ULTRASTRUCTURAL ASPECTS OF NERVOUS-SYSTEM AND STATOCYST MORPHOGENESIS DURING EMBRYONIC-DEVELOPMENT OF CONVOLUTA-PSAMMOPHILA (TURBELLARIA, ACOELA)

The notion that statocysts originated from an infolding of ectoderm lined by ciliated sensory cells has been challenged with evidence of 'capsule'-limited, non-ciliary statocysts in several independent phyla. Statocysts in turbellarians primitively lack cilia and are embedded within or closely adjoined to the cerebral ganglion; they are likely to be derived from nervous tissue. We investigated the development of the simple statocyst in an acoel turbellarian, a statocyst consisting of three cells. Observations of serial TEM sections of embryos at different stages of development support the hypothesis of an inner (non-epithelial) origin of the statocyst. First, a three-cell complex is delimited by a basal lamina; it then undergoes cavitation by swelling, autophagy, and fluid secretion. The statocyst becomes discernible within the precursor ganglion cells while they still contain yolk inclusions. The two outer (parietal) cells, enclosed together by a 10-nm-thick basal lamina, arrange themselves in an ovoid of about 10-mu-m diameter and surround the inner statolith-forming cell. The statolith is formed later within vacuoles of the statolith-forming cell.