Larval and post-larval development of the branchiopod clam shrimp Cyclestheria hislopi (Baird, 1859) (Crustacea, Branchiopoda, Conchostraca, Spinicaudata)

The larval and post-larval development of Cyclestheria hislopi is examined by SEM. There are at least nine stages (excluding the adult) - six larval a nd three post-larval stages. The first four stages are passed within the eg g-membrane. The larval and the post-larval phase are separated by a profoun d change in morphology that takes place between stages VI and VII. The larv a shifts from a dorso-ventrally flattened 'larval' appearance up to stage V I to a laterally flattened, more 'adult' appearance from stage VII. New mor phological data have been revealed by this study, including (1) a large and globular larval dorsal organ; (2) the carapace starts its development from the segments of the first and second maxillae; (3) the anterior ramus of t he second antenna in adult Cyclestheria hislopi is the endopod, and the pos terior ramus the exopod. Direct development of the brood in Cyclestheria hi slopi unique among conchostracans - is compared with that of the Cladocera. If Cyclestheria is the sister group to the Cladocera, as favoured in this work, the classical neoteny theory of the Cladocera must be reconsidered, a s there is no particular similarity between any adults of the Cladocera and any of the larval stages of Cyclestheria. It is suggested that Cyclestheri a displays the type of development present in a cladoceran ancestor. A comp arison between Cyclestheria and the Upper Cambrian 'Orsten' fossil Rehbachi ella kinnekullensis reveals a remarkable similarity in the endite morpholog y of the trunk limbs.