Search results
Results from the WOW.Com Content Network
All chordates share a common ancestor which lost its echinoderm stereom calcite skeleton. However, later revisions of the theory had each separate lineage losing its calcitic skeleton independently, [ 6 ] as it evolved from its own mitrate ancestor, making the chordates a paraphyletic group [ citation needed ] .
Hemichordates ("half chordates") have some features similar to those of chordates: branchial openings that open into the pharynx and look rather like gill slits; stomochords, similar in composition to notochords, but running in a circle round the "collar", which is ahead of the mouth; and a dorsal nerve cord—but also a smaller ventral nerve cord.
The Cambrian chordates are an extinct group of animals belonging to the phylum Chordata that lived during the Cambrian, between 538 and 485 million years ago. The first Cambrian chordate known is Pikaia gracilens , a lancelet -like animal from the Burgess Shale in British Columbia , Canada.
Pikaia gracilens is an extinct, primitive chordate marine animal known from the Middle Cambrian Burgess Shale of British Columbia.Described in 1911 by Charles Doolittle Walcott as an annelid, and in 1979 by Harry B. Whittington and Simon Conway Morris as a chordate, it became "the most famous early chordate fossil", [1] or "famously known as the earliest described Cambrian chordate". [2]
The notochord is the defining feature (synapomorphy) of chordates, and was present throughout life in many of the earliest chordates. Although the stomochord of hemichordates was once thought to be homologous or from a common lineal origin, it is now viewed as analogous, convergent , or from a different lineal origin. [ 22 ]
Main page; Contents; Current events; Random article; About Wikipedia; Contact us; Help; Learn to edit; Community portal; Recent changes; Upload file
Early screens to identify genes involved in cuticle development led to the discovery of a class of genes that was necessary for proper segmentation of the Drosophila embryo. [ 3 ] To properly segment the Drosophila embryo, the anterior - posterior axis is defined by maternally supplied transcripts giving rise to gradients of these proteins.
The upper limit on prey size is set by the mouth size, which in the largest genus Bathochordaeus is around 1–2 mm wide for a trunk length of 1–3 cm. [24] In some species, houses are discarded and replaced regularly as the animal grows in size and its filters become clogged; in Oikopleura , a house is kept for no more than four hours before ...