crustacean is an animal that has a segmented body.
crustacean is an animal that has a segmented body.
what is an animal that has a segmented body?
crustacean
developmental biologists seek to understand how genes control the development of structural features through a cascade of processes in which key genes produce morphogens, chemicals that diffuse through the body to produce a gradient that acts as a position indicator for cells, turning on other genes, some of which in turn produce other morphogens. a key discovery was the existence of groups of homeobox genes, which function as switches responsible for laying down the basic body plan in animals. the homeobox genes are remarkably conserved between species as diverse as the fruit fly and humans, the basic segmented pattern of the worm or fruit fly being the origin of the segmented spine in humans. the field of animal evolutionary developmental biology ('evo devo'), which studies the genetics of morphology in detail, is rapidly expanding with many of the developmental genetic cascades, particularly in the fruit fly "drosophila", catalogued in considerable detail.
body plan
the discovery of "anomalocaris" ("abnormal shrimp") was a comedy of errors. the name was initially given to a fossil that looked like the rear end of a shrimp-like crustacean. walcott classified a ring-like fossil he called "peytoia" as a kind of jellyfish, and another poorly preserved fossil he called "laggania" as a holothurian (sea cucumber). after many plot twists, derek briggs started dissecting another ill-defined fossil in very thin slices and found a pair of "anomalocaris"-like structures on one end of a specimen of "laggania", which also had a specimen of "peytoia" attached just behind those of "anomalocaris". after dissecting more specimens and finding similar configurations, briggs and whittington concluded that the whole assemblage represented a single animal, which was named "anomalocaris" because that was the earliest name assigned to any of its parts. this animal's body was fragile and usually disintegrated before it could be fossilized. but the complete animal had tough grasping appendages ("anomalocaris"), a tough, ring-like mouth with teeth on the inner edge ("peytoia") and a long, segmented body ("laggania") with flaps on the sides that enabled it to swim with a mexican wave motion, and perhaps to turn quickly by putting the flaps on one side into reverse. this monster was over long when other animals were only a few inches at most. nedin suggested in 1999 that the animal was capable of taking heavily armored trilobites apart, possibly by grabbing one end of their prey in their jaws while using their appendages to quickly rock the other end of the animal back and forth, causing the prey's exoskeleton to rupture and allowing the predator to access its innards. in 2009 hagadorn found that anomalocarid mouthparts showed little wear, which suggests they did not come into regular contact with mineralised trilobite shells. computer modeling of the "anomalocaris" mouthparts suggests they were in fact better suited to sucking on smaller, soft-bodied organisms. although whittington and briggs concluded that "anomalocaris" did not fit into any known phylum, research since the 1990s has concluded that it was closely related to "opabinia" and to the ancestors of arthropods. in 2009 a fossil named "schinderhannes bartelsi", an apparent relative of "anomalocaris", was found in the early devonian period, about 100 million years later than the burgess shale.
fossils of the burgess shale