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.
The pterothoracic nota (comprising the meso- and metathoracic segments) have two main divisions - the anterior wing-bearing alinotum and the posterior phragma-bearing postnotom.
[1] The phragma, or endotergite, is a transverse infolding of the intersegmental sclerite, and its main function is to provide a reinforcing basis of attachment for the dorsal longitudinal muscles.
However, there are other groups of insects in which, for reasons other than flight, the pronotum is hypertrophied, such as in all beetles (Coleoptera), and most treehoppers (family Membracidae, order Hemiptera).
Nudibranchs are a large group of sea slugs, shell-less marine gastropod mollusks in the clade Nudibranchia.