Head Kidneys in Hatchlings of Scoloplos Armiger (Annelida: Orbiniida): Implications for the Occurrence of Protonephridia in Lecithotrophic Larvae

Abstract
It is generally believed that lecithotrophic larvae of annelids do not possess functional excretory organs. However, as in certain annelids the planktotrophic trochophora larva has been secondarily modified into a lecithotrophic developmental stage and because protonephridia are characteristic for the trochophora, lecithotrophic developmental stages should also possess such organs. To test this assumption hatchlings of the orbiniidan Scoloplos armiger, which develops directly without a free-living larval stage, were investigated ultrastrucrurally. Each hatchling possesses a pair of protonephridia which lie caudal to the eyes and almost level with the frontal margin of the foregut. Each organ consists of three multiciliated cells, a terminal cell, a duct cell and a nephropore cell. The terminal cell bears a distally oriented hollow cytoplasmic cylinder, which surrounds the cilia. Adherens junctions connect this structure to the duct cell. Several clefts and pores perforate the wall of the hollow cylinder. Extracellular material covers the pores and clefts and thus may function as a molecular sieve during filtration. A comparison with the protonephridia of other annelid larvae reveals: (1) that one pair of protonephridial head kidneys consisting of a terminal cell, a duct cell and a nephropore cell must be assumed for the trochophore in the ground pattern of annelids and (2) that these organs are preserved when lecithotrophic larval stages evolved within the Annelida