Have you ever wondered why jellyfish are transparent?

Because the body wall of these primitive creatures is formed just of two layers of cells...

But even so, they come with surprising characters for such simple creatures...

For example, the highly venomous box jellyfish has a set of special eyes, similar to the human eyes own, that enable them to avoid obstacles as they swim close to the ocean floor.

Unlike the true jellyfish, which are drifted by the ocean current, box jellyfish swim actively and can make rapid 180-degree turns and dart with speed between objects.

Their ability is suspected to be due to one set of their 24 eyes that offer them an image of the objects they meet.

"Behavior-wise, they're very different from normal jellyfish," said study leader Anders Garm of Lund University in Sweden.

The box jellyfish's eyes are placed on cup-like organs hanging from their cube-shaped bodies.

Vertebrates (human included) have multi-purpose eyes that detect color, size, shape and light intensity, but box jellyfish presents four different varieties of special-purpose eyes.

The simplest set perceives just light levels, but the most complex set of eyes records the color and size of objects.

One complex eye is found on the top of the cup-like structure, the other on the bottom, so that the jellyfish has "an extreme fish-eye view, so it's watching almost the entire underwater world," said Garm.

To test the performances of these eyes, Garm experimented with the jellyfish put in a flow chamber endowed with various objects to assess if the jellyfish could detect them.

The jellyfish was able to avoid things of various colors and shapes, but they cold not detect transparent objects.

"They can't respond to the see-through ones. Because jellyfish belong to one of the first groups of animals to evolve eyes (the phylum Cnidaria), understanding how their eyes operate will show scientists what eyes were like early in evolutionary time," said Garm.