From the Glass Cabinet

Congratulations to Ben and Carrie for guessing the mouth part (Aristotle's lantern) of a sea biscuit (specifically Clypeaster rosaceus). Here is another photo showing the teeth of the mouth structure.

photo copyright 2011 by Heather Stoker

This Aristotle's lantern came from a sea biscuit that was (sadly) crushed when it was pulled up in a dredge offshore North Carolina. I don't have a picture of the animal, but it essentially looks like a really fat or bloated sand dollar. It is covered in short spines that are quite prickly on bare hands. This Aristotles' lantern was covered with algae that it had been eating.
The sea biscuit is considered an irregular urchin because, unlike regular urchins, it does not have its asshole on top of its head. That statement is totally unscientific and not very accurate. Let me try that again. The anus (and gonopores) are on the aboral surface (the side you see when the animal is creeping along the bottom) and the mouth is on the oral surface (the side you don't see). So the regular urchins have a mouth and anus in a direct line with each other- like the axis of their pentaradial symmetry. The irregular urchins have a mouth AND an anus on the oral surface, and just gonopores (where sperm or eggs are released) on the aboral surface. So really more bilaterally symmetrical.

Here's two photos of the aboral surface of an irregular urchin test/"skeleton"(a keyhole urchin):

and a regular urchin test (purple sea urchin):

photos copyright 2011 by Heather Stoker

A little hard to see, but the keyhole urchin has little dots (gonopores) and no larger hole (anus) in the middle. The regular urchin has an anus surrounded by the gonopores. (See January 3rd Invertebrate Limerick post for a nice close-up of this structure.)

Only irregular urchins from Clypeaster's order (and one other order) have the lantern and it is different from those of regular urchins in that it can not protrude and angle in different directions and is constructed of many fewer parts. Your classic sand dollars and the keyhole urchin shown above do NOT have this structure.

Here is a link to a scientific paper that will tell your more about the sea biscuit's Aristotle's lantern. It even has mathematical formulas and other information that is way over my head to explain about the breaking strength of this mouth structure. Here it is- very informative and interesting: http://www.bowdoin.edu/faculty/a/ajohnson/pdfs/EllersTelfordJawForcesJEB1991.pdf

Anyway, Congratulations again!