The Ultrastructure of the Presumed Chemoreceptor Aesthetasc “Y” of a Cypridid Ostracode

The aesthetasc “Y” of cypridid ostracodes is divided into three parts: distal, intermediate, and proximal. Distally, it is equipped with five pores containing electron‐dense plugs. The distal part shows a peculiar striated pattern formed by two types of cuticle. The intermediate and proximal parts are smooth with no pores or other substructures. 14 cilia run in the large receptor‐cavity of the distal part. They originate from 14 bipolar sensory cell bodies within the antenna. From the intermediate part, the cilia are grouped in five cuticular sheaths, surrounded by five sheath cells arranged mesaxonally. At the level of the ciliary junctions, the sheath cells form cavities in which the cilia join the dendrites. Before molting, a new aesthetasc is formed in the antenna by hypodermal cells. The cuticle is secreted in a fold around the sheath cells, which in a way act as moulds. The cilia of the old aesthetasc run through the five pores of the new aesthetasc. The main function of the pores is apparently to enable the animal to obtain information from the surroundings also during the molting period. On morphological grounds, the aesthetasc “Y” is believed to be a chemoreceptor.