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

The enchytraeid Grania americana has a dorsal statocyst within the peristom ium. It is connected to the anterior end of the cerebral ganglion. Transmis sion electron and light microscopy were used to study this region of the he ad. The statocyst is divided into four membrane-bound chambers; each contai ns an electron dense statolith surrounded by numerous sensory cilia, so tha t it is a multi-chambered or compound statocyst. Cilia within the statocyst possess a 9x2 + 0 microtubule arrangement, with one of the doublets in a c entral position. All cilia that were observed in the statocyst and in the c erebral ganglion had this arrangement of microtubules. This is the first de scription of a statocyst in the Clitellata. The statocyst of G. americana i s compared to those described in polychaetes and may provide useful informa tion for phylogenetic and taxonomic studies of the genus, of clitellates, a nd of annelids.