Friday, October 19, 2012

Midterm exam

Chromodoris by David Doubilet
Well done, everyone!  I have a thick stack of exams to grade, and will get a good start on them this weekend.  What follows is a minor rewrite of what I posted at this point in the term last time I taught this class.  As my students recover from turning in their midterms and from the various viruses that seem to be sweeping campus, this slideshow of photos taken by David Doubilet  might be therapeutic.  But don't get so dazzled by those lurid colors and such that you stop thinking altogether. Remember that one of the special things about gastropods is that they exhibit torsion, a 180-degree twist of the visceral mass (including most internal organs and nervous system) relative to the foot. Among many consequences of torsion is the loss of the left post-torsional gonad and displacement of all the pipes that empty into the mantle cavity over to the right, downstream of the gills, which is a better place to dump your urine, feces, and gametes, after all. While nudibranchs and their sea-sluggy opisthobranch kin are detorted (untwisted) as adults, notice they all retain a penis that emerges from the right side. Yes, they ALL have a penis, being simultaneous hermaphrodites, and it's on the right side for all of them. A remnant of torsion past, both ancestrally and developmentally.

No comments: