Evolution & Systematics
Echiura's phylogeny is complex and controversial; the main point of disagreement is about segmentation. As morphologists place Echiura as an independent phyla, geneticists believe they are a taxon of Annelida. Many traits such as paired genital sacs, paired chaete as well as a similar cleavage pattern as Annelida suggests a secondary loss of segmentation (Ruppert et al. 2004). Additionally, homologies in the nervous system development and organization (Hessling et al. 2002) and segmentation of the nerve cord (Hessling, 2003) as well as molecular, DNA and RNA analysis indicate a high probability that Echiura should be phylogenetically placed within Annelida.
This position is starting to be accepted by most scientists, however, as many unknowns persists, it still requires further investigation.
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Drawing by Lucie Malard, Adapted from Westheide et al. 2006
The most recent classification of Echiura within the phylum Annelida
In addition to unclear phylogenetic position of the Echiuran, the family classification within the phylum is also highly controversial and many questions are still raised.
The first and older classification puts Echiuridae as the primitive taxon with three body "segments" and chaetae. The other ones as being derived due to the only anterior pair of chaetae (Ruppert, 2004).
The second classification was made using mitochondrial DNA analysis but also by studying the sexual dimorphism and the feeding type. The first group (Bonellidae and Ikedidae) are together because they both present sexual dimorphism. The second group (Echiuridae and Urechidae) has anal chaetae which suggests they are close families (Goto et al. 2013) and are absent in any other family.
Many other characteristics can be taken into account and therefore, this classification is not definite, in the following years, many changes might be made, as these families are better understood.
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Drawing by Lucie Malard, Adapted from Ruppert, 2004
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Drawing by Lucie Malard, Adapted from Goto et al. 2013 |