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A diverse array of animals was found in fossils from the Burgess Shale site in British Columbia, dating from the Cambrian period of the Phanerozoic eon. They include: Pikaia (swimming eel-like chordate) Hallucigenia (with toothpick-like spikes on seafloor Anomalocaris (large predator with hooked claws) Marella (arthropod swimming at left) Ottoia (burrowing U-shaped worm)

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Ecdysozoa (Arthropoda, Nematoda) have exoskeletons that are shed in a process called ecdysis. This molting cicada is in the process of emerging from its old exoskeleton. The animal then secretes a new, larger exoskeleton.

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Lophotrochozoa (Mollusca, Annelida) are distinguished by a ciliated feeding structure (lophophore) or a trochophore larva.

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In protostome development, the mouth forms from the blastopore.
In deuterostome development, the mouth forms from a secondary opening.

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Acoelomates such as flatworms lack a body cavity between the digestive tract and outer body wall.

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All of these diverse organisms are animals.

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Bilateral symmetry. has a single plane of symmetry that divides the body into right and left sides, as well as a dorsal (top) side and a ventral (bottom) side, an This also allows for cephalization, the development of a head, where a nervous system and sensory organs can be stored.

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Kingdom Animalia can be classified on morphological traits such as being multicellular (Metazoa), possessing tissues (Eumetazoa), bilaterial symmetry (Bilateria), and gastrula development (Deuterostomia). Molecular evidence, on which this tree is based, indicates that these traits may not reveal cladistic relationships.

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In most animals, the fertilized zygote undergoes cleavage, a succession of mitotic cell divisions without cell growth, leading to the formation of a multicellular hollow ball of cells called a blastula. The blastula then undergoes a process of differntiation called gastrulation, producing a gastrula with layers of embryonic tissues that will develop into adult body parts.

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Coelomates such as earthworms have a true coelom, a body cavity completely lined by tissue derived from mesoderm and surrounding a digestive tract derived from endoderm. The coelom separates and suspends the organs from the body, providing cushioned support and room to grow, as well as separate openings for the mouth and anus.

deuterostome.html: 32_09DeutDevelopment.jpg
Deuterostomes such as echinoderms and chordates exhibit radial cleavage.
Deuterostomes form their coelom by infolding of the archenteron.
Deuterostomes develop their anus from the blastopore of the gastrula.

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Diploblastic animals have two germ layers: an outer ectoderm and an inner endoderm.
The jellyfish has a single opening for a mouth and an anus.

embryonic.html: 32_02AnimalEmbryoDevel.jpg
Early embryonic development in animals.
Cleavage of the zygote by successive mitotic divisions results in the formation of a blastula, a hollow ball of cells.
In gastrulation, the embryo folds in at the blastopore toward the blastocoel (cavity), forming a gastrula with germ layers of embryonic tissues: the ectoderm and the endoderm; some animals have an additional layer, mesoderm.

porifera.html: 32_03Choanoflagellate.jpg
The closest living relatives to Animalia are the colonial protist choanoflagellates.
These cells possess a collar (choanos in Greek) around a flagellum.
They resemble the specialized choanocytes of multicellular sponges.

protostome.html: 32_09ProtDevelopment.jpg
Protostomes such as molluscs and annelids exhibit spiral cleavage.
Protostomes form their coelom by splitting of the mesoderm.
Protostomes develop their mouth from the blastopore of the gastrula.

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Pseudocoelomates such as nematodes have a body cavity partially lined by tissue derived from mesoderm.
Note the digestive tract is not covered by mesoderm tissue.

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Radial symmetry.
The parts of a radial animal, such as a sea anemone (phylum Cnidaria), radiate from the center. Any imaginary slice through the central axis divides the animal into mirror images.

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Tissues, such as epithelial, connective, or muscle tissue, are made of similar cells that act as a functional unit and are isolated from other tissues by membranous layers.

triploblastic.html: 32_09ProtDeutDevelopment.jpg
Triploblastic animals such as humans have three germ layers: an outer ectoderm surrounds a mesoderm; the inner endoderm folds inward at the blastopore, forming the embryonic digestive tube called the archenteron.

trochophore.html: 32_13Trochophore.jpg
A trochophore larva.