DESCRIPTION OF THE PHYLUM ACOELOMORPHA (EHLERS 1986)
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PHYLUM ACOELOMORPHA LINKS
| Acoelomorpha (a-see-lo-MOR-fa) is derived from three Greek roots that mean without (a-α) a hollow (koilos-κοῖλος) form (morfi- μορφή ). The reference is to an animal that has no internal opening. The term was coined by Ehlers (1986) in his revision of the Platyhelminthes. |
| INTRODUCTION TO THE ACOELOMORPHA Organisms in this group are very simple bilaterally-symmetrical animals. They superficially resemble members of the Platyhelminthes, with which they have been classified. Like the free-living flatworms (Turbellaria), the acoelomorphs are dorso-ventrally flattened, have a ciliated epithelium, a ventral mouth, and are hermaphroditic (see Figure 1 for a diagram of Isodiametra). However, like the Xenoturbellarida, the acoelomorphs have an anterior sensory structure, a statocyst that likely senses the pull of gravity. Furthermore, they have a simple nerve net rather than a central nervous system and a solid endocellular mass (the central syncytium) where digestion takes place. The Acoela and Nemertodermata were considered the most primitive orders of the Platyhelminthes (Pechenik 2005; and Brusca and Brusca 2002). Problems with the monophyly of the free-living flatworms (Turbellaria) began to be voiced regarding fundamental structures. The acoelomorphs differed regarding the flagellum in the spermatozoa, the ciliary root structure in the epidermis, and the protonephridia (e.g. Smith et al. 1986). Subsequent molecular phylogenetic studies seemed to confirm that the Acoela and Nemertodermata were not members of the Platyhelminthes (Carranza et al. 1997; Ruiz-Trillo et al. 1999; Baguña and Riutort 2004; Hooge et al. 2002; Phillippe et al. 2007; Mwinyi et al. 2010). A new picture of basal bilaterian relationships began to emerge and is abstracted in Figure 2. In this new view, the Acoelomorpha is the most basal of the extant bilaterian phyla and sister to the Deuterostomata and Protostomata. If this is the case, Acoelomorpha does not have a home and exists in exile within the Animal Kingdom. |
FIGURE 1. Morphology of Isodiametra pulchra.
st = statocyst; cs = central syncytium; de = developing eggs
Image from: de Mulder et al. (2009)
FIGURE 2. The emerging view of the Acoelomorpha (Acoela + Nemertodermata) as the most basal of the living bilaterians and sisters to the Deuterostomata and Protostomata.
| CHALLENGES TO THE ACOEL HYPOTHESIS Though molecular phylogenetic treatments place the Acoelomorpha at the base of the bilateria, some morphological characters still seem to connect the acoels to the Platyhelminthes, especially the Rhabdocoel flatworms (Egger et al. 2009). Both groups have an unusual type of stem cell in the somatic and gonadal tissues. Also, both the free-living flatworms and the acoels have a similar method of epidermal cell renewal. Egger et al. (2009) question whether such morphological and developmental characters could have evolved twice. Also, they question the accuracy of the molecular phylogenies, which are problematic due to artifacts of long branch attraction. Telford (2008) reviews work that places the acoels at the base of the bilaterians and within the deuterostomes. The acoels also have what Marletaz and Le Parco (2010) referred to as a signature gene. GAMT (guanidinoacetate N-methyltransferase) is found in pre-bilaterian lines as well as in the deuterostomes. However, it is not found in the protostome lines. That GAMT occurs in the acoels is consistent with their basal status, but it does not rule out their inclusion in the deuterostomes where they would be allied with the Xenoturbellarida. Another important problem was presented by Wallberg et al. (2007), who found that the Acoela and the Nemertodermata were paraphyletic. Their analysis did indicate that the taxa were basal within the bilaterian line; however, Acoela was a sister group to the Nemertodermata + other bilateria (e.g. Deuterostomata + Protostomata). If this is the case, the Acoela and Nemertodermata would have to be described as separate higher taxa. Bilaterians are not only bilaterally symmetrical, but they also are also triploblastic. Furthermore, all bilaterian lines (e.g. Deuterostomes and Protostomes) have taxa that are segmented and have complete digestive tracts. This suggests that such characters were primitive in the bilaterians (Tautz 2004), and the last common ancestor of the deuterostomes + protostomes (P-D LCA) also must have been complex. That the acoels are anything but complex may mean that their divergence preceded the P-D LCA. They may have become secondarily simplified, or they do not occupy a basal position at all. |
| ACOELOMORPHA AND THE ORIGINS OF THE BILATERIA The acoelomorpha do resemble the bilaterially-symmetrical planula larva of the Cnidaria. Perhaps, the bilaterians emerged from the cnidarians through paedomorphosis (Boero et al. 2007). Then, the bilaterians became modular through the development of the homeotic gene systems, which also could have given rise to segmentation. If the acoelomorpha are basal, then the evolution of body cavities (different types of coelomic cavities), would have appeared independently in the various lines of bilaterians. |
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| By Jack R. Holt. Last revised: 09/22/2015 |
