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Hesslring, Rene, Westheide, Wilfried. 2002. Are echiura derived from a segmented ancestor? Immunohistochemical analysis of the nervous system in developmental stages of Bonellia viridi. 252:100-113- Echiura have many homologous characteristics with the Annelids, although they lack any segmentations. The author sets out to discover, are Echiura derived from asegmented ancestor?
- There are many debates as to how Echiura were derived as well as what their phylogenetic position is. There are two different hypotheses considered for this phylum: The Echiura are considered a different taxon from Annelida, or the Echiura are a subtaxon of the Annelida.
- Immunohistochemical analysis shows serially segments units in the nervous system of the species Bonellia viridis. Using confocal laser-scanning mircoscopy, many similarities were noticed between Echiurans and Annelids: a spcialized supraesophageal ganglion was not clearly distinguishable from very similar circumsophageal connectives in Echiurans. In both phylums the ventral nerve cord develops as two separated trunks and then develop into one.
- Using only the homology of segmentation would not justify classifying the Echiurans as a subtaxon of annelida, however the others mentioned adds support. The results indicate that there is support for including Echiura in Annelida
Saiz-Salinas, Jose I., Dean, Harlan, K. Cutler, Edward, B. 2000. Echiura from the antarctic and adjacent waters. Polar Biology. 23:661-670.- There has been approximately 555 Echiura discovered in the Antarctic seas, with 9 species found. Zoogeographical analysis shows differential distribution along horizontal gradients and depth being important, but is this true?
- Over 25 years of research has been done in collecting the Echiura, however some species have onylbeen classified by their proboscis, for example and many important characters have been ignored. So re-examination is in order to discover whether the above question is true.
- Samples came from different depths as well as latitudes, using trawls, dredges and grabs, and dissecting microscopes and techniques for analysis. Eight species were examined. Some of the characters looked at were rings of anal setae, gonopores, gonoducts, ventral setae, and anal vesicles. It was noted that many species had not been properly classified.
- The results showed that differential distribution along longitudinal and latitudinal gradients were not shown,, but depth remained an important factor.
Guidici, M de Nicola. 1984. Defence Mechanisms of Bonellia viridis. 78:271-273- May marine species have microorganisms and benthic animals living on their body, do the Echiuran, bonellia viridis have such organisms as well?
- Many sutdies have shown that these worms do not have such life forms, however they live in an environment filled with these types of organisms.
- 14 Echiurans were collected throughout different months of the year. No life forms were found on any of the samples. This suggests that the pigment bonellin present in the worm may be a chemical defence system. This chemical is known to stop embryonic development in echinoderms, however it makes the worm very sensitive to light.
- bonellin is, quite reasonably the cause of the lack of any microorganisms on Bonellia viridis.
The Echiura are not often seen as fossils; they do not have any hard parts, so are not fossilized easily. Burrows have been found, however there is always the possibility that they belong to another type of animal. What has been found is seen in the Cambrian time period, where the oldest fossil is Pennslyvanian.
For many years, Echiura had been classified under the annelids due to their laterally branched ventral nerve cord giving indications of having been segmented in the past. However, these animals are now thought to be considered their own phylum, a sister taxa to the annelids. Within this group, there are 5 classes and approximately 150 species known to date.
Echiurans, known as Spoonworms, for their anterior prostomium that can roll into a gutter-like shape, are closely related to the annelids. They are found only in marine water, and even in depths of up to 10 000 feet. They are cylindrical in shape and are ventrally ciliated, and their thin collagenous cuticle is unsegmented. Some species contain tiny hooks called chatae along the anterior ventral side used for digging when extended, while others have two circles of chatae around the posterior of their trunk for an anchor. Echiurans move their bodies using peristaltic waves to move themselves through the burrow, and their ciliated prostomium is able to creep, thus their dorso ventral and longitudinal muscles are well developed. These worms have no mechanism of defence as they are soft-bodied, however the pigment in their skin, called bonellin is very toxic to predators, and is intended to paralyse them. Echiurans have many of the same features of annelids, including a ventral nerve ring a through gut. All Echiurans contain a siphon for transporting any water through their digestive system as well as a hemal system; the only species known to not have a hemal system is Erechis caupo. The sexes are separate, and the gametes are kept in genital sacs, which squeeze them out when the animals are spawning. If any larvae land on a female, a special pigment in their skin, called bonellin (which is toxic to predators) will activate development of male worms, and if the larvae land anywhere else, they will develop into females. The majority of Echiurans are deposit feeders, living in u-shaped burrows, however the Urechis caupo is a filter feeder. Urechis are able to use the peristaltic movements for pumping food through their body. These unusual animals feed on mostly plankton and are commensal hosts to a few species of tiny clams, polychaete scale worms and the goby fish. Deposit feeders on the other hand stretch their prostomium out of the burrow, which can stretch up to 10 times the length of their trunk, to collect particles from the soil.
