Oral glands underwent substantial modification during the origin and diversification of snakes. Oral glands have provided rich data for snake systematics, and for informing evolutionary scenarios about the adaptive radiation of snake feeding. However, sampling has been patchy, and many questions remain about gland homology, function and evolution. This chapter addresses labial (supra- and infralabial), temporomandibular, rictal, sublingual, premaxillary, accessory and dental (= venom and Duvernoy’s) glands. We review and synthesize developments and data and present new histological sections and high-resolution tomography of some snakes and lizards, providing descriptions and illustrations of oral glands and associated structures. We comment on labial and dental glands of some toxicoferan and non-toxicoferan lizards, and report the first observation of a possible infralabial gland in a dibamian lizard. There are insufficient data to resolve all outstanding questions about gland homology across lizards and snakes, but the ancestral snake possibly had rictal and lacked dental (venom) glands, the latter perhaps evolving only within colubroidean caenophidians.

Venom, a specialized form of poison, is actively injected by the venomous organism into its target animal to facilitate several quotidian functions. Over a hundred convergent origins of this remarkable functional trait, along with intricate mechanisms of venom delivery, have been documented across animals. Pinpointing the emergence of venom in squamate reptiles has important implications for understanding the evolutionary history of snakes, but it has been challenging. Several competing hypotheses have been put forth to explain the evolutionary origin of squamate venom, including assertions of single, dual and multiple origins. In this chapter, in addition to a summary of this ongoing dialogue, we provide an overview of ecology, composition, delivery mechanisms, and evolutionary models that explain the possible origin and diversification of venom in squamate reptiles.