Copulatory organ musculature in Childia (Acoela) as revealed by phalloidin fluorescence and confocal microscopy.

Copulatory organs of eight species of the monophyletic taxon Childia were investigated in detail, using phalloidin fluorescence method and confocal microscopy. Childia species were shown to have one, two or several tubular stylets, conical to cylindrical in shape, composed of few to numerous needles. The musculature varied greatly, from the absence of seminal vesicle to extensively developed seminal vesicles with several additional types of specialized muscles. Ten copulatory organ characters were coded and mapped on the total evidence tree. The data obtained permitted to follow the evolution of the Childia stylet and to demonstrate that the structure of the stylet apparatus is largely consistent with the phylogeny of the group (CI=0.75). Possible function of different muscle specializations was discussed.

A phylogenetic analysis of myosin heavy chain type II sequences corroborates that Acoela and Nemertodermatida are basal bilaterians.

Bilateria are currently subdivided into three superclades: Deuterostomia, Ecdysozoa, and Lophotrochozoa. Within this new taxonomic frame, acoelomate Platyhelminthes, for a long time held to be basal bilaterians, are now considered spiralian lophotrochozoans. However, recent 18S rDNA [small subunit (SSU)] analyses have shown Platyhelminthes to be polyphyletic with two of its orders, the Acoela and the Nemertodermatida, as the earliest extant bilaterians. To corroborate such position and avoid the criticisms of saturation and long-branch effects thrown on the SSU molecule, we have searched for independent molecular data bearing good phylogenetic information at deep evolutionary nodes. Here we report a phylogenetic analysis of DNA sequences from the myosin heavy chain type II (myosin II) gene from a large set of metazoans, including acoels and nemertodermatids. Our study demonstrates, both for the myosin II data set alone and for a combined SSU + myosin II data set, that Platyhelminthes are polyphyletic and that acoels and nemertodermatids are the extant earliest bilaterians. Hence, the common bilaterian ancestor was not, as currently held, large and complex but small, simple, and likely with direct development. This scenario has far-reaching implications for understanding the evolution of major body plans and for perceptions of the Cambrian evolutionary explosion.

Organisation of the nervous system in the Acoela: an immunocytochemical study.

In order to broaden the information about the organisation of the nervous system in taxon Acoela, an immunocytochemical study of an undetermined Acoela from Cape Kartesh, Faerlea glomerata, Avagina incola and Paraphanostoma crassum has been performed. Antibodies to 5-HT and the native flatworm neuropeptide GYIRFamide were used. As in earlier studies, the pattern of 5-HT immunoreactivity revealed an anterior structure composed mainly of commissures, a so-called commissural brain. Three types of brain shapes were observed. No regular orthogon was visualised. GYIRFamide immunoreactive cell clusters were observed peripherally to the 5-HT immunoreactive commissural brain. Staining with anti-GYIRFamide revealed more nerve processes than did staining with anti-FMRFamide. As no synapomorphies were found in the organisation of the nervous system of the Acoela and that of the Platyhelminthes, the results support the view that the Acoela is not a member of the Platyhelminthes.

The brain of the Nemertodermatida (Platyhelminthes) as revealed by anti-5HT and anti-FMRFamide immunostainings.

The taxa Nemertodermatida and Acoela have traditionally been considered closely related and classified as sister groups within the Acoelomorpha Ehlers 1984 (Platyhelminthes). Recent molecular investigations have questioned their respective position. In this study, the 5-HT and FMRFamide immunoreactivity (IR) in the nervous system of two nemertodermatids, Nemertoderma westbladi and Meara stichopi, is described. The 5-HT immunoreactive pattern differs in the two nemertodermatids studied. In M. stichopi, two loose longitudinal bundles of 5-HT-immunoreactive fibres and an basi-epidermal nerve net were observed. In N. westbladi the 5-HT-IR shows a ring-shaped commissural structure, different from the commissural brain of acoels. In both nemertodermatids, FMRFamide immunoreactive nerve fibres followed the 5-HT-immunoreactive fibres. It is demonstrated that the Nemertodermatida have neither a 'commissural brain' structure similar to that of the Acoela, nor a 'true', ganglionic brain and orthogon, typical for other Platyhelminthes. The question of the plesiomorphic or apomorphic nature of the nervous system in Nemertodermatida cannot yet be answered. The neuroanatomy of the studied worms provides no synapomorphy supporting the taxon Acoelomorpha.