The larval crustacean dorsal organ and its relationship to the trilobite median tubercle

Abstract

In the dorsal median anterior region of the head, the larvae of decapod Crustacea possess a discrete organ: a glandular-sensory complex which is not present in post-larval juvenile forms. Its external topography reveals the presence of one or two central pores (Brachyura, Anomura) or a poreless central area (Macrura) surrounded by four equidistant plate-pits which contain a central pimple or ''nipple''. The Macrura plate-pits lack the central pimple but develop a row of diminutive pegs. No definite function can yet be ascribed to the organ. The dorsal organ described here in Decapoda larvae closely resembles the median sensory tubercle found on the cephalon of some trilobites, especially Trinucleidae. In view of the possible homology between these structures in the two groups, previous interpretation of the trilobite organ as a median eye tubercle must be rejected.