It is found in fish that feed in waters where sandy or muddy debris is present and in fish whose bulging eyes would otherwise be an impediment to streamlining. The tertiary spectacle is an all together separate layer of tissue over a complete eye, formed by the fusion of the eyelids which subsequently become transparent. This most advanced spectacle is found in the snakes and other reptiles as an adaptation to the constantly blowing dust and sand of their desert environment. The three different types of spectacle are examples of analogies - adaptations that perform the same function, but evolved independently through different evolutionary pathways. This material may be copied for use in student classroom projects and reports.
Limited permission is provided for teachers and educational web sites.
No other use of this material is allowed without written permission.