The Acoela are a distinct assemblage of marine worms ranging in size from the microscopic to a little over a centimetre in length. The name 'Acoela' means 'without a cavity', and refers to the lack of a proper gut in these animals. Instead, the mouth (which is situated on the underside of the animal) leads to a central vacuole surrounded by a syncytium (a multinucleate mass that is not divided into individual cells) that takes in nutritive particles by phagocytosis (engulfment by membrane folding, like how an Amoeba feeds). In many of the larger acoels, feeding is supplemented (or even largely superseded) by the presence of endosymbiotic algae that provide nutrients for their host worm (it is the algae that give the worms in the photo above their bright green colour). One particular family of acoels, the Solenofilomorphidae, live in anoxic sulfide sands, an environment that until the late 1960s was thought inimicable to animal life.
Acoels have received most attention in recent years due to debate about their relationships to other animals. Long included among the Platyhelminthes (flatworms), recent studies have indicated that acoels are not close relatives of that group (which is now placed as a derived lineage of Lophotrochozoa). It is generally accepted that acoels are most closely related to a similar group of marine worms called Nemertodermatida, and a few recent studies have also connected them with another marine worm Xenoturbella (Philippe et al. 2011). All three groups lack a through-gut, and a relationship between them seems credible. As regards the connection between this total group and other animals (a subject that I've discussed in a previous post), there are three main options currently on the table: (1) Acoels and their relatives are the sister group (or, in some studies, successive sister groups) to all other bilaterians. This is perhaps the most popular position, and suggests that the simple morphology of acoels relative to other animals may be comparable to the ancestral morphology for bilaterians as a whole. (2) Some recent studies (e.g. Philippe et al. 2011) have recovered an alternative position for acoels within the deuterostomes, as sister to echinoderms + hemichordates. If so, the simple morphology of acoels would probably be derived from more complex ancestors. (3) Finally, there are still some studies (e.g. Dunn et al. 2008) whose results place acoels together with the Platyhelminthes among the Lophotrochozoa (generally as the sister to other Platyhelminthes). However, most current researchers seem to regard such results as due to long-branch attraction between the two groups.
Within the Acoela themselves, recent studies have supported the association of larger species as a clade nested among the smaller species (Hooge & Tyler 2006). Many characters used in earlier classifications of the group, such as the presence of a pharynx or endosymbionts or the placement and numbers of genitalia*, have not always related well to recent molecular phylogenies, but other features such as the arrangement of muscles in the body wall have. One particularly notable feature has been the arrangement of microtubules in the spermatozoa, with successive reductions in the number of central microtubules (from the ancestral 9 + 2 arrangement, to 9 + 1, to 9 + 0) correlating strongly with molecular data (Hooge & Tyler 2006).
*I believe I've noted it before, but for all that organisms such as microscopic worms are usually regarded as morphologically 'simple' and 'conservative', you don't find much variation among more 'complex' animals like mammals in the number of penises they have.
Dunn, C. W., A. Hejnol, D. Q. Matus, K. Pang, W. E. Browne, S. A. Smith, E. Seaver, G. W. Rouse, M. Obst, G. D. Edgecombe, M. V. Sørensen, S. H. D. Haddock, A. Schmidt-Rhaesa, A. Okusu, R. M. Kristensen, W. C. Wheeler, M. Q. Martindale & G. Giribet. 2008. Broad phylogenomic sampling improves resolution of the animal tree of life. Nature 452: 745-749.
Hooge, M. D., & S. Tyler. 2006. Concordance of molecular and morphological data: the example of the Acoela. Integrative and Comparative Biology 46 (2): 118-124.
Philippe, H., H. Brinkmann, R. R. Copley, L. L. Moroz, H. Nakano, A. J. Poustka, A. Wallberg, K. J. Peterson & M. J. Telford. 2011. Acoelomorph flatworms are deuterostomes related to Xenoturbella. Nature 470: 255-258.