressed in the ventral hindbrain in vertebrates, is expressed in pharyngeal ventral endoderm in S. kowalevskii, close to the gill slit (Fig. 4b), and that foxA is expressed throughout endoderm but repressed in the gill slit region (Fig. 4b). The co-expression of this ordered cluster of the four transcription factors during pharyngeal development strongly supports the functional importance of their genomic clustering.

The presence of this cluster in the crown-of-thorns sea star, an echinoderm that lacks gill pores, and in amniote vertebrates that lack gill slits, suggests that the cluster’s ancestral role was in pharyngeal apparatus patterning as a whole, of which overt slits (perforations of apposed endoderm and ectoderm) were but one part, and the cluster is retained in these cases because of its continuing contribution to pharynx development. Genomic regions of the pharyngeal cluster have been implicated in long-range promoter–enhancer interactions, supporting the regulatory importance of this gene linkage (see Supplementary Note 9) 40 . Alternatively, genome rearrangement in these lineages may be too slow to disrupt the cluster even without functional constraint. Here we propose that the clustering of the four ordered transcription factors, and their bystander genes, on the deuterostome stem served a regulatory role in the evolution of the pharyngeal apparatus, the foremost morphological innovation of deuterostomes.

11: Deuterostomes - Biology

Unit Five. Evolution of Animal Life

All the animals we have met so far have essentially the same kind of embryonic development. Cell divisions of the fertilized egg produce a hollow ball of cells, a blastula, which indents to form a two-layer-thick ball with a blastopore opening to the outside. In mollusks, annelids, and arthropods, the mouth (stoma) develops from or near the blastopore. An animal whose mouth develops in this way is called a protostome (figure 19.26, top). If such an animal has a distinct anus or anal pore, it develops later in another region of the embryo.

A second distinct pattern of embryological development occurs in the echinoderms and the chordates. In these animals, the anus forms from or near the blastopore, and the mouth forms subsequently on another part of the blastula. This group of phyla consists of animals that are called the deuterostomes (figure 19.26, bottom).

Figure 19.26. Embryonic development in protostomes and deuterostomes.

Cleavage of the egg produces a hollow ball of cells called the blastula. Invagination, or infolding, of the blastula produces the blastopore. In protostomes, embryonic cells cleave in a spiral pattern and become tightly packed. The blastopore becomes the animal's mouth, and the coelom originates from a mesodermal split. In deuterostomes, embryonic cells cleave radially and form a loosely packed array. The blastopore becomes the animal's anus, and the mouth develops at the other end. The coelom originates from an evagination, or outpouching, of the archenteron in deuterostomes.

Deuterostomes represent a revolution in embryonic development. In addition to the fate of the blastopore, deutero-stomes differ from protostomes in three other features:

1. The progressive division of cells during embryonic growth is called cleavage. The cleavage pattern relative to the embryo’s polar axis determines how the cells array. In nearly all protostomes, each new cell buds off at an angle oblique to the polar axis. As a result, a new cell nestles into the space between the older ones in a closely packed array (see the 16-cell stage in the upper row of cells). This pattern is called spiral cleavage because a line drawn through a sequence of dividing cells spirals outward from the polar axis, (indicated by the curving blue arrow at the 32-cell stage).

In deuterostomes, the cells divide parallel to and at right angles to the polar axis. As a result, the pairs of cells from each division are positioned directly above and below one another (see the 16-cell stage in the lower row of cells) this process gives rise to a loosely packed array of cells. This pattern is called radial cleavage because a line drawn through a sequence of dividing cells describes a radius outward from the polar axis (indicated by the straight blue arrow at the 32-cell stage).

2. In protostomes, the developmental fate of each cell in the embryo is fixed when that cell first appears. Even at the four-celled stage, each cell is different, containing different chemical developmental signals and no one cell, if separated from the others, can develop into a complete animal. In deuterostomes, on the other hand, the first cleavage divisions of the fertilized embryo produce identical daughter cells, and any single cell, if separated, can develop into a complete organism.

3. In all coelomates, the coelom originates from mesoderm. In protostomes, this occurs simply and directly: The mesoderm cells simply move away from one another as the coelomic cavity expands within the mesoderm. However, in deuterostomes, the coelom is normally produced by an evagination of the archenteron—the main cavity within the gastrula, also called the primitive gut. This cavity, lined with endoderm, opens to the outside via the blastopore and eventually becomes the gut cavity. The evaginating cells give rise to the mesodermal cells, and the mesoderm expands to form the coelom.

Key Learning Outcome 19.11. In protostomes, the egg cleaves spirally, and the blastopore becomes the mouth. In deuterostomes, the egg cleaves radially, and the blastopore becomes the animal's anus.

Diversity Is Only Skin Deep

Perhaps the most important lesson to emerge from the study of animal diversity is not the incredible variety of animal forms, from worms and spiders to sharks and antelopes, but rather their deep similarities. The body plans of all animals are assembled along a similar path, as if from the same basic blueprint. The same genes play critical roles throughout the animal kingdom, small changes in how they are activated leading to very different body forms.

The molecular mechanisms used to orchestrate development are thought to have evolved very early in the history of multicellular life. Animals utilize transcription factors, like those discussed on page 247, to turn on or off particular sets of genes as they develop, determining just what developmental processes occur, where, and when.

In many cases the same gene is found to control the same developmental process in many, if not all, animals. For example, a gene in mice called Pax6 encodes a transcription factor that initiates development of the eye. Mice without a functional copy of this gene do not make the transcription factor and are eyeless. When a gene was discovered in fruit flies that caused the flies to lack eyes, this gene was found to have essentially the same DNA sequence as the mouse gene—the same Pax6 master regulator gene was responsible for triggering eye development in both insects and vertebrates. Indeed, when Swiss biologist Walter Gehring inserted the mouse version of Pax6 into the fruitfly genome, a compound eye (the multifaceted kind that flies have) was formed on the leg of the fly! It seems that although insects and vertebrates diverged from a common ancestor more than 500 million years ago, they still control their development with genes so similar that the vertebrate gene seems to function quite normally in the insect genome.

Pax6 plays this same role of releasing eye development in many other animals. Even marine ribbon worms use it to initiate development of their eye spots. The Pax6 genes of all these animals have similar gene sequences, suggesting that Pax6 acquired its evolutionary role in eye development only a single time more than 500 million years ago, in the common ancestor of all animals that use Pax6 today.

A more ancient master regulator gene called Hox determines basic body form. Hox genes appeared before the divergence of plants and animals in plants they modulate shoot growth and leaf form, and in animals they establish the basic body plan.

All segmented animals appear to use organized clusters of Hox genes to control their development. After the sequential action of several "segmentation” genes, the body of these early embryos has a basically segmented body plan. This is true of the embryos of earthworms, fruit flies, mice— and human beings. The key to the further development of the animal body is to now give identity to each of the segments—to determine whether a particular segment will become back, or neck, or head, for example. In fruit flies and mice, similar clusters of Hox genes control this process. Flies have a single set of Hox genes, located on the same chromosome, while mice have four sets, each on a different chromosome (it appears the vertebrate genome underwent two entire duplications early in vertebrate evolution). In the illustration here, the genes are color-coded to match the parts of the body in which they are expressed.

How does a cluster of Hox genes work together to control segment development? Each Hox gene produces a protein with an identical 60-amino-acid segment that lets it bind to DNA as a transcription factor and, in so doing, activate the genes located where it binds. The differences between each Hox gene of a set determine where on DNA a Hox protein binds, and so which set of genes it activates.

Hox genes have also been found in clusters in radially symmetrical cnidarians such as hydra, suggesting that the ancestral Hox cluster preceded the divergence of radially and symmetrical animals in animal evolution.

Embryological Characteristics of Protostomes and Deuterostomes

1. Cleavage of Protostomes are spiral, determinate or mosaic. Bilateral-mosaic cleavage of some protostomes (e.g., nematodes).

2. In Protostomes, the blastopore either becomes the mouth, the anus forms a new structure (e.g., most of molluscs) or the blastopore gives rise to both mouth and anus (e.g., some molluscs, polychaetes, onychophores).

3. In Protostomes, modes of mesoderm development is from mesentoblast (some-times called 4d it indicates the blastomere in 8-cell stage and by proliferation, develops mesoderm).

4. Coelom formation in protostomes is schizocoely (splitting of mesodermal bands).

5. Type of larva of protostomes is trochophore.

Difference # Deuterostomes:

1. Cleavage of Deuterostomes is radial, inderminate or regulative of many deuterostomes. Bilateral-mosaic cleavage of some deuterostomes (e.g., sea squirts).

2. In Deuterostomes, the blastopore usually be-comes the anus and mouth forms a new structure.

3. Deuterostomes develops as out-folds of the archenteron.

4. Coelom formation in Deuterostomes is enterocoely (coelom is segregated as epithelial out pockets of the archenteron).


This tutorial presented the phylum Chordata and the subphyla Urochordata, Cephalochordata, and Vertebrata. Individuals within the phylum all have a notochord, a dorsal nerve cord, pharyngeal slits, and a postanal tail. The notochord may or may not persist in the adult, and the pharyngeal slits are modified in various ways in the different groups. The Urochordates are the probable ancestors of vertebrates.

Vertebrates have a dorsal nerve cord that is protected by a vertebral column, and the anterior portion of this nerve cord is protected by a cranium. There are several basal features found in vertebrates, and they are used to distinguish the lineages. The presence or absence of jaws, teeth, lungs, legs, amniotic eggs, and hair are more derived features that arose in succession, and they give the vertebrates the diversity seen today.

Watch the video: Cleavage and blastocyst formation (February 2023).