Platyhelminthes
The nature of a controversial phylum

by Seth Tyler ©2003
University of Maine

Representative worms from the three major monophyletic groups within the phylum Platyhelminthes: the Acoelomorpha (represented by an acoel turbellarian), the Catenulida (represented by a retronectid turbellarian), and the Rhabditophora (represented by a digenetic trematode)

The relatively small worms that are commonly called flatworms are classified in the phylum Platyhelminthes, whose Greek roots mean, in fact, "flat worm." In general, the phylum encompasses worms that

They thus stand distinct from the major phylum of worms, the Annelida, whose members have a complete gut (with anus as well as mouth), segmented bodies with fluid-filled coelomic compartments, a cuticle-covered body wall, and muscles that arise from epithelial mesodermal tissue, and monoflagellated sperm (as do most other animals). But other phyla of small worms share some of the characters that otherwise set platyhelminths apart. Hermaphroditism, with reproductive organs as complicated as those of the flatworms, appear in the Gnathostomulida and Gastrotricha, for instance. Lack of a cuticle covering the body wall is also a feature of the Gnathostomulida (though the jaws of gnathostomulids are true cuticular elements which are completely lacking in platyhelminths), and so, too, is a sack-like gut a feature of the gnathostomulids (but it appears to be derived from a complete gut by virtue of evidence for a vestigial anus in gnathostomulids). Other worm phyla have cuticle (Gastrotricha, Nematoda, Nematomorpha, Kinorhyncha, Priapulida, etc.), multiciliated epidermal cells (all worm phyla but the Gnathostomulida), solid (acoelomate) bodies (Gastrotricha and miscellaneous representatives of Nematoda, Annelida, etc.), and fiber-form muscle cells. And though obviously a convergent similarity, the character of biflagellate spermatozoa is seen in some groups of fishes.

Nevertheless, the characteristics we can list for the phylum Platyhelminthes are not absolute distinctions--none apply to all groups together to the exclusion of other animals. Because no overarching characteristics (synapomorphies) for the phylum as a whole can be identified unequivocally, there is ground for considering the Platyhelminthes not to be a valid monophyletic phylum (Smith et al., 1986). At the moment, the largest monophyletic groups that can be identified among flatworms are three, the Acoelomorpha, the Catenulida, and the Rhabditophora, and while any two of these may be related as sister groups, the characters we could use to tie those two would exclude the third from falling into a logical phylogenetic relation with them. For example, the Catenulida and Rhabditophora appear to share homologies in the structure of the epidermal ciliary rootlets, in the mechanism by which they replace their epidermal cells, and in having protonephridia, but these homologies do not extend to the Acoelomorpha. The Acoela, on the other hand, appears to share with the Rhabditophora the homology of biflagellate spermatozoa, but since catenulids don't have such sperm, this homology would negate those homologies listed between Catenulida and Rhabditophora. The Acoela shares no apomorphies with both the Catenulida and Rhabditophora except possibly the presence of special stem cells called neoblasts (Rieger and Ladurner, 2001).

Turbellarian platyhelminths; representatives of 4 of the 11 orders of turbellarians

The current classification system for the Platyhelminthes is a cladistic one recognizing these three monophyletic groups and dispensing with the traditional division of the phylum into the four classes Turbellaria, Monogenea, Trematoda, and Cestoda. Turbellaria is, in particular, considered an invalid class because it is paraphyletic (Ehlers, 1985). (That is, because the parasitic classes arose from an ancestor that would be classified within the Turbellaria, the group Turbellaria is not monophyletic, not all of its descendants being encompassed within it.) Turbellarians are the largely free-living flatworms, those that don't parasitize other animals, while the other classes encompass the obligate parasites. Eleven orders of turbellarians are recognized in the commonly used classification that Hyman championed (Tyler, 1999): Nemertodermatida, Acoela, Catenulida, Haplopharyngida, Macrostomida, Polycladida, Lecithoepitheliata, Prolecithophora, Rhabdocoela, Proseriata, and Tricladida). Monogeneans are largely ectoparasites on vertebrates like fishes; trematodes are the flukes, most of which live inside the organs of vertebrates as adults; and cestodes are the tapeworms, living in the intestines of vertebrates as adults. The term "turbellarian" is still validly applied to the free-living flatworms, but the term "Turbellaria" (that is, the official, capitalized taxon name) should be written in quotation marks to indicate its paraphyletic status. Not all turbellarians are free-living, and so "free-living plathelminths," another name often applied to them by cladists wishing to avoid "Turbellaria," has some disadvantages; there are some highly specialized parasites among virtually all subgroups of the turbellarians.

Turbellarian platyhelminths; representatives of another three orders of turbellarians

The quintessential parasitic flatworms are the monogeneans, trematodes, and cestodes, and these constitute monophyletic groups, but by a cladistic classification, these monophyletic groups would not have the rank of class. Instead, if any monophyletic groups are to be considered classes, they would have to be the three major groups constituting the former "Turbellaria," namely the Acoelomorpha, Catenulida, and Rhabditophora. The major parasitic groups lie within the Rhabditophora, specifically within the monophyletic taxon Neodermata in that class. They are clearly closely related, descended from a common ancestor among the turbellarians.

Representatives of the three major groups of Neodermata (a tapeworm, a digenetic trematode, and a monogenean)

Exactly where the Neodermata came from among turbellarian platyhelminths is not at all clear; a number of potential ancestral groups have been proposed. The major advancement that allowed them to adopt a parasitic existence seems to be the neodermis which arises as the parasites attack a new host and metamorphose from the free-living larva. That larva has an epidermis much like that of turbellarians, composed of ciliated cells. When it locates a host, the larva sheds this ciliated epidermis and a new epidermis, the neodermis, emerges from cells situated below the muscle layer of the body wall. These cells fuse to create a syncytial covering over the entire body. The neodermis, thus, is an uninterrupted layer of syncytium whose nuclei lie in cytons below the body-wall musculature; on its apical surface facing the environment are specialized microvilli-like projections whose shape is specialized in each of the neodermate groups. The neodermis must offer advantages in a parasitic existence, allowing the parasite to absorb nutrients from the host (cestodes have, in fact, dispensed with the mouth and gut and gain all their nutrients from the host by absorption through the neodermis) and probably serving a dynamic role in defeating host immune reactions. Platyhelminths seem to be preadapted to developing such an epidermis in that even the turbellarians go through successive generations of epidermis in their embryonic development, and as adults they regenerate their epidermis by repacing cells that are lost with cells that migrate into it from the parenchyma below the muscles (Tyler and Tyler 1996).





The Acoela and its relationship to the Platyhelminthes

Convoluta pulchra Smith and Bush 1991, a typical mud-inhabiting acoel that feeds on diatoms

This group of quite small worms is so-named because of their lack of a gut cavity ("A" for non; "coela" for cavity [gut, in this case, not coelom]). Their digestive tissue is a syncytium that encompasses food items in large vacuoles rather than within an epithelially lined lumen as do the guts of other animals. Because no epithelium delimits the gut contents, they appear solid-bodied (acoel). (Some other turbellarian groups are even named according to the shape of their gut--the triclads with three-branched guts, the polyclads with many-branched guts, for instance.) By some older, no longer accepted theories, the syncytial nature of the gut in acoels allied them with the ciliate protozoans or with a hypothetical protozoan-derived phagocytic ancestor, but electron microscopy has shown that this syncytiality arises secondarily from fusion of progenitor cells among so-called wrapping cells in the parenchyma surrounding it. The digestive tissue in some acoels is cellular, with spaces between these cells serving as a sort of lumen but not an epithelium-bounded lumen as in other turbellarians (Smith and Tyler, 1985).

Almost all acoels live in marine habitats, mostly in between the grains of marine sediments but some swim in the plankton or creep on algae and other marine substrates. One feature that is immediately apparent in acoels is the statocyst, a glass-like sphere at the anterior end of the body in the brain, with a capsule surrounding a hemispherical concretion, presumably used to sense the direction of gravity and other acceleration. Also prominent in acoels are parts of the reproductive system, which, as in other flatworms, is a complicated hermaphroditic system with both male and female components. As with many other animal groups, classification of the acoel species is based on the variety of form in these reproductive organs. Like other flatworms, the sperm in acoels are biflagellate (having two tails or flagella).

Aphanostoma bruscai Hooge and Tyler 2003, schematic drawing by Matt Hooge of the internal organs reconstructed from sections

The seemingly primitive and varied nature of the nervous system in many acoels (that is, a nerve net and in many species without a well-differentiated brain) makes them good candidates for models of the ancestral condition of the nervous system among flatworms and other bilaterians (Reuter et al., 2001). But acoels have some peculiar features that set them off strongly from other flatworms as well as from other phyla of animals. No other animals have such a digestive syncytium instead of an epithelial gut, and the extreme reduction (if not complete absence) of the extracellular matrix is peculiar; acoels have no basement membrane under their epidermis or other epithelia, something that has parallels only among a few other groups among the lower rhabditophoran and catenulid flatworms, for instance. The cilia of acoels, the locomotory organelles by which they swim, have a unique ultrastructure as well. Unlike cilia of other metazoans, the epidermal cilia of acoels are interconnected by a complicated system of rootlets, and they also have a distinctive shape to their tips--a sort of abrupt narrowing set off from the main part of the cilium by a sharp shelf (Hendelberg and Hedlund, 1974; Tyler, 1979; Lundin, 1997).

Indications that these peculiar features are secondary specializations come from comparison of acoels with their closest relatives, the Nemertodermatida. Nemertodermatids resemble acoels in many respects and were once classified in a family within the Acoela. Their statocyst is double, however (having two stones within it), and their gut, while not as distinctly sack-shaped as other turbellarians, is epithelially lined (Smith, 1981; Smith and Tyler, 1985). Together the Acoela and Nemertodermatida are sister groups constituting the Acoelomorpha. Their sister-group relationship is strongly supported by similarities in their ciliation (nemertodermatids have their cilia interconnected by ciliary rootlets and have shelfed tips), and similarities in body-wall structure (reduction of extracellular matrix), reproductive organs (lack of sperm ducts and lack of female ducts), and the relation of the statocyst to the nervous tissue. In all of these characters, it is evident that the Nemertodermatida stands as the plesiomorphic sister group to the Acoela. The peculiar nature of the gut in acoels, therefore, is derived from an epithelial condition.

Studies that have tried to better place the Acoela phylogenetically using techniques of molecular systematics have relied on nucleotide-sequence data in the small and large subunits (SSU and LSU) of rDNA genes and, more recently, genes for myosin and fragments of the COI and Cytb genes in the mitochondria. Surprisingly, the rDNA data seem to indicate that the acoels are not at all related to the other platyhelminths, not even related to the nemertodermatids. Ruiz-Trillo et al.'s (1999) first study of this kind was sensational enough, the editors of Science thought, to warrant publication in that widely read journal, probably because the authors went so far as to claim that acoels are the most basal of all bilaterians--i.e., that all other bilaterally symmetrical animals (all animals but the sponges and cnidarians) arose from an ancestor that would be classified as an acoel. Such a claim is actually not new: Haszprunar (1996) also proposed a basal position for the Acoelomorpha on the basis of the lack of paired cerebral ganglia, lack of an orthogonal nervous system, and lack of ultrafiltratin nephridia. (With his cladistic analysis, he also splits the remaining platyhelminths into the separate monophyletic groups Rhabditophora and Catenulida and dismisses the phylum as paraphyletic because of its having descendants in the higher spiralians, among others.) Ruiz-Trillo et al. (1999) as well as Littlewood et al. (1999) base their conclusions on SSU-rDNA sequence data. All but one of the 18 acoel species these authors included in this study appeared to have such rapidly evolving SSU rDNA that they could not be used in the analysis that compared the Acoela to other phyla of animals. The only species that appears not to fall under this restriction is Paratomella rubra, which has, incidentally, been shown by morphological studies (Smith 1981, Smith and Tyler, 1985) to be a phylogenetically important species, so the conclusions about relationships of the Acoela to other flatworms and other phyla rest entirely on it---sequences of other acoels are apparently unreliable.

This conclusion was criticized for its lack of consideration of the wealth of morphological characters that contradict it (Tyler et al., 1999). Particularly the notion that the Nemertodermatida is unrelated to the Acoela is denied by many morphological synapomorphies that unite these taxa (Tyler and Rieger, 1977; Smith and Tyler, 1985) as mentioned above. That shortcoming in the molecular studies has since been mitigated somewhat by analyses of longer sequences (Jondelius et al., 2002; Telford et al., 2003), so that the Nemertodermatida is at least now acknowledged by molecular systematists as more closely related to the Acoela though, still, not considered its sister taxon. The rDNA implications have also been challenged on the basis of sequence data for the EF1-alpha (Berney et al., 2000), but that molecule, too, may not be a reliable indicator of relationships (Telford et al., 2003)

The basal position of Paratomella in molecular phylogenies has implications for the nature of the bilaterian ancestor, of course. Baguñà et al. (2001) and Ruiz-Trillo et al. (2002) have even gone so far as to argue that this ancestor had the same life cycle (direct, without a larva) and morphology (lacking gut and coelom) as the acoels. This notion that acoels can serve as models for the origin of the Bilateria is contradicted by the highly specialized nature of these worms (Smith and Tyler, 1985). They make rather poor intermediates between diploblasts and bilaterians. For example, the highly specialized nature of the digestive tissue in acoels is hardly intermediate between the gastrodermis of cnidarians and that of higher animals. So, too, the body wall of acoels,
Amphiscolops gardineri, a relatively large (2-4 mm), colorful acoel
with its lack of extracellular matrix (basement membrane), unique ciliary rootlets, and mesenchymal muscles can hardly be seen as intermediate between that of cnidarians (which have monociliated epitheliomuscular cells resting on a well-developed basement membrane) and such bilaterian phyla as the Annelida and Mollusca. Adopting acoels as intermediates between diploblasts and other bilaterians requires reversal and secondary derivation of many characters in epithelia, musculature, gut, and reproductive systems. Many of the coelomate bilaterians have epitheliomuscular cells in their coelomic lining, for instance, and if acoels are intermediate then such epitheliomuscular cells would have to have been reinvented in these higher animals after having been lost from the cnidarian ancestor of the acoels (Rieger, 1986). Neither can the complex hermaphroditic reproductive system of the acoels and its involvement in internal fertilization through copulation and a direct life cycle be seen as intermediate between lower diploblasts and the higher animals, many of which, like the cnidarians, have external fertilization through free-spawned gametes and pelagic larvae. The pelago-benthic life cycle, alternating pelagic larvae with benthic adults, is widely used by many animals and must be more primitive than the direct life cycle of the acoels (Rieger, 1986, 1994), a cycle that is likely a specialized adaptation to their wholey benthic existence in sediments.

The contradiction between the molecular data and the morphological may have arisen because the Acoela is such an ancient group, or because its rDNA sequence is so rapidly evolving that it has changed to a greater extent than is otherwise the case among animal groups--that is, it is subject in phylogenetic analyses to the so-called long-branch attraction problem (Joffe and Kornakova 2000; Ruiz-Trillo et al. [1999] report that acoels have rates of nucleotide substitution three to five times that of most other animals). Similar results from 18S rDNA have emerged in studies of other phyla, showing, according to Cavalier-Smith (1998), "how grossly misleading the rRNA tree can sometimes be."

Blair et al. (2002) have showed the flaw of revising phylogenies on the basis of one or a few genes or sequence signals like those applied to the Acoela. They tested the hypothesis of the Ecdysozoa which, also on the basis of 18S rDNA sequence data, placed the nematodes and insects together in a common clade, one sharing a more recent ancestor than either group has in common with other animals (Aguinaldo et al., 1997). A few other studies of some other genes have leant support to this hypothesis, and it has even been accepted in some textbooks and in a recent summary of the Tree of Life project (Pennisi, 2003) despite rather overwhelming evidence to the contrary from morphology. By expanding the basis of comparison to use more than 100 gene sequences now available from the genome studies of the nematode Caenorhabditis elegans, the insect Drosophila melanogaster, and mankind (Homo sapiens), Blair et al. (2000) found that the Ecdysozoa hypothesis is not supported. The erroneous concept of Ecdysozoa is, they say, apparently the result of rate bias, compositional bias, or some other artifact. The same could well be true, as morphological studies indicate, of the argument for a basal position of the Acoela separate from other platyhelminths.

The rDNA gene is highly useful, nevertheless, when it comes to deciphering relationships within the Acoela, that is, at lower levels of the taxonomic hierarchy. Hooge et al. (2002) have found, with a larger database of 18S-rDNA sequences from more species of acoels, a branching pattern in the gene tree that is well supported by morphological characters of sperm and body-wall musculature (see pdf of the correlation between the gene tree and morphological characters.



Conclusions

The Platyhelminthes may well prove to be polyphyletic, but conclusive evidence for that is not yet available. The most promising character potentially serving as an autapomorphy for the flatworms (and thus keeping them together as a phylum) is the stem-cell system of neoblasts (Rieger and Ladurner, 2001; Tyler and Hooge, 2003). In any event, it is likely that all three platyhelminth clades (Acoelomorpha, Catenulida, and Rhabditophora) originated through a process of progenesis from a large-bodied, coeolomate ancestor (Rieger, 1986, 1994; Tyler, 2001) even if those are independent origins. No other hypothesis of origins accounts for the histological structure of flatworms. Progenesis has played a major role in much of the evolution of the intersititial fauna (Westheide, 1987), and while this process can be readily recognized among a few taxa in which sufficient intermediates are known (the interstitital annelids, for instance), that for the flatworms seems to have produced such a drastic revamping of morphology that the origins have become obscurred. Because of this morphological obfuscation, the group is a prime candidate for molecular phylogenetics, but given their ancient nature, resolution of their phylogenetic position will probably have to await more complete knowledge of their genomes. Unfortunately, available data from the few genes we do now know is simply not sufficient.

Could the Acoela be the most basal group of the Bilateria? Yes, certainly, at least among extant animals. Did the ancestor of the Bilateria look or live like an acoel flatworm? Not likely. The Acoela are a specialized branch from that ancestor, and while they may be genetically more closely related to that ancestor than other living animals, that ancestor was probably a large-bodied coelomate organism. Present-day acoels are reduced in body size and have changed their mode of reproduction (and show all the morphological modifications those changes require) probably in adaptation to a benthic life in sediments. Perhaps other platyhelminths adapted similarly from another ancestor, but many of their similarities are rather difficult to explain as convergence. At the moment, the phylum Platyhelminthes, with its three clades Acoelomorpha, Catenulida, and Rhabditophora, still stands as the best solution to classifying these worms.



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2002 version of this article (For archival purposes)