SENSORY RECEPTORS AND EPIDERMAL STRUCTURES OF A MEIOFAUNAL TURBELLARIAN (PROSERIATA, MONOCELIDIDAE, MINONINAE)

The senogram (set of sensory receptors) of a proseriate turbellarian f rom 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, a nd at least eight (and possibly eleven) types of epidermal receptors: A, an anterior and antero-lateral receptor with basal bodies and stron gly branched processes (modified cilia); B, an 'insunk' collar recepto r in all parts of the body surface, with eight microvilli surrounding the single cilium; C, a ciliary bundle arising from basal bodies at th e base of a deep pit, in anterior third of body; D, a bundle of dendri tes, reaching the surface, in anterior third of body, possibly a proto nephridial pore; E, a multiciliate receptor with long branched ciliary rootlets, in anterior and antero-lateral part of body; F, as for E bu t with a single rootlet; G, as for F but with a vertical and an obliqu e rootlet arising from the basal body (F and G are possibly modificati ons of E); H, a receptor with a single cilium and short rootlet, somew hat raised above the epidermal surface; I, a pharyngeal receptor with a short cilium; J, a pharyngeal receptor with a long cilium; K, an ant erior receptor with a bulbous cilium. A single cilium or up to three c ilia surrounded by a cytoplasmic tube may constitute a further recepto r type. Most epidermal perikarya are 'insunk', and epidermal cilia hav e large anteriorly directed, very thin vertical rootlets and short 'sp urs' at the basal body. Bundles of rootlets of epidermal cilia converg e towards their tips, some tips located close to the epidermal surface ; it is suggested that they may be directly stimulated by mechanical s timuli. The variety of receptors indicates that the animal must respon d to a complex array of stimuli. It is stressed that morphological stu dies of sensory receptors are the basis for an understanding of their function and of the ecological requirements of a species.