XIV. The Lantern and Girdle of Some Recent and Fossil Echinoidea

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Abstract
Two centuries ago Klein (1734) proposed a classification of the Echinoidea based upon the position and character of the “ mouth. ” The clumsy terms “ Emmesostomi ” and “ Apomesostomi ” that he applied to the two main sections of the class have fortunately lapsed ; but the situation of the peristome has been recognized as fundamental in all subsequent schemes of classification. Like Klein’s collateral scheme based on the position of the periproct, this principle has the advantage of being applied with equal ease to recent and fossil forms. In a general way, a central position of the peristome may be taken to imply the presence of jaws, while an eccentric position implies their absence. Thus a classification on that basis divides the Echinoids into two sections which have very different habits. The types that have jaws live in the open, scrambling among rocks, while those without jaws shuffle over silt or may burrow into it. The gnathostomatous condition precedes the atelostomatous both in palaeontology and ontogeny, although both conditions are abundantly represented in the modern fauna.

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