Symmetry
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photograph of a sea urchin's testAlong with most echinoderms, sea urchins are radially symmetrical, that is, built on a circular plan, rather than a bilateral one.  Advantages of radial symmetry in an organism include food-catching, predator-sensing, and locomotion done equally well in any direction. Disadvantages are that in the absence of a head end, a foremost cluster of sensory devices and nerves (brain) is lacking. If you look carefully at the test of sea urchin in the photograph, you may be able to see the madreporite near the central hole at the 7o'clock position, which is the only disruption in an otherwise perfect radial symmetry of the test.

NOTE
  symmetry is the arrangement of body parts into geometric design.  Imagine that you’re having apple pie with a cup of tea.  With straight-down cuts of a knife you can slice your pie into an infinite number of like-halves (i.e., symmetrical halves): the pie has radial symmetry.  In comparison, your teacup can be divided into like halves with only one slice – straight down through the top of the cup and bisecting the handle. The cup has bilateral symmetry

NOTE lit. “two sides” L.; bilateral invertebrates, such as crabs and worms generally have a head end, on which are clustered things like mouths, eyes, antennae, tentacles, and so on. In association with these, there are nerve clusters within the head known as cerebral ganglia or, if they are big enough, as brains.  For these reasons, bilateral animals tend to move with their heads in front, the better able to sense their environments

 

 

 


 

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Research study 2

 

photograph of a sea urchin Strongylocentrotus franciscanus and a sea cucumber Parastichopus californicusDespite the absence of a head end and associated sense organs, sea urchins are by no means sensory-limited.  Vision is lacking, but special cells embedded in the skin provide light/dark perception, and responses to shadows can be quite sensitive.  Other sensory cells in the skin provide perception of water-borne chemicals, and of pain.  Since sea urchins will right themselves after being flipped over, there must also be balance receptors, although these have yet to be described.  A sea urchin is able to distinguish different tastes of foods, can perceive predators from a distance, and is sensitive to the presence of gametes in the surrounding seawater. Without a centralised nervous system such as a brain, sea urchins cannot learn from experience.


Two different evolutionary strategies: a radially symmetrical red urchin Strongylocentrotus franciscanus and a secondarily derived bilaterally symmetrical sea cucumber Parastichopus californicus. Although they lack a head, sea cucumbers generally move with their mouth end foremost 0.25X

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