What is the evolutionary relationship of arthropods and annelids
They are both segmented animals. That means they both have bodies that are constructed of repeated segments. BTW, so do humans. Phylogenetic relationships of Annelids, Molluscs, and Arthropods evidenced from molecules and morphology. Article in Journal of Molecular Evolution. Both Annelida and Arthropoda are phyla in the Linnaean classification system, the scientific categorization of all living creatures according to their relationships .
The results suggest that ecdysis moulting arose once and support the idea of a new clade, Ecdysozoa, containing moulting animals: No support is found for a clade of segmented animals, the Articulata, uniting annelids with arthropods.
The hypothesis that nematodes are related to arthropods has important implications for developmental genetic studies using as model systems the nematode Caenorhabditis elegans and the arthropod Drosophila melanogaster, which are generally held to be phylogenetically distant from each other. Ultrastructure of nuchal organs in polychaetes Annelida - New results and review.
Acta Zoologica 78 2 Nuchal organs are epidermal sensory structures present in most polychaetes. They are situated at the posterior edge of the prostomium and may extend posteriorly onto the peristomium.
Although there is considerable external variation, they all consist of ciliated supporting cells, bipolar primary sensory cells and retractor muscles. They are innervated directly from the brain by paired nerves. The sensory cells are usually monociliated; their sensory processes lie in subcuticular spaces, the olfactory chambers.
Phylogeny of the Annelida and allies
Structural variability is to be observed in the location of the sensory cells, the course of the nuchal nerve, position of nuchal ganglia as well as in cytological features of sensory and supporting cells. These differences provide useful characters for phylogenetic considerations to establish supraspecific taxa within the phylogenetic system of the Annelida. Special emphasis is laid on the problem of whether the nuchal organs represent an autapomorphy of the Polychaeta or the Annelida and thus whether the lack of nuchal organs in Clitellata is primary or secondary.
As is discussed, the probability of a loss of the nuchal organs in Clitellata is higher, which favours the second hypothesis: The rearticulation of annelids. None of the trees generated supported a clade in which Polychaeta and Clitellata were separated. We furthermore hope that coming investigations will consider the possibility the Clitellata is in fact an ingroup of the Polychaeta, which would make Annelida and Polychaeta synonymous.
Rearticulating with extra assumptions; a response to Eibye-Jacobsen and Nielsen. Zoologica Scripta 26 1 What is required is further evidence and study now that the possibility of the paraphyly of the Annelida has been raised. Journal of Natural History. The phylogenetic systematics of the polychaetes, i.
The main competition is between two diametrically opposed hypotheses: A third hypothesis, which is based primarily on certain morphological peculiarities and the presence of exclusively monociliary cells in Owenia, and which postulates a sessile stem species, is mentioned only peripherally: From a decision in favour of Hypothesis 2 it would follow that the Clitellata should be considered the most primitive annelids, so that the possession of parapodia and many extremely differentiated chaetae, for instance, would be interpreted as a highly derived character state.
The consequence for the phylogenetic systematics of the Polychaeta is that oligochaete-like taxa would have to be considered more primitive than, for example, nereidid- like taxa. On the basis of Hypothesis 1, the evolution of these structures would have proceeded in the opposite direction, and polychaete systematics would have the reverse arrangement.
The most important evidence for Hypothesis 2 comes from functional morphological considerations; namely the inference that metamerism has arisen from a burrowing mode of life. It is shown here that 1 this hypothesis rests partly on ignorance of the close relationship between reproductive biology and morphology in the clitellates, 2 the notion that metamerism, and hence the stem species of the Articulata, originated from a burrowing life in the marine environment is unconvincing, and 3 the origin of metamerism can be explained quite differently with reference to modern ultrastructural findings.
According to these findings, septa, which are the fundamental structural elements for annelid segmentation, evolved as a morphological prerequisite for the development of transversely running blood vessels; other purposes of septa e. A highly complex blood vascular system may have been the consequence of the development of lateral parapodia-like appendages. Thus, parapodia are assumed to be part of the ground pattern of the Articulata and hence were present in the stem species of the Annelida.
This is consistent with the traditional interpretation of annelid systematics, which places the errant polychaete taxa at the base of the system Hypothesis 1. McHugh, Dahmnait Molecular evidence that echiurans and pogonophorans are derived annelids. The Annelida, which includes the polychaetes and the clitellates, has long held the taxonomic rank of phylum.
How Are the Annelid & the Arthropod Different? | Animals - cypenv.info
The unsegmented, mud-dwelling echiuran spoon worms and the gutless, deep-sea pogonophoran tube worms including vestimentiferans share several embryological and morphological features with annelids, but each group has also been considered as a separate metazoan phylum based on the unique characters they display. Phylogenetic analyses of DNA sequences from the nuclear gene elongation factor-1alpha place echiurans and pogonophorans within the Annelida.
This result, indicating the derived loss of segmentation in echiurans, has profound implications for our understanding of the evolution of metazoan body plans, and challenges the traditional view of the phylum-level diversity and evolutionary relationships of protostome worms.Simple Animals: Sponges, Jellies, & Octopuses - Crash Course Biology #22
Zoologica Scripta 26 2: In this paper, we first demonstrate the historical background for the current unsatisfactory state of systematics of the polychaetes. We then briefly discuss our knowledge of internal and external structures. A review of the polychaete families makes up the third section; 81 families are treated in detail. Five families have been recently synonymized with others, and six families are too poorly known to be sufficiently characterized.
Fossil polychaetes are briefly mentioned, with specific attention to problems associated with incorporating them in recent systematics. The traditional separation in 'errant' and 'sedentary' polychaetes has increasingly become recognized as being unsatisfactory; however, the current trend towards grouping the polychaetes in many orders without specifying the relationships among the orders, is no more satisfactory.
The lack of consistent morphological information is a major source of uncertainty. Intensive morphological studies should remove terminological ambiguities and alleviate some of the problems. A series of cladistic analyses assess the status and membership of the taxon Polychaeta. As well as the polychaete families, non- polychaete taxa such as the Echiura, Euarthropoda, Onychophora, Pogonophora as Frenulata and VestimentiferaClitellata, Aeolosomatidae and Potamodrilidae are included in the analyses.
All trees are rooted using the Sipuncula as outgroup. Characters are based on features where present such as the prostomium, peristomium, antennae, palps, nuchal organs, parapodia, stomodaeum, segmental organ structure and distribution, circulation and chaetae. A number of analyses are performed involving different ways of coding and weighting the characters, as well as the number of taxa included.
Transformation series are provided for several of these analyses. One of the analyses is chosen to provide a new classification. The Annelida is found to be monophyletic, though weakly supported, and comprises the Clitellata and Polychaeta. The Polychaeta is monophyletic only if taxa such as the Pogonophora, Aeolosomatidae and Potamodrilidae are included and is also weakly supported.
The Pogonophora is reduced to the rank of family within the Polychaeta and reverts to the name Siboglinidae Caullery, The new classification does not use Linnaean categories and the Polychaeta comprises two clades, the Scolecida and Palpata.
The Palpata has the clades Aciculata and Canalipalpata. The Aciculata contains the Phyllodocida and Eunicida. The Canalipalpata has three clades; the Sabellida including the Siboglinidae Spionida and Terebellida. The position of a number of families requires further investigation.
In the chapter, The Annelida. Rouse presents the same higher classification, incorporates the pogonophorans as Family Siboglinidae within the polychaetes, and has Echiura retained outside annelids meantime.
Kojima,Shigeaki Paraphyletic status of Polychaeta suggested by phylogenetic analysis based on the amino acid sequences of elongation factor-1 alpha. Polychaeta, Oligochaeta and Hirudinea.
How Are the Annelid & the Arthropod Different?
The first two classes consist of worms differentiated primarily by the use of parapodia for movement; the third includes leeches. The Arthropoda phylum contains five classes: Crustacea shrimp and lobsterArachnida spiders and scorpionsChilopoda centipedesDiplopoda millipedes and Insecta roaches and beetles.
According to the University of California's Museum of Paleontology website, the Annelida phylum contains about 9, species. The Arthropida phylum comprises an estimated 6 million to 9 million species of arthropods. Body Structures Members of the Annelida and Arthropoda phyla differ in their body formations.
Annelid species typically have tubelike bodies consisting of numerous segments, known as somites, covered in stiff hairlike structures called setae that help the annelid move forward.
Leeches have somites but not setae; instead, they have suckers on both ends of their flatter bodies. On the other hand, arthropods have more complex segmented bodies covered in an exoskeleton that protects their organs and muscles. Most arthropods also have visible distinctions between their heads, bodies and legs; this is not the case for annelids.
Reproduction Methods Reproduction among arthropods usually involves a male and a female of the species mating in order to fertilize eggs in the female, who then lays the eggs. Some arthropods fertilize their eggs externally. Most species are dioecious, meaning they have two distinct sexes. The polychaete class of annelids is also dioecious, but most members of the other two classes are not.