Mantis Shrimp Eyes! And some other bits.
In addition to their amazing arms, the Stomatopods crustaceans enjoy a number of other unique and amazing features. Nannosquilla decemspinosa, for example, will curl into a circle and roll away after retreating water if left behind by the tide. And the planktonic larva of some species are golf-ball sized spheres with 4 razor-sharp spikes sticking out.
But it's their eyes - their amazing, unmatchable eyes - that continue to stun researchers.
Not only is each eye perched on a movable stalk, but the facets on each are so arranged that the thumbsplitter can watch you from three different directions from each eye. This gives them incredibly good triangulation on their prey, or your finger.
Further, the eye is filled with an incredible assortment of pigments. Human eyes can detect a mere 3 colours, and combine the results to give us what we laughably call full-colour vision.
The midband of each eye consists of six parallel strips - the first four have eight different types of light-sensitive cell, with pigments that respond to different wavelengths of light. With these, the prawn-killer's vision extends from the ultraviolet to the infra-red. And they've got tunable filters to match light conditions.
Rows 5 & 6 have photoreceptors for polarised light - that is, light vibrating in just one direction - like a piece of string tied to a hook and wriggled up and down. We already knew about their ability to detect linear polarisation - indeed, lots of animals use it - bees for example use the polarisation pattern of the sky to determine bearings even on partly clouded days. But mantis shrimp go one step further - and are the only animals that can detect circularly polarised light - light that twists in a helix, one way or the other.
This gives them even more advantages over other predators, and animals that would try to eat them. Also, by signaling to each other with circularly polarized pigment patterns on their bodies, they have a communication channel that their predators won't be able to detect - a private channel for them to use for such messages as "Hey baby, come check out my pleopods". You'll be pleased to know, Rose, that your Peacock Shrimp is even more sensitive to this secret channel than most species of stomatopod. But I hope that you weren't planning on keeping anything else in the same tank, because they're not very community-minded beasts.
Details and more info here, at Not Exactly Rocket Science