Sunday, March 16, 2008

A thing I did not know this morning

Iguanas have three eyes.

We were at a pet expo today. Older Monkey gets all excited about critters with not enough fur, not enough legs etc so he was haunting the reptile table. While he was debating the wisdom of adopting a rat snake [1] I peered at an iguana the size of a Germen Shepherd and thought that, hunh, that thing on it's forehead looks like an eye.

And it is!
The two species of lizard within the genus Iguana possesses a dewlap, a row of spines running down their back to their tail, and a third eye on their head. This eye is known as the Parietal eye, which looks just like a pale scale on the top of their head.

The parietal eye is a part of the epithalamus, which can be divided into two major parts; the epiphysis (the pineal organ, or pineal gland if mostly endocrine) and the parietal organ (often called the parietal eye, or third eye if it is photoreceptive). It arises as an anterior evagination of the pineal organ or as a separate outgrowth of the roof of the diencephalon. In some species, it protrudes through the skull.[4] The parietal eye uses a different biochemical method of detecting light than rod cells or cone cells in a normal eye.[5


[1] It's home, in an aquarium tank, in his room. I'm not a huge fan of the thing but it's small and harmless, he spent his own money on it and it's not going to make messes on the floor so what the hey? Plus if it gets loose and lost in the house it can spend it's days in the basement killing rodents - it's a win-win!
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