Prechordates {stem chordate} had notochord in both larval and adult stages, allowing muscle attachment, providing long body axis, and affecting reproduction.
germ layers
Prechordate embryos had three cell layers. Endoderm is inner tube, mesoderm is between, and ectoderm is outer tube. Ectoderm becomes senses, nerves, and outer skin. Mesoderm becomes muscles and glands. Endoderm becomes digestive tract.
coelom
Prechordates had tube-shaped body structures, with digestive tube inside main tube.
deuterostome
During development, inner tube opens to outside anus first.
head and tail
Having coelom makes longitudinal bodies, from which head, trunk, and tail can separately evolve. Head holds central ganglia and mouth.
bilateral symmetry
Body parts and appendages have pairs, one right and one left. Body also has front and back. Bilateral symmetry [-590000000] resulted from having coeloms.
segmentation
Prechordates had repeated body structures, allowing different lengths and requiring coordination among body segments. Segments can vary independently.
development stages
Prechordates had streamlined larvae with cilia for swimming in mobile stage. Adult sessile stage did not move. Prechordate ciliated larvae evolved to become stem chordates.
bone
Stem chordates calcified tissue to make bone, allowing better muscle attachment, more shapes, and more textures.
respiration
Stem chordates had external respiration by gills, allowing efficient oxygen uptake and carbon-dioxide removal from blood. Body-side gill slit openings allowed water to flow into mouth and through gills, resulting in better respiration.
nervous system
Prechordates had main head ganglion, with peripheral nerves to tail. Head ganglion provided unified control for all body segments and allowed swimming, burrowing, and defense. Stem chordates had dorsal hollow nerve, so all nerves have same pathway from head to periphery. Cerebrospinal fluid was in dorsal hollow nerve. Nerves were bilateral sense and motor paths. Interneurons coordinated neurons.
hormones
Hormones from glands affect neurons and other tissues.
Chordates {chordate}| (Chordata) are deuterostomes and have bilateral symmetry.
types
Amphioxus is a living chordate, has no jaw, is flat, and is small. Pikaia was an ancient chordate [Bone, 1979].
evolution
Hemichordata ancestors and Echinodermata and Pterobranchia ancestors split. Chordata ancestors and Hemichordata ancestors split. Chordates developed from prechordate larval forms.
notochord
Early chordates had one firm cartilage segmented rod down back along body long axis, allowing increased swimming efficiency by providing places for muscle attachment. Adult notochords allowed reproductive-method changes.
external respiration
Early chordates had paired pharyngeal gill slits. Side openings allowed greater water flow into mouth, over gills, and out body. Blood oxygen uptake and carbon-dioxide removal became more efficient with gills.
filter feeding
Skin calcification made dermal bone that allowed structures for catching small organisms in water flowing into mouth. Filter feeding gathered more food and calcium.
nervous system
Dorsal hollow nerve lay along back under notochord, from periphery to head, and had sense and motor pathways. Cerebrospinal fluid formed in middle.
brain
Head ganglion unified control over all body segments and other ganglia, coordinating sense input and motor output. Brain allowed better swimming, burrowing, and defense and more coordinated behavior. Eye, pineal gland, hypothalamus, and hindbrain began in chordates. Chordates had serotonin neurons, which later evolved to brainstem.
senses
Sense cells detected motions and stationary patterns.
Skin-tissue calcification {dermal bone, chordate} allows structures for filter feeding.
Dermal bone allows structures for catching small organisms in water flowing through mouth {filter feeding}, which gathers more calcium to make bone and allows more energy and larger sizes.
Chordates have flexible straight cartilage {notochord}| down back.
Stem chordates have head as unique body segment, whose ganglion {main ganglion} provides unified control for all body segments and allows swimming, burrowing, and defense.
Stem chordates have a tube of nerves down back {dorsal hollow nerve}, so all nerves have same pathway from head to periphery.
Prechordates have ciliated larvae that swim {mobile stage, prechordate}.
Prechordates have mobile larval stage that has movable hairs {ciliated larvae} and swims.
Prechordate adult stage {sessile stage, prechordate} does not move and evolved little.
Stem chordates have external respiration {gill, chordate}|, allowing efficient oxygen uptake into blood and carbon-dioxide removal from blood.
Body-side openings {gill slit}| allow water to flow into mouth and through gills, for better respiration.
Gills allow blood and water to exchange oxygen and carbon dioxide {external respiration} efficiently.
Embryonic bodies have endoderm alimentary canal within ectoderm tube {coelom, chordate}|, with mesoderm between tubes.
Ectoderm coelom surrounds endoderm tube from mouth to anus {alimentary canal}|, with mesoderm between tubes, allowing better digestion.
Chordates {cephalochordate} (Cephalochordata) can have body segments, be small, be fish-like, strain seawater for food, and have no brain. Adults have chordate characteristics.
540 million years ago, later chordates {prevertebrate} calcified skin {dermal bone, skin} and formed cranium bone around brain, allowing more muscle-attachment sites and better protection. Prevertebrates had structures for filter feeding.
Chordates {tunicate}| {sea squirt} (Urochordata) can live in tropical oceans, be sessile or floating, and have translucent cellulose covers, with hole for incurrent siphon and hole for excurrent siphon. They filter-feed to catch phytoplankton. They can bud. Larvae have chordate characteristics, but adults have gill slits. Vertebrates evolved from tunicate larvae.
Floating tunicates {salp} can have barrel shapes and live in colonies, making tube strings. Salp feces sink to bottom, carrying phytoplankton carbon molecules from carbon dioxide.
Swimming tunicates {larvacean} {apendicularian} (Larvacea) can have oval bodies, movable tails, and notochords. Every few hours, they make 2-centimeter-diameter gelatin mass around body, in which they trap plankton. Mucous mass sinks to bottom, carrying phytoplankton carbon molecules from carbon dioxide. Larvaceans make no buds, only use sexual reproduction, and are mostly hermaphrodites. Sperm release first. Breaking body wall releases eggs and causes death.
Tunicates have translucent cellulose covers {tunic, tunicate}, with hole for incurrent siphon and hole for excurrent siphon.
Outline of Knowledge Database Home Page
Description of Outline of Knowledge Database
Date Modified: 2022.0225