(OSTRACODA: MYODOCOPA) FIRST ANTENNA

The halophore, halothalium, and s-seta are termed herein and occur on the myodocopine first antenna, probably the most systematically significant myodocopine appendage. The morphology of the exoskeleton and distribution of these structures are described, following scanning electron and light microscopic analyses and literature comparisons. The movements of these and other myodocopine first antennal structures are studied, using video recordings of an exemplary species. Halophores are setules with a characteristic exoskeletal ultrastructure comprising a layer of very fine rings, with walls approximately circular in cross section, covered by an outer, probably elastic, layer or sheath. There is a single pore at the halophore tip. This organization permits great flexibility. If dendrites innervate halophores, the flexibility may aid in sampling for water-borne chemicals or mechanoreception. The s-seta (formerly the sensory seta) is a seta arising from the fifth article of the myodocopine first antenna that frequently possesses numerous long halophores. The sseta is always and only present in the Myodocopina. The collective halophores distributed over the whole of one first antenna are termed the halothalium. Analogies of the structures described in this study are made with other crustacean structures.