Plathelminthes and Plathelminthomorpha — paraphyletic taxa

The concept of monophyly of Plathelminthes and Plathelminthomorpha (Plathelminthes and Gnathostomulida) is critically re-analysed. Based on new morphological and molecular evidence, it is concluded that both taxa are paraphyletic assemblages. The Acoelomorpha (Xenoturbellida?, Nemertodermatida and Acoela) are considered as the earliest offshoot of the bilaterian stem line, primarily lacking paired cerebral ganglia, an orthogonal nervous system and ultrafiltration nephridia. They are followed by Plathelminthes sensu stricto (i.e. the rhabditophoran turbellarians and Neodermata), Catenulida, and Gnathostomulida, the latter group is probably the sister group of the Syndermata (Rotifera and Acanthocephala). The respective characters, as well as the implication for the proposed stem species of the Bilateria are outlined and discussed.     

The basic organization of the Plathelminthes

The basic organization of the Plathelminthes is summarized. Special attention is given to epidermal structures, musculature, extracellular matrices, nervous system and sensory structures, digestive system, totipotent stem cells, protonephridia, reproductive system and life cycle. The latest common ancestor of the Plathelminthes lacked any parenchymal cells and tissues; Plathelminthes do not represent Parenchymia. Discussions concerning the relationships of the Plathelminthes with other Metazoa must be based on the characteristics of the plathelminth stem species.     

Notentera ivanovi Joffe et al., 1997: a contribution to the question of phylogenetic relationships between ‘turbellarians’ and the parasitic Plathelminthes (Neodermata)

Notentera ivanovi is a parasitic platyhelminth found in the gut of a polychaete, Nephthys ciliata. N. ivanovi has no mouth, pharynx, or intestine; the dorsal epidermis of adult animals forms a thick pad which is very similar to gut epithelia even at the light microscopic level. Structure of ciliary rootlets and dermal glands with peculiar striated secretion bodies point to close relationships with the Fecampiidae, though other important characters imply placing the genus Notentera in a separate family. In mature sperm of N. ivanovi, the axonemes are fully incorporated; they are directed from proximal to distal, as in the Neodermata. We argue that new family, Notenteridae, should be included in the taxon Fecampiida. We also suggest that all Plathelminthes with neodermatan type of spermiogenesis (Fecampiidae, Urastomidae, and Neodermata) form a monophyletic branch within the Plathelminthes Neoophora. This revised version was published online in July 2006 with corrections to the Cover Date.     

Glyoxylic acid induced fluorescence in the nervous system of Gyratrix hermaphroditus (Kalyptorhynchia,Polycystididae)

Catecholamines (CAs) are found in the neuropile of the brain, in 3 pairs of longitudinal nerve cords, in the transverse ventral commissure, in anterior ventral and dorsal nerves, in two pharyngeal nerve rings and in 24 neurons in the nervous system of Gyratrix hermaphroditus. The CA distribution pattern in compared with those of other neuroactive substances. Homology of neurons in the family of Polycystididae and in Plathelminthes in general is discussed.     

Plathelminths in tropical intertidal sediments of northeastern Australia

Assemblages of free-living interstitial plathelminths are described from the tropical coast of northeastern Australia. About one hundred species were recorded from sediments of mangroves, tidal flats, and seagrass beds. The majority (88%) of the plathelminths occurred in only one havitat. Richest in species were Kalyptorhynchia (39), followed by Acoela (17) and Typhloplanoida (16). Over 50% of the species found were represented by one to three individuals, but a few species were frequently encountered. The preferred habitat was sand with 86% of all species, while abundances were higher in muddy sediments. The mean species density was 7 in 10 cm^–2 of sediment, and abundances ranged from 8 to 52 individuals 10 cm^–2 with an average plathelminth abundance of 25 individuals 10 cm^–2. Predatory plathelminths were predominant with up to 77% of the individuals. A comparison is made to plathelminth assemblages in temperate tidal flats.     

Northern circumpolar distribution of brackish-water plathelminths

The composition of brackish-water faunas on both sides of North America shows a high degree of similarity with the corresponding faunas in European brackish waters in terms of species of the Plathelminthes. Twenty species are common to both the Atlantic coast of Canada and Northern Europe; a further nine brackish-water plathelminths are common to the Pacific coast of Alaska and Europe; and two species occur in all three areas. These patterns of distribution must be regarded as the result of past or present connections between the American and European populations of brackish-water species. The most parsimonious explanation is the hypothesis that a species-rich community of brackish-water plathelminths has a northern circumpolar distribution.     

Comparative morphology of statocysts in the Plathelminthes and the Xenoturbellida

The general fine-structural organization of statocysts in Catenulida, Nemertodermatida, Acoela, Proseriata, Lurus (Dalyellioida), and Xenoturbella are summarized. In lithophorous (statocyst-bearing) members of the Catenulida, the statocysts exhibit a few parietal cells and one or several movable statoliths within a spacious intracapsular cavity. Statocysts in the Nemertodermatida have several parietal cells and two lithocytes, each equipped with one statolith, whereas those of the other acoelomorphan taxon, the Acoela, always have two parietal cells and one movable lithocyte. The statocysts of lithophorous members of the Proseriata represent more sophisticated systems: each has two clusters of accessory cells in addition to several parietal cells and a voluminous lithocyte in which the statolith is movable. In catenulids and proseriates, processes of outer neurons penetrate the capsule of the statocyst, whereas such innervations have not been found in the Nemertodermatida and Acoela. I conclude that the different types of statocysts have evolved independently within the Plathelminthes. Xenoturbella displays an intraepidermal statocyst with many monociliary parietal cells and several mobile cells (lithocytes) within the central cavity of the statocyst. Each of these mobile cells carries a statolith-like structure and one prominent cilium. The statocyst of Xenoturbella does not correspond to any type of plathelminth statocyst.     

The ultrastructure of the eyes of<Emphasis Type="Italic"> Paravortex karlingi</Emphasis> (Plathelminthes,Rhabdocoela)

Both eyes of Paravortex karlingi belong to the rhabdomeric type. Each eye consists of a single pigment cup cell and three sensory cells. Modified mitochondria lie in three protrusions of the cup cell, and several of these mitochondria form giant mitochondrial derivates. The centres of the derivates include electron-dense substances. These modified structures may have lenticular functions. Furthermore, it is shown that giant mitochondrial derivates develop by fusion of smaller ones. Embryos have many small mitochondria in the developing eyes whereas the adult possesses less numerous, but giant forms. This circumstance leads to the assumption that all such lenticular structures within the rhabdocoels result from identical processes.     

REDUCED MALE ALLOCATION IN THE PARTHENOGENETIC HERMAPHRODITE DUGESIA POLYCHROA

Parthenogenetic lineages that arise in a hermaphroditic, sexual population will inherit the male function from their sexual progenitors. Natural selection then acts to reduce male allocation of the parthenogens, freeing resources presumably for the female function. Depending on age and the available genetic variation, one therefore expects to find reduced male allocation in naturally occurring parthenogenetic lineages. We investigated the allocation to sperm production in the hermaphroditic flatworm Dugesia polychroa in three lakes containing a sexual (S), a (pseudogamous) parthenogenetic (P), and a mixed sexual-parthenogenetic population (M). Parthenogenetic lineages from M were assumed to be relatively young due to recurrent origins from the coexisting sexuals, whereas those from P were assumed to be older on biogeographical grounds. As predicted, we found drastically reduced sperm production in parthenogens compared to sexuals, even in the parthenogenetic lineages from M, which may be younger. M parthenogens did not have more testes, but produced more sperm than individuals from the purely parthenogenetic population (P). However, the latter result could not be reproduced with laboratory-raised animals and therefore may be a consequence of different ecological conditions in the different lakes, for example, differences in mating rates. To study the behavioral component of male allocation, copulation frequencies were recorded for sexuals from M and for parthenogens from P. Compared to the drastic reduction in sperm production, copulation frequency was less reduced in parthenogens. This may be a consequence of allosperm limitation in pseudogamous parthenogenetic populations.     

Ultrastructure and formation of the bursa mouthpiece of Philocelis cellata (Plathelminthes,Acoela)

Philocelis cellata has a strengthened bursa mouthpiece which is arranged in front of the male copulatory organ. The main components of the bursa mouthpiece are numerous ring-shaped bursa mouthpiece cells whose central parts contain strengthened elements forming a tube around the sperm duct. Each of the peripheral areas of the bursa mouthpiece cells is separated by similarly ring-shaped gap cells. The end of the bursa mouthpiece towards the bursa is formed by a so-called sorting apparatus which consists of different cells; opposite the bursa the sperm duct ends in a globe-shaped sperm vestibule. The bursa mouthpiece is differentiated successively, beginning at the distal part at the bursa and proceeding proximally.