Sensory Receptors and Epidermal Structures of a Meiofaunal Turbellarian (Proseriata, Monocelididae, Minoninae)

Abstract

The senogram (set of sensory receptors) of a proseriate turbellarian from the meiofauna of a high-energy beach in south-eastern Australia is described, based on serial ultrathin sections. It comprises a pair of rhabdomeric eyes that lack a pigment cup, a statocyst located ventro-anteriorly to the brain and consisting of one lithocyte with a single ovoid statolith in the electron-lucent cytoplasm, several parietal and accessory cells and nerve fibres penetrating the statocyst capsule, and at least eight (and possibly eleven) types of epidermal receptors: A, an anterior and antero-lateral receptor with basal bodies and strongly branched processes (modified cilia); B, an 'insunk' collar receptor in all parts of the body surface, with eight microvilli surrounding the single cilium; C, a ciliary bundle arising from basal bodies at the base of a deep pit, in anterior third of body; D, a bundle of dendrites, reaching the surface, in anterior third of body, possibly a protonephridial pore; E, a multiciliate receptor with long branched ciliary rootlets, in anterior and antero-lateral part of body; F, as for E but with a single rootlet; G, as for F but with a vertical and an oblique rootlet arising from the basal body (F and G are possibly modifications of E); H, a receptor with a single cilium and short rootlet, somewhat raised above the epidermal surface; I, a pharyngeal receptor with a short cilium; J, a pharyngeal receptor with a long cilium; K, an anterior receptor with a bulbous cilium. A single cilium or up to three cilia surrounded by a cytoplasmic tube may constitute a further receptor type. Most epidermal perikarya are 'insunk', and epidermal cilia have large anteriorly directed, very thin vertical rootlets and short 'spurs' at the basal body. Bundles of rootlets of epidermal cilia converge towards their tips, some tips located close to the epidermal surface; it is suggested that they may be directly stimulated by mechanical stimuli. The variety of receptors indicates that the animal must respond to a complex array of stimuli. It is stressed that morphological studies of sensory receptors are the basis for an understanding of their function and of the ecological requirements of a species.