Blue crabs have eyes.
Peduncles are common in molluscs and crustaceans such as crabs, lobsters, and snails. The eye stalk is a long, thin appendage that extends from the animal’s head and ends with an eye. It can be easily moved so that the animal can look in any direction and can be extended or retracted at will.
When an animal with an eyestalk feels threatened, such as when the eyestalk 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 filled with blood, it extends, and when blood is drawn, it retracts.
Land snails, most of which are classified in the order Stylommatophora, have cobs.
In addition to providing an expanded field of vision, the eye stalk contains a number of hormones that affect different aspects of the creature’s life. Experiments were done in which the eyes of different animals were removed, and the removal affected much more than just visual ability. The specific hormones and their functions vary somewhat by species and sex, but in general hormones are related to the growth and reproductive activities of animals.
Blue crabs, for example, rely on hormones in their eyes to help regulate their growth. Since crabs have an exoskeleton, they need to change to grow. The blue crab eye affects how much and how fast these crabs grow, releasing hormones that regulate how often they shed their outer shells.
In spider crabs, the eyestalk has a significant impact on the ability to reproduce. 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.
In other creatures, the eyes perform a different function. Males of certain insect species, such as the stem-eye fly, use their eyes to attract a mate, and the larger the male’s eyes, the more likely he is to reproduce. It can also provide a lodging place for parasites. The amber snail is sometimes seen with a large striped eye. This is because it has been infected by a parasitic worm that settles there, causing the stalk of the eye to swell and making it impossible for the snail to retract.