Anfibios

Zoomorphology (2011) 130:97–106 DOI 10.1007/s00435-011-0124-1

ORIGINAL PAPER

Ultrastructure and phylogenetic signiWcance of the head kidneys in Thalassema thalassemum (Thalassematinae, Echiura)
Chiharu Kato · Janina Lehrke · Björn Quast

Received: 15 January 2011 / Revised: 4 March 2011 / Accepted: 7 March 2011 / Published online: 24 March 2011 © Springer-Verlag 2011

Abstract Recent molecular analyses consistently resolve the “spoon worms” (Echiura) as a subgroup of the Annelida, but their closest relatives among annelids still remain unclear. Since the adult morphology of echiurans yields limited insight into their ancestry, we focused on characters of their larval anatomy to contribute to this discussion. Electron microscopical studies of the larval protonephridia (so-called head kidneys) of the echiuran species Thalassema thalassemum revealed distinct correspondences to character states in serpulid polychaetes, although a close relationship between Echiura and Serpulidae is not supported by any phylogenetic analysis. The larval head kidneys of T. thalassemum consist of only two cells, a terminal cell and a duct cell. The terminal cell forms a tuft of six cilia projecting into the lumen of the terminal cell. The cilia are devoid of circumciliary microvilli. A Wlter structure is formed by two to three layers of elongate microvilli that surround the lumen of the terminal cell in a tubular manner. A thin layer of extracellular matrix (ECM) encloses the outer microvilli of the tubular structure. The tips of the microvilli project into the lumen of the adjacent duct cell but are not directly connected to it. However, mechanic coupling is facilitated by the surrounding ECM and abun-

dant hemidesmosomes. The distal end of the multiciliary duct cell forms the external opening of the nephridium; a specialized nephropore cell is absent. Apart from the multiciliarity of the duct cell, details of the head kidneys in T. thalassemum reveal no support for the