Ultrastructure of the statocyst of the marine enchytraeid Grania americana (Annelida: Clitellata)

The enchytraeid Grania amrricana has a dorsal statocyst within the peristomium. It is connected to the anterior end of the cercbral ganglion. Transmission electron and light microscopy were used to study this region of the head. The statocyst is divided into four membrane-bound chambers; each contains an electron dense statolith surrounded by numerous sensory cilia, so that it is a multi-chambered or compound statocyst. Cilia within the statocyst possess a 9 × 2 + 0 microtubule arrangement, with one of the doublets in a central position. All cilia that were observed in the statocyst and in the cerebral ganglion had this arrangement of microtubules. This is the first description of a statocyst in the Clitellata. The statocyst of G. americana is compared to those described in polychaetes and may provide useful information for phylogenetic and taxonomic studies of the genus, of clitellates, and of annelids.