Passage 1 Scientists trying to determine where a starfish's head is have come to a startling conclusion: it is effectively the whole animal. This finding helps us understand how evolution generates the dramatic diversity of animal forms. Starfish, also known as sea stars, belong to a group of animals called echinoderms, which includes sea urchins and sea cucumbers. Their strange body plans have long puzzled biologists. Most animals, including humans, have a distinct head end and tail end, with a line of symmetry running down the middle of their body, dividing it into two mirror-image halves. Animals with this two-sided symmetry are called bilaterians. Echinoderms, on the other hand, have five lines of symmetry radiating from a central point and no physically obvious head or tail. Yet they are closely related to animals like us and evolved from a bilaterian ancestor. Even their larvae are bilaterally symmetrical, but radically reorganise their bodies as they metamorphose into adults. Laurent Formery at Stanford University says, “The morphology cannot tell you anything, almost.” He and his colleagues decided to look at a set of genes known to direct the head-to-tail organisation of all bilaterians. In these animals, the genes are turned on, or expressed, in stripes in the outer layer of the developing embryo. The genes that are expressed in each stripe define which point on the head-to-tail axis it will become. The team wanted to see if the gene expression patterns would reveal a “molecular anatomy” in starfish. To their surprise, the genes that determine the head end in bilaterians were expressed in a line running down the middle of each arm on the underside of a starfish. The next “head - most” genes were expressed on either side of this line. Even more strangely, the genes normally expressed in the trunk of bilaterians were missing in the outer layer of the starfish. This suggests that starfish have jettisoned their trunk regions and freed up the outer layer to evolve in new directions. The findings show that “the body of an echinoderm, at least in terms of the external body surface, is essentially a head walking about on its lips on the seafloor,” says Thurston Lacalli at the University of Victoria in Canada. What conclusion have scientists reached about starfish A. Starfish have a clearly defined head and tail. B. The whole starfish can be seen as its head. C. Starfish evolved from a non - bilaterian ancestor. D. Starfish larvae have no symmetry. What is the difference between echinoderms and bilaterians A. Echinoderms have a line of symmetry running down the middle of their body. B. Bilaterians have five lines of symmetry radiating from a central point. C. Echinoderms don't have an obvious head or tail physically. D. Bilaterians' genes are expressed in a more complex way. What did Formery's team find in their research on starfish A. The gene expression patterns in starfish are the same as those in bilaterians. B. The genes determining ... ...
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