What is an eyestalk?

Blue crabs have eyes.

Eyestalks are common in molluscs and crustaceans such as crabs, lobsters and snails. The eye stalk is a long, slender appendage that extends from the animal’s head and ends with an eye. It can be easily moved so the animal can look in any direction and can be extended or retracted at will.

When an animal with an eye stalk feels threatened, such as when the stalk is touched, it retracts its eyes, bringing them closer to the head to protect them. It does this by pumping blood into or out of the eye. When it is filled with blood, it is extended, and when blood is removed, it is retracted.

Land snails, most of which are classified in the order Stylommatophora, have cobs.

In addition to providing an extended field of vision, the eye stalk contains a number of hormones that affect different aspects of the creature’s life. Experiments were done where eyes were removed from different animals, and the removal affected much more than just visual ability. Specific hormones and their functions vary somewhat by species and sex, but in general hormones relate to the growth and reproductive activities of animals.

Blue crabs, for example, rely on hormones in their eyes to help them regulate their growth. Since crabs have an exoskeleton, they need to change in order to grow. The blue crab’s eye stalk affects how much and how quickly these crabs grow, releasing hormones that regulate how often they shed their outer shells.

In spider crabs, the eye stalk has a significant impact on the ability to breed. When the scientists removed the eyes of some of these crabs, the females produced significantly more eggs than those with intact eyes. This led scientists to conclude that this part of the spider crab produces a hormone that regulates reproductive activities and egg production.

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In other creatures, the eyes perform a different function. The males of certain insect species, such as the stalk-eye fly, use their eyes to attract a mate, and the larger the male’s eyes, the more likely he is to be able to breed. It can also provide a hosting location for parasites. The amber snail is sometimes seen with a large striped eye. This is due to the fact that it has been infected by a parasitic worm that takes up residence there, causing the eye stalk to swell and making it impossible for the snail to retract.

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