The fine structure of the statocyst of the prosobranch molluse Pomacea paludosa.

Abstract

Ultrastructural observations on the statocysts of the prosobranch gastropod Pomacea paludosa are reported in this paper. 1. The static nerve consists of a great number of naked axons with a diameter of 0.2–0.5 μm. Several axons are joined together to bundles by glia cells (Fig. 1). 2. The epithelium of the statocyst contains 2500–3000 hair (= sense) cells, each with an area of at most 75 μm2 (Fig. 2). This great number of hair cells is compared with the number of receptor cells in other gastropod statocysts. 3. The hair cells are surrounded by smaller supporting cells. The significance of the interlacing of hair and supporting cells and of the possible contact between hair cells is discussed. 4. Supporting cells possess microvilli and sometimes one modified cilium (Fig. 5). The hair cells bear microvilli and 30–40 cilia. These cilia have the typical 9+2 arrangement of the filaments, striated roots to one side of the basal body and a basal foot to the other (Fig. 3). 5. For each sense cell and even for larger areas of the statocyst wall the basal feet point nearly in the same direction (Fig. 4). Hence it is suggested (a) a directional sensitivity for each hair cell and (b) a correlation between the areas of same polarisation and the “groups” of nerve fibers. 6. Different types of vesicles in the nerve layer of the cyst wall (Fig. 6) and stimulation experiments give some evidence for an efferent innervation of the statocysts.