Hesiolicium gen.n. (Turbellaria Acoela) and Observations on Its Ultrastructure

Abstract

Crezée, M. (University of Florida Marine Laboratory, Gainesville, FL 32611, USA) and Tyler, S. Hesiolicium gen.n. (Turbellaria Acoela) and observations on its ultrastructure. Zool. Scr. 5 (5): 207–216, 1976. —Hesiolicium inops gen.n., sp.n. is a thin and elongate protandric hermaphrodite from the marine interstitial environment. The male pore, with no accessory organs, lies just posterior to the mouth, with no pharynx. The dorsal ovary, lateral testes, adhesive papillae and a caudal organ are other features. The posterior and midventral adhesive papillae are bundles of haptocilia up to 22 μm long. In a crook near their base these adhesive cilia have three microtubule doublets displaced from the rest of the axoneme, giving the crook region a figure‐eight cross section. The caudal organ is a complex of cilium‐bearing sensory cells, large gland cells and modified muscle and epidermal cells. The rootlets of epidermal cilia and the two axial units of the sperm are as described for other acoels. Principally because of the haptocilia, Hesiolicium is provisionally placed in the Paratomellidae.

Morphology of the Turbellaria at the ultrastructural level

“…Some information is also available on more complex glandular-sensory organs in Turbellaria (Rieger & Tyler 1974;Crezee & Tyler 1976) but very little is still known about the structure of the complex frontal organ, which according to Ax (1963) and Karling (1974) is a characteristic feature of the Turbellarian archetype (see Ruppert 1978, for fine structure of frontal organ in polyclad larvae).…”

“…Although fine structural information was used early in combination with light microscopy for a species description (Reisinger 1968), it is only recently that this approach has been used more frequently (e.g. Sterrer & Rieger 1974;Rieger & Sterrer 1975;Crezee 1975;Crezee & Tyler 1976;Lanfranchi 1978). Today about one third of the ultrastructural information deals with some aspect of the body wall, about one fifth with gametes and another fifth with regeneration.…”

Comparative ultrastructure of the epidermal ciliary rootlets and associated structures in species of the Nemertodermatida and Acoela (Plathelminthes)

“…Some of the most significant ultrastructural characters of species of the Acoela and the Nemertodermatida are the shelf-like structure on the distal end of the epidermal cilia and the system of interconnected epidermal ciliary rootlets. The distal shelf of the cilia was first described in members of the Acoela by Tyler (1973) and Crezee and Tyler (1976). The presence of a similar distal ciliary shelf in species of the Nemertodermatida was described by Tyler and Rieger (1977).…”

A commissural brain! The pattern of 5-HT immunoreactivity in Acoela (Plathelminthes)

“…The current knowledge about the neuroanatomy of Acoela has been based on studies using conventional staining techniques (Westblad 1948;Dörjes 1968Dörjes , 1970Reisinger 1976), the acetylcholinesterase (AChE) method (Mamkaev and Kotikova 1972;Boguta 1976;Raikova and Kotikova 1991) and ultrastructural studies (Ferrero 1973;Crezée and Tyler 1976;Ehlers 1985;Bedini and Lanfranchi 1991;Raikova 1991). Noticeable morphological differences in the organisation of the nervous system of closely related Acoela have been observed (Ivanov and Mamkaev 1973).…”