What the difference between an inverted and everted retina?

Question

I'm looking specifically for examples and comparisons, because the definitions of inverted and everted are remarkably similar; e.g. from Dictionary.com/Reference.com:

everted
verb (used with object) to turn outward or inside out.

http://dictionary.reference.com/browse/everted

vs. (esp. #5)

inverted verb (used with object) 1. to turn upside down. 2. to reverse in position, order, direction, or relationship. 3. to turn or change to the opposite or contrary, as in nature, bearing, or effect: to invert a process. 4. to turn inward or back upon itself. 5. to turn inside out.

http://dictionary.reference.com/browse/inverted

I'm curious because of this:

Structurally, the cephalopod eye differs from the vertebrate eye. Cephalopods do not have a cornea, and the retina is everted so that the distal end of photoreceptive cells lies directly behind the lens and points toward incoming light. As a result, photoreceptor cells connect to the optic nerve behind the retina (Packard 1972).

From: Evolution: Education and Outreach October 2008, Volume 1, Issue 4, pp 439-447

http://link.springer.com/article/10.1007%2Fs12052-008-0084-1 http://link.springer.com/article/10.1007%2Fs12052-008-0084-1/fulltext.html

via

https://en.wikipedia.org/wiki/Cephalopod#cite_note-Serb2008-12

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If you have the rep please tag this with cephalopod; the tag doesn't exist and I don't have enough rep to create it. Thanks.

Answer 1

Based on your title and description, all that means is that in vertebrates the retina is inverted in a sense that light sensing cells sit at the back of the retina meaning light has to pass through layers of neurons and capillaries before reaching light sensing cells, whereas cephalopod has the photoreceptors at the front side of the retina, with processing neurons and capillaries behind them hence retina is everted i.e. inside out. See this page (http://en.wikipedia.org/wiki/Retina) under "In other animals" section.

Hope this helps!