The nauplius eye and frontal organs of the non-Malacostraca (Crustacea)

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Abstract
The present work is a continuation of the morphological studies of the nauplius eye and frontal organs of the Crustacea. The previous papers on this topic comprised the Decapoda (ELOFSSON, 1963) and the Malacostraca (ELOFSSON, 1965). Its has been found that the nauplius eye and frontal organs of the Crustacea are separated into four different types. These comprise the Phyl-lopoda, Anostraca, Maxillipoda-Ostracoda and Malacostraca. Frontal organs do not appear in the maxillopod-ostracod group, but are present as paired dorsal and ventral frontal organs in the Malacostraca, paired ventral frontal organ in the Anostraca, and paired distal and unpaired posterior medial frontal organ in the Phyllopoda. The frontal organs of the different groups are not homologous. Their nature as reduced frontal eyes is maintained. The nauplius eye (and frontal organs when present and developed as eyes) shows, for instance, the following dissimilarities. The sensory cells of the eyes of the Malacostraca are everse with rhabdomeres at their sides combining into rhabdoms. The remaining groups have inverse sensory cells and rhabdomeres around the distal tip of the cells. Each cup in the maxillopod-ostracod line has tapetal and lens cells which are not found in the other types. The phyllopod group has a unique arrangement of the eye and frontal organs, depending on the typical and special development of the organs. Due to the structural dissimilarities and lack of transitional forms the frontal eyes (= nauplius eye and frontal organs) of the four different groups are considered non-homologous, and this view is discussed with regard to morphological, phylogenetical and functional viewpoints.