Chapter 32, 33, 34

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Chapter 32 – An Introduction to Animal Diversity
All animals share a common ancestor, sponges are basal animals, eumetazoa is a clade of animals with true tissues, most animal phyla belong to the clade bilateria, and vertebrates and some other phyla belong Animals share a unique homeobox-containing family of genes, known as the Hox genes, suggesting that this gene family evolved in the eukaryote lineage that gave rise to animals. Hox genes play important roles in the development of animal embryos, controlling the expression of dozens or even hundreds of other genes. Hox genes can thus control cell division and differentiation producing different morphological features of animals. Some calculations based on molecular clocks estimate that the ancestors of animals diverged from the ancestors of fungi as far back as 1.5 billion years ago. The Cambrian explosion is a period where animal diversification appears to have accelerated dramatically between 542 and 525 million years ago. The first hypothesis is that new predator-prey relationships that emerged in the Cambrian period generated diversity through natural selection. The second hypothesis focuses on the rise in athmospheric oxygen that preceded the Cambrian explosion. The third hypothesis holds that the evolution of the Hox gene complex provided the development flexibility that resulted in variations in morphology. XxTtYy

Radial symmetry is the form found in a flowerpot while bilateral symmetry is a two-sided symmetry seen in a shovel. Many radial animals are sessile (living attached to a substrate) or planktonic (drifting or weakly swimming, such as jellyfish). In contrast, bilateral animals generally move actively from place to place. Coelomeate are triploblastic that possess a true coelom. A coelom is a body cavity that is a fluid-filled space separating the digestive tract from the outer body wall. A so called true coelom forms from the tissue derived from mesoderm. Pseudocoelomates have a body cavity formed from the blastocoel rather than from mesoderm. Such a cavity is called a pseudocoelom. Despite its name, a pseudocoelom is not false; it is a fully functional body cavity. Acoelomeates lack a coelom altogether.

Dipoblastic and tripoblastic
Animals that have only two germ layers are dipoblastic; animals with three layers are tripoblastic. Spiral and radial cleavage
Spiral cleavage is when the planes of cell division are diagonal to the vertical axis of the embryo. Radial cleavage planes are either parallel or perpendicular to the vertical axis of the egg. Determinate and interdeterminate cleavage

Determinate cleavage of some animals with this development pattern rigidly casts the developmental fate of each embryonic cell very early. Interdeterminate cleavage means that each cell produced by early cleavage divisions retains the capacity to develop into a complete embryo. Schizocoelous and enterocoelous development

As the archenteron forms in protosome development, initially solid masses of mesoderm split and form the coelomic cavity; this pattern is called schizocoelous development. In contrast, formation of the body cavity in deuterostome development is described as enterocoelous: The mesoderm buds from the wall of the archenteron, and its cavity becomes the coelom.

Pattern of cleavage
Protostome – spiral and determinate cleavage and deuterostome – radial and indeterminate cleavage Fate of the blast pore
Protostome – mouth forms from the blastopore and deuterostome – the mouth forms from a secondary opening Coelom formation
Protostome – the coelom forms from splits in the mesoderm and deuterostome – the coelom forms the mesodermal outpocketings of the archenteron All animals have a common ancestor.
Sponges are basal animals.
Eumentazoa is a clade of animals with true tissues.
Most animal phyla belong to the clade Bilateria.
Vertebrates and some other phyla belong to the clade Deuterostomia. Several recent molecular studies generally assign two...
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