A Cladogram of Molting Animals - Research Paper Example

New findings go a long way in supporting earlier calls for the subdivision of the phylum into two. More empirical data on molecular analysis in particular of 18S rRNA/DNA was provided which showed that there is a stronger affinity between calcareans and ctenophores than that which is between calcareans and other poriferans. Poriferans (sponges) have always been regarded as the basal living metazoans that are monophyletic as far as phylogenetic studies are concerned. The various relationships of organisms at the metazoan tree base remain largely unknown even though new trends of triploblast systematics are emerging which provides a clear picture of the lineage. According to previously done classifications, these basal metazoan organisms have been put in different relationships using several markers except one relationship; monophyly. Earlier analyses of the basal metazoans (sponges, placozoans, cnidarians, and ctenophores) have almost unanimously agreed that cnidarians and ctenophores have more close ties or relations with triploblasts than they have with the sponges (poriferans) (Hooper and Willenz 11). 

Aschelminthes was used to refer to an assemblage of polyphyletic meiofaunal-sized animals which included several phyla. However, the legitimacy of Aschelminthes as a taxon was questioned based on a lack of morphological as well as molecular evidence. Currently, these organisms have been declassified into separate phyla. It is sometimes useful to use the term Aschelminthes to refer to all previous organisms that were classified under it. On top of this, it has not been agreed upon whether the formerly known organisms in this group make up a monophyletic group. To complicate matters further, it has not been decided on which phyla to place the Aschelminthes. According to recent morphological studies, the Aschelminthes were described as possibly having two clades (Aguinaldo 490). The two clades are gnathiferans that are hypothesized to contain a newly formed taxon Micrognathozoa while the other clade is Introverta. The second clade of Introverta is thought to be a possible link between Scalidophora and Nematoida. There is a possible remote relation between the introvertans and the panarthropods but for the gnathiferan clade, it has not yet been established if it falls within the bilateral organisms for certainty. Both gastrotrichs and chaetognaths' phylogenetic placements are equally unsettled owing to unsettled issues in phylogenetic analyses. There has been a relationship between Gastrotrichs and Nematoida, gnathiferans, and introverts. However, a more close affinity between Gastrotrichs and introverts than the others has been recognized and led to the formation of a clade called Nemathelminthes or Cycloneuralia. Nemathelminthan autapomorphies may be interpreted to be belonging or as synapomorphies of the Ecdysozoa clade which is much larger and includes panarthropoda. The reasoning behind this is that tardigrades have terminally positioned mouths while all panarthropods have foreguts that are cuticularized (Balavoine and Adoutte 145).

In the past, the Onychophora (velvet worms) were considered a possible (intermediate) evolutionary link between arthropods and annelids. The Onychophora are tropical invertebrates with an estimated 75 extant species that live on the forest floor. These organisms are segmented just like annelids and have a pair of nephridia (excretory organs). Both the female and male reproduction organs are found together (monoecious). Just like true arthropods, they shed cuticles periodically (molting) which is necessary for growth. Besides these characteristics, velvet worms share other many characteristics with annelids which were used to show that connection. However, the current prevailing opinion among taxonomists is that Arthropoda and Onychophora are representatives of sister groups. Some contentions still exist but it has been shown that embryological development, locomotion, and the leg structure of Onychophora are most comparable to those of myriapods (Garey 612). A more recent analysis of nucleotide sequences of the similarities and differences of ribosomal tRNA of Onychophora and arthropods has shown a very close affinity in these two groups. It is therefore natural to assume that Onychophora is related to both the annelids and arthropods but certainly not the link between them. Regardless of the classification that is adopted, it is reasonable to imagine that the ancestors of the current arthropods had distinct worm-like features, appearance, and structure. However, they were different in one way from worms. They used to have an exoskeleton that was secreted by the epidermal cells.  The exoskeleton of those ancestors was more rigid than the cuticle found in annelids but it permitted membranous joints that allowed mobility. The jointed appendages and exoskeleton have variously become arthropods’ hallmarks (Aguinaldo 193).