The notum (plural nota) is the dorsal portion of an insect's thoracic segment, or the dorsal surface of the body of nudibranch gastropods. The word "notum" is always applied to dorsal structures; in other words structures that are part of the back of an animal, as opposed to being part of the animal's ventral surface, or underside. This word is used in entomology, the study of insects, and in malacology, the study of mollusks. In malacology the word is used to describe the back of the body of the taxonomic group of marine, shell-less gastropods that are known as nudibranchs.