30.1 What Is an Animal?

• Animals form a monophyletic group, sharing key traits:

  • All animals are multicellular eukaryotes whose cells lack cell walls but have an extensive extracellular matrix (ECM)

  • All animals are heterotrophs, meaning that they obtain carbon compounds from other organisms

  • All animals move under their own power at some point in their life cycle.

  • All animals other thαn sponges have neurons and muscle cells

• The radiation of animals began around 550 million years ago during an event called the Cambrian explosion

30.2 What Key Innovations Occurred during the Origin of Animal Phyla?

• There are four main sources of evidence:

  • Fossils

  • Comparative morphology

  • Comparative development

  • Comparative genomics

• Both choanoflagellates and sponges are benthic, meaning that they live at the bottom of aquatic environments, and sessile, meaning that adults live permanently attached to a substrate rather than moving freely.

• The beating flagella of choanoflagellates and specialized cells in sponges called choanocytes trap bacteria and other organic debris.

• Colonies are groups of individuals that are attached to each other.

• Epithelium is a layer of tightly joined cells that covers the interior and/or exterior surface of the animal.

• New genomic data have revealed a number of important insights at apply to the entire animal lineage:

  • The evolution of αnimαls is more complicated than α smooth transition from simple to complex.

  • Many key innovαtions did not αrise αll αt once.

  • Evolution did not stop within any of the lineages.

• Sponges do not have complex tissues, groups of similar cells that are organized into tightly integrated structural and functional units.

• Animals whose embryos have two types of tissue are called diploblasts

• Animals whose embryos have three types of tissue are called triploblasts

• The embryonic tissues are organized in layers, called germ layers

• In diploblasts 1ese germ layers are called ectoderm (“outer skin”) and endoderm (“inner-skin”)

• In most cases the outer and inner “skins” of diploblast embryos are connected by a gelatinous material called mesoglea that may contain some cells.

• In triploblasts, however, there is a germ layer called mesoderm (“middle-skin”) between the ectoderm and endoderm.

• Ctenophores, many cnidarians, and some sponges have radial symmetry (“spoke symmetry”), which means they have at least two planes of symmetry.

• Many ctenophores are often described as having a special kind of radial symmetry called biradial symmetry-two planes of symmetry, not more.

• Cnidarians have nerve cells at are mostly organized into a diι fuse arrangement called a nerve net

• Some research shows that ctenophores also have clusters of nerves, called ganglia.

• The nervous systems of bilaterians are diverse, ranging from a nerve net to a more complex central nervous system, or CNS.

• Cephalization is the evolution of a head, or anterior region, where structures for feeding, sensing the environment, and processing information are concentrated.

• The large mass of neurons that is located in the head, and that is responsible for processing information to and from the body, is called the cerebral ganglion or brain.

• Lophotrochozoans grow continuously when conditions are good.

• Ecdysozoans grow by shedding their external skeletons or outer coverings and expanding their bodies.

• Segmentation is defined as the division of the body or a part of the body into a series of similar structures.

• A segmented backbone is one of the defining characteristics of vertebrates, a monophyletic group within the Chordata that includes fishes, reptiles (including birds), amphibians, and mammals

30.3 What Themes Occur in the Diversification within Animal Phyla?

• Several variables have been hypothesized to play a role in 1e adaptive radiation of animals:

  • Higher oxygen levels

  • Rise of a higher-quality food source

  • The evolution of predation

  • New niches beget more new niches

  • Modified genes, modified bodies

• Animals can typically be classified as:

  • detritivores that feed on dead organic matter

  • herbivores that feed on plants or algae

  • carnivores that feed on animals

  • omnivores that feed on a variety of organisms

• Most carnivores are predators that kill their prey.

• Endoparasites live inside their hosts and usually have simple, wormlike bodies.

• Ectoparasites live on the outside of their hosts.

• Suspension feeders, also known as filter feeders, employ a wide array of structures to trap suspended particles

• Many deposit feeders digest organic matter in sediments; their food consists of sand or mud-dwelling bacteria, archaea, protists, and fungi, along with detritus that settles on the surface of sediments.

• Fluid feeders range from butterflies and moths that feed on nectar with a straw-like proboscis to vampire bats that feed on blood.

• Mass feeders ingest chunks of food.

• Viviparous (“live-bearing”) species such as humans and most other mammals nourish embryos inside the body and give birth to live young.

• Oviparous (“egg-bearing”) species such as chickens and crickets deposit fertilized eggs. The embryos within are nourished by yolk.

• Ovoviviparous (“egg-live-bearing”) species such as guppies and garter snakes retain eggs inside the body during early development, but the growing embryos are nourished by egg yolk instead of by nutrients transferred directly from the mother, as in viviparous species.

• Metamorphosis is a drastic change of form from one developmental stage to another.

• During indirect development, embryogenesis produces larvae

• Through the process of metamorphosis, larvae transform into juveniles

• After a period of growth and maturation, juveniles become adults, the reproductive stage in the life cycle.

30.4 Key Lineages of Animals: Non-Bilaterian

• Spicules are stiff spikes of silica or calcium carbonate (CaC03) that, along with collagen fibers, give structural support to the ECM.

• Many cnidarians have a life cycle that includes both a sessile polyp form that reproduces asexually and a free-floating medusa that reproduces sexually