Spore and seed plants, animal evolution

Animals arose from unicellular flagellate eukaryotes; from colonies came two-layered cnidarians (jellyfish, hydra). Flatworms gained bilateral symmetry, three layers, and digestive, excretory and nervous systems; roundworms a body cavity and a complete gut (mouth and anus); annelids a body divided into segments, circulation and a nerve cord; molluscs a shell, gills or lungs and a heart; arthropods a chitinous exoskeleton and jointed legs. Chordates developed the notochord, a tubular nervous system, gill slits and a closed circulation; fish the jaw, cartilage then bony skeleton, fins and a two-chambered heart; amphibians five-toed limbs, lungs and a three-chambered heart; reptiles internal fertilisation, the amniotic egg and dry skin; birds a four-chambered heart, warm-bloodedness and flight adaptations; mammals the uterus, hair, mammary glands and a well-developed cerebral cortex. According to newer data: non-bird dinosaurs died out about 66 million years ago mainly because of a large asteroid impact, with massive volcanism as a contributing factor (the textbook’s «cooling climate» is only partly right: the climate cooled sharply after the impact), birds descend from theropod dinosaurs and mammals from synapsids in the Triassic, and chordates are thought to descend not from annelids but from a group closer to echinoderms (deuterostomes).