ABSTRACT
Heterobranch land snails include some of the most endangered animals on earth, accounting for a large proportion of known extinctions. This chapter outlines aspects of the diversity, natural history, structure, evolution, and classification of the Heterobranchia. The concept of the Heterobranchia is relatively new, being originally proposed by Gerhard Haszprunar. The name Heterobranchia is widely accepted, although an alternative name Heterostropha is used by a few palaeontologists. Most phylogenetic treatments of heterobranchs focused on either ‘pulmonates’ or ‘opisthobranchs’, so questions remained about relationships within the Euthyneura. The sister group relationships of the major heterobranch groups, and of the Heterobranchia as a whole, has been, and remains, controversial. The origin of heterobranchs is probably located deep in gastropod evolution and separate from, although possibly sister to, the caenogastropods. While most of the ‘lower heterobranchs’ closely resemble other shelled gastropods, at least in external features, the euthyneuran heterobranchs have undergone more departures from the original gastropod ground plan than any other group.
