The California sea hare, Aplysia californica, is a very large marine gastropod which can sometimes reach 75 cm in length. This species has been given priority in sequencing of the whole genome by the National Human Genome Research Institute. It is the first and so far the only marine gastropod which has this level of significance for genetics.
The individual in this image has paused and is rearing up, perhaps to investigate its surroundings. Normally the front part of the foot of the animal is kept on the substrate.
This sea hare has a small and soft internal shell that is made of conchiolin. The two horn-like or ear-like structures on the top of the head (which cause the resting animal to slightly resemble a hare or rabbit) are the rhinophores, which are used primarily for chemoreception.