Sep 24, 2026
Brittle stars are relatives of sea stars and have no conventional head or centralised brain. Some species have light-sensitive cells distributed across their arms, allowing the entire body surface to contribute to sensing the environment. Their unusual visual system can help them respond to changes in light.
Source: wikimedia commons
Many flatworms have no clearly defined head in the way vertebrates do, yet some possess simple eyespots called ocelli. These light-sensitive structures are usually concentrated towards the front end of the body and help the animals detect changes in light intensity, particularly when searching for darker environments.
Source: wikimedia commons
Jellyfish have no brain and no head, but some species possess surprisingly sophisticated light-sensing structures. Box jellyfish, for example, have 24 eyes arranged around their bell. Their eyes include structures capable of detecting light, obstacles and movement, helping them navigate their environment.
Source: wikimedia commons
Scallops have no obvious head, but their mantle contains dozens to hundreds of tiny eyes. These eyes can detect changes in light and movement, helping the animal respond when predators approach. Their unusual eyes are built around a mirror-like structure rather than the lens arrangement familiar in human eyes.
Source: wikimedia commons
Sea cucumbers have no head or conventional eyes, but they can detect changes in light through photoreceptor cells in their skin. Some species can respond by hiding or changing their behaviour when exposed to bright light, demonstrating that seeing does not always require conventional eyes.
Source: wikimedia commons
Sea stars do not have a head or brain, but they can still detect light. Tiny eyespots are located at the tips of their arms, allowing them to distinguish between light and dark and perceive rough shapes. Their distributed nervous system helps them respond to their surroundings without a centralised head.
Source: wikimedia commons
Sea urchins have no head or eyes in the usual sense, yet researchers have found that their entire body can function as a kind of visual system. Light-sensitive cells are distributed across their skin, allowing them to detect light and respond to visual stimuli without forming traditional eyes.
Source: wikimedia commons
The crown-of-thorns starfish has no head or central brain, yet it can still sense its surroundings through light-sensitive structures associated with the tips of its arms. Its unusual visual system helps it respond to changes in light and navigate its environment without the conventional head-and-eye arrangement found in many animals.
Source: wikimedia commons
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