lpetrich
Contributor
Adult starfish (sea stars), at least. Along with adults of some other echinoderms: sea urchins (echinoids) and sea lilies (crinoids).
Starfish Are Heads--Just Heads | Scientific American
noting an article in Nature magazine: the animals are actually almost all head and no trunk
This seems like an odd sort of thing, because starfish don't seem to have any head: just a body that is the center of the arms, usually 5, sometimes more. Furthermore, adult starfish have radial symmetry, like other echinoderms, while their larval phases have bilateral symmetry.
This result was found by examining the expression of genes involved in nose-to-tail (anteroposterior) patterning, genes that include the famous Hox genes. The proteins from their expression then control the expression of other genes, producing identity along this axis. This nose-to-tail patterning system is found over most of the animal kingdom, and most of the time, it is for patterning the main body axis.
But starfish are different, this research shows.
The most forward expressed genes are actually expressed around the central tubes of the water vascular system: a ring in the "body" with radial tubes in the arms. These tubes are on the ventral (downward) side of the animal. Next most forward genes are expressed at the tube feet, a little sideways from the radial tubes, and next forward, but still in the head, genes are expressed a little bit further sideways.
So these tubes are "frontmost" and outward from these tubes, like sideways in the arms, is "backward" from "frontmost", but still "head".
Looking at other echindoderms, sea urchins are like starfish with their arms pulled upward, surrounding the "body". Sea lilies are like upside-down starfish on stalks.
Working out how this body plan emerged is a very challenging task, and some fossils of early echinoderms may have some clues.
Starfish Are Heads--Just Heads | Scientific American
noting an article in Nature magazine: the animals are actually almost all head and no trunk
This seems like an odd sort of thing, because starfish don't seem to have any head: just a body that is the center of the arms, usually 5, sometimes more. Furthermore, adult starfish have radial symmetry, like other echinoderms, while their larval phases have bilateral symmetry.
This result was found by examining the expression of genes involved in nose-to-tail (anteroposterior) patterning, genes that include the famous Hox genes. The proteins from their expression then control the expression of other genes, producing identity along this axis. This nose-to-tail patterning system is found over most of the animal kingdom, and most of the time, it is for patterning the main body axis.
But starfish are different, this research shows.
The most forward expressed genes are actually expressed around the central tubes of the water vascular system: a ring in the "body" with radial tubes in the arms. These tubes are on the ventral (downward) side of the animal. Next most forward genes are expressed at the tube feet, a little sideways from the radial tubes, and next forward, but still in the head, genes are expressed a little bit further sideways.
So these tubes are "frontmost" and outward from these tubes, like sideways in the arms, is "backward" from "frontmost", but still "head".
Looking at other echindoderms, sea urchins are like starfish with their arms pulled upward, surrounding the "body". Sea lilies are like upside-down starfish on stalks.
Working out how this body plan emerged is a very challenging task, and some fossils of early echinoderms may have some clues.