1. Deuterostomes Principles of Biology
Scientists classify animals into deuterostomes or protostomes according to features of early embryological development.
Principles of Biology

2. Deuterostome Diversity
Deuterostomes
Deuterostome Diversity
The deuterostomes are a very diverse group that includes the largest and most complex animals.
Deuterostomes and protostomes differ in four key steps during early development: the role of the blastopore, the direction of cell cleavage, cell fate before differentiation, and development of the coelom.
During gastrulation, cells migrate to various parts of the embryo according to their ultimate fate. During migration, an opening called the blastopore forms.
In protostomes, the blastopore eventually becomes the mouth. An opening at the opposite end of the embryo develops into the anus.
In deuterostomes, the blastopore develops into the anus, whereas the opening at the opposite end develops into the mouth.
During embryonic cell division in deuterostomes, cleavage occurs radially.
During embryonic cell division, as the cells divide, each daughter cell is added to the blastula parallel or perpendicular to the cell's vertical axis.
In protostomes, cleavage occurs spirally. Daughter cells are added to the blastula at a diagonal to the axis of the embryo.
In deuterostomes, undifferentiated embryonic cells are indeterminate-each embryonic cell has the capacity to develop into a complete organism.
In protostomes, cells are determinate, so the fate of embryonic cells is determined early in development, before differentiation.
In deuterostomes, the coelom develops from folds of mesoderm in the archenteron.
In protostomes, the mesoderm splits away to form the coelom.
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3. Hemichordates and xenoturbellids are deuterostome worms.
Deuterostomes
Hemichordates and xenoturbellids are deuterostome worms.
Hemichordates are the closest relatives that link the chordates and other invertebrates, suggesting one possibility for their common ancestor.
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4. Echinoderms are a large group of marine invertebrates.
Deuterostomes
Echinoderms are a large group of marine invertebrates.
Humans and other vertebrates share a common bond of embryological development and hence evolutionary history with echinoderms-which include sea stars, sea cucumbers, and sea urchins-although there is not much anatomical similarity.
Most adult echinoderms have a hard, stiff covering composed of calcite plates called ossicles.
Most adult echinoderms have tube feet, also called podia. These tiny protrusions are often shaped like suction cups, although they usually do not adhere by suction.
Echinoderms have a water vascular system that operates a simple, decentralized nervous network. It is also involved in respiration and digestion.
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5. Figure 1 Podia and spines of an echinoderm.
Deuterostomes
Figure 1 Podia and spines of an echinoderm.
The delicate and colorless podia line the oral groove of echinoderms such as this giant sea star. Thick white spines made of calcium carbonate protrude from the ossicles under the skin on the aboral surface of the sea star.
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6. Echinoderms are a large group of marine invertebrates.
Deuterostomes
Echinoderms are a large group of marine invertebrates.
The larval stage of all echinoderms is bilaterally symmetric, however, justifying their inclusion with higher vertebrates as Bilateria. An object with bilateral symmetry has mirror image halves, left and right.
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7. Deuterostomes
Figure 2 Pentaradially symmetrical adult with bilaterally symmetrical larva.
The adult stages of echinoderms are radially symmetric, but the larval stages are bilaterally symmetric. The larvae remain in the planktonic form for a week or two before metamorphosing and settling down to a largely sessile life as pentaradial adults.
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8. Figure 3 Echinoderms are metazoans that date back to the Cambrian.
Deuterostomes
Figure 3 Echinoderms are metazoans that date back to the Cambrian.
Lineages marked with an asterisk are extinct. The remaining lineages are the five major groups that remain today.
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9. Figure 4 Echinoderm feeding behavior.
Deuterostomes
Figure 4 Echinoderm feeding behavior.
Echinoderms such as this sea star can feed on prey that would not fit in their mouths, greatly increasing the size and types of prey available to them. After digestion, the sea star releases waste via the aboral anus.
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10. Figure 5 A field of common sand dollars (Echinarachnius parma).
Deuterostomes
Figure 5 A field of common sand dollars (Echinarachnius parma).
Although circular in shape, sand dollars demonstrate their pentaradial symmetry with five pores in the endoskeleton test. A thin skin covers sand dollars, and when they die, the white test remains as in this specimen.
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11. Deuterostomes
Chordates are a diverse group containing all vertebrates and some invertebrates.
Four morphological features characterize the chordates: pharyngeal gill slits, the nerve cord, a notochord, and a postanal tail.
Pharyngeal gill slits-repeated segments through which water is filtered for food-are present in the embryos of all chordates, including humans.
In tetrapods, the foremost pharyngeal arches, bones that once supported gill slits are modified into the jaws.
The nerve cord or neural tube is a hollow dorsal tube running anteriorly-posteriorly along the body of all chordates.
During early development of the embryo, the ectoderm folds over to form the nerve cord. In vertebrates, this structure is modified to form the brain and spinal cord.
The notochord lies ventral to the nerve cord and, in most vertebrates, forms the bones of the vertebral column-the backbone.
The notochord supports the postanal tail, which extends beyond the anus, to provide locomotion for marine vertebrates.
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12. Deuterostomes
Figure 6 A phylogeny of the vertebrates shows the key adaptations that define each major group.
Egg-laying mammals provide milk to their young, so they are classified as mammals. Placental mammals (not shown) are therefore a subgroup of mammals.
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13. Deuterostomes
Chordates are a diverse group containing all vertebrates and some invertebrates.
Groups of vertebrates have lungs or homologous organs (such as the swim bladder in fish). The lungs of lobefin fishes enabled the animal to gulp air and thereby colonize shallow anoxic waters.
Along with the fleshy lobefins that supported the body, lungs paved the way for the eventual colonization of land by the tetrapods. Amphibians are the modern relatives of early tetrapods.
The amniotic egg, combined with internal fertilization, freed animals from the need for open water to reproduce.
Mammals retain the egg in the female's body allowing development to continue for a longer period of time. The placenta is a disk-shaped organ that integrates female tissue with the embryo's umbilical cord.
Besides allowing the female to protect the embryo inside her body during development, the placenta provides nutrients and removes waste for the entire period of gestation.
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