Carolsfeld, Joachim2024-08-152024-08-1519841984https://hdl.handle.net/1828/19601The behavioural repetoires of the two olindiad hydromedusae Gonionemus vertens and Eperetmus typus were tabulated, extracellular recordings of events associated with their behaviour were made, and an ultrastructural survey of all their tissues was carried out. Many of the control mechanisms of their behaviours are similar to those hypothesis ed for o t her hydromedusae, as summarized by Spencer & Schwab (1982), but some differences are present: 1) Both species lack the excitable exumbrella and endoderm which in other hydromedusae are involved in the mediation of a protective "crumpling" response. The behavioural pattern is absent in G. vertens but in E. typus it is present and is mediated by neuronal pathways in the bell margin and radii. The structure and distribution of gap junctions and septate junctions in the tissues of G. vertens are similar to those reported for hydromedusae with excitable epithelia, so these characteristics of the intercellular junctions appear independent of excitability. 2) Turning during swimming makes use of a unique subumbrellar sphincter muscle. 3) Nerves are present in the swimming muscle of the subumbrella and velum of both species. Circular conduction of the swimming event in the subumbrella of G. vertens is more rapid and more sensitive to excess Mg++ ions than is radial conduct ion, suggesting the event is propagated by neuronal conduction in a circular direction and myoid conduction in a radial direction. No capacity for independent spontaneous activity of the muscle sheets was observed. 4) In tentacles of G. vertens the events associated with local activity are distinct from those associated with coordinated activity both in shape and conduction parameters. This suggests double innervation of the tentacle muscles is present to permit both local and coordinated contractions. 5) Coordinated contractions of the tentacles are far less common in G. vertens than in E. typus and most other anthomedusae. "Cryptic" tentacle pulses are more common in G. vertens, indicating that the behavioural difference is due to electrical activity in the epitheliomuscular cells of the tentacle ectoderm that does not elicit visible contractions. 6) Feeding of G. vertens relies on a number of possible "coordinating" mechanisms and a large amount of local activity. Pointing appears to involve coordination by the "activation" of local pacemakers; sensory input seems to coordinate the swimming component of feeding; and enhanced general "excitation levels" in the animal are reflected by overall increased tentacle activity and "coordination" of radial muscle contractions. 7) Somersaulting is unique to G. vertens, but there is no evidence for unique musculature or nervous structures. This behavioural pattern probably relies on the shape and density of the medusa's bell. 8) Tentacle anchoring in G. vertens seems to result from a coordinated control of secretion of an adhesive. Anchor detachment is apparently not centrally coordinated. Other morphological characters of these two species that have not been reported in hydromedusae before include mesogleal spurs and supportive cells at the base of the velum, possible paddle cilia on the velum and gonads of G. vertens, ciliated chambers in the exumbrellar bell margin of E. typus, and supportive cells between the "motor giants" of the inner nerve ring of E. typus. These observations suggest that hydromedusae harbour far greater morphological and neurophysiological diversity than is commonly recognized.301 pagesAvailable to the World Wide WebThesis