Chapter 1, The Reptilian Spectacle - UWSpace - University of ...

Vision has been credited with sparking the incredible phyletic diversification of animals during the
Cambrian explosion 545 million years ago (Nilsson 1996; Land and Nilsson 2002). Given the
remarkable value of light perception and image formation, it is perhaps no wonder that most species
have evolved structures, behaviours and biochemical mechanisms to protect the integrity of their eyes
during the courses of their life cycles. While many invertebrates have eyes supported by hard chitinous
material (eg. arthropods) and others are able to regenerate damaged or excised eyes (eg. gastropods,
Flores Scarsso and Pellegrino de Iraldi, 1973), vertebrates have comparatively fragile eyes in that the
optically transmissive window to the outside world, the cornea, is rather delicate compared to their
integument and has limited regenerative abilities beyond the renewal of the epithelium and scarring of
the stroma. Vertebrates have thus evolved protective extra-ocular structures, with eyelids and
nictitating membranes (i.e. “third” eyelids) being the most familiar examples among terrestrial species.
Another protective structure that evolved among some vertebrates, terrestrial and aquatic alike,
consists of a layer of transparent integument overlaying the eyes, which acts as a permanent,
immovable shield against the external environment. These integumentary “spectacles” are found in
some fishes (which typically lack eyelids altogether) and a few amphibians, but find their greatest
terrestrial presence among reptiles, with some lizards having them, and snakes in particular being
ubiquitously equipped with them.
Given that there is no analogue to the spectacle in mammals (or birds), and that it thus
precludes spectacled animals as models for most human ocular conditions, it is perhaps no source of
wonder that gaps exist in our knowledge of it in such diverse areas as its anatomy, physiology, optics,
evolution, and its implications to ecology and ethology. This review aims to bring together the current
state of knowledge of the reptilian spectacle -- its anatomy and physiology, its adaptive significance,
and its evolution -- and in doing so to highlight areas of limited knowledge, some of which will be
addressed by the experiments described in the 3 chapters that follow. This review will begin with a
description of the anatomy of spectacles, from a somewhat historical perspective, to provide a context
for further discussions of their other biological characteristics.
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