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.
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Date Modified: 2022.0225