Nature Blog Network

Tuesday, August 14, 2007

Tuesday Toon

This weeks toon is brought to you by one of my readers, cow_2001. Click on image to go to the cartoon's webpage and view a larger version.


  1. Can you please clear something up to me on the matter of sea cucumbers?

    My invertebrates professor told us that, like starfish and sea urchin, the cucumber is spherically symmetrical, because it was like taking a sea urchin and elongate it.
    But it doesn't seem reasonable to me because they have a dorsal side and a ventral side, or they wouldn't be able to move on the sea bed, no? Surely, they're of bilateral symmetry.

    I'd be very grateful if you'll riddle me this, it's been bugging me since my first year's first semester...

  2. Good question meirav,

    Sea cucumbers are in fact much like urchins that are elongated along the oral-aboral axis. Remember that urchins and sea stars have a dorsal and ventral side too. The ventral side is where their tube feet and mouthparts are. Sea cucumbers also have tube feet and mouthparts on their ventral side, but their tube feets are modified for feeding and are located around the mouth.

    The Echinoderms are radially symmetrical, not spherical, meaning that a radius is drawn out from a central point of origin. Sea cucumbers are 5-rayed, like urchins and most sea stars. You can see this best in cross-section. These 5 rays are really longitudinal lines, called ambulacria, also with tube feet. These 5 ambulacria, 5 rows of tube feet, and the presence of ossicles in their in their leathery skin (reduced to small spicules which make them more felxible) are the other reminder of their sea star ancestry.

    I hope this helps, feel free to ask anything else about marine invertebrates.


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