You’re sleepin’ in the rain, and you’re always late for supper
Jellyfish have traditionally been considered simple and primitive. When you gaze at one in an aquarium tank, it is not hard to see why.
Like its relatives the sea anemone and coral, the jellyfish looks like a no-frills animal. It has no head, no back or front, no left or right sides, no legs or fins. It has no heart. Its gut is a blind pouch rather than a tube, so its mouth must serve as its anus. Instead of a brain, it has a diffuse net of nerves.
A fish or a shrimp may move quickly in a determined swim; a jellyfish pulses lazily along.
But new research has made scientists realize that they have underestimated the jellyfish and its relatives - known collectively as cnidarians (pronounced nih-DEHR-ee-uns). Beneath their seemingly simple exterior lies a remarkably sophisticated collection of genes, including many that give rise to humans’ complex anatomy.
These discoveries have inspired new theories about how animals evolved 600 million years ago. The findings have also attracted scientists to cnidarians as a model to understand the human body.
“The big surprise is that cnidarians are much more complex genetically than anyone would have guessed,” said Dr. Kevin J. Peterson, a biologist at Dartmouth. “This data have made a lot of people step back and realize that a lot of what they had thought about cnidarians was all wrong.”
Renaissance scholars considered them plants. Eighteenth-century naturalists grudgingly granted them admittance into the animal kingdom, but only just. They classified cnidarians as “zoophytes,” somewhere between animal and plant.
It was not until the 19th century that naturalists began to understand how cnidarians developed from fertilized eggs, their body parts growing from two primordial layers of tissue, the endoderm and ectoderm.
Other animals, including humans and insects, have a third layer of embryonic tissue, the mesoderm, wedged between the ectoderm and the endoderm. It gives rise to muscles, the heart and other organs not found in cnidarians.
Cnidarians also have a simpler overall body plan. Fish, fruit flies and earthworms all have heads and tails, backs and fronts, and left and right sides. Scientists refer to animals, including humans, with this two-sided symmetry as bilaterians. In contrast, cnidarians seem to lack such symmetry completely. A jellyfish, for example, has the symmetry of a bicycle wheel, radiating from a central axis.
print { Constance Jacobson }
