Hemichordata (half cord)
Graptolites, sea squirts, and acorn worms, all marine animals, fall within this phylum.
Hemi chordates are similar to chordates (vertebrate animals) in that they have gill slits, a dorsal nerve cord, and a dorsal notocord-like structure (a stiffening rod) in the same location as the vertebrate spinal cord. The hemichordate body is elongate
and bilaterally symmetrical. They appeared in the Middle Cambrian (ca. 505 million years ago); sea squirts and acorn worms live in modern oceans.
Graptolithina (class) - Graptolite fossils usually appear in rocks as black, saw-toothed, pencil-like marks. This pencil mark-like appearance gives this group its name ("graptolite" means "rock writing") and is due to the way they were fossilized—by carbonization. Each "tooth" shows the shape of the protective covering in which the individual animals lived. These coverings, called theca, are arranged along a common stalk to form a colony. The colony grew by asexual budding. Graptolites were marine and were usually buoyed up by various structures attached to the colony. Graptolites lived from the Middle Cambrian (505 million years ago) through the Middle Devonian (370 million years ago).
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