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Most annelids have a pair of coelomata (body cavities) in each segment, separated from other segments by septa and from each other by vertical mesenteries. Each septum forms a sandwich with connective tissue in the middle and mesothelium (membrane that serves as a lining) from the preceding and following segments on either side.
In parasitic annelids, flatworms and roundworms, suckers are the organs of attachment to the host tissues. In tapeworms and flukes , they are a parasitic adaptation for attachment on the internal tissues of the host, such as intestines and blood vessels. [ 1 ]
Pleistoannelida is a group of annelid worms that comprises the vast majority of the diversity in phylum Annelida.Discovered through phylogenetic analyses, it is the largest clade of annelids, comprised by the last common ancestor of the highly diverse sister groups Errantia and Sedentaria (Clitellata and related polychaetes) and all the descendants of that ancestor.
Historically the group has been assigned to the echinoderms, barnacles, annelids and mollusks. Relationships to other Cambrian forms (such as the Halkieriids) have been proposed and discounted. [8] In 2008, the discovery of a fossil preserving soft tissue (including chaetae and parapodia) established an annelid affinity.
[14] [15] Learning and memory using nociceptors have been described in the sea hare, Aplysia. [16] [17] [18] Mollusk neurons are able to detect increasing pressures and tissue trauma. [19] Neurons have been identified in a wide range of invertebrate species, including annelids, molluscs, nematodes and arthropods. [20] [21]
Parazoa (Parazoa, gr. Παρα-, para, "next to", and ζωα, zoa, "animals") are a taxon with sub-kingdom category that is located at the base of the phylogenetic tree of the animal kingdom in opposition to the sub-kingdom Eumetazoa; they group together the most primitive forms, characterized by not having proper tissues or that, in any case, these tissues are only partially differentiated.
This list of prehistoric annelids is an attempt to create a comprehensive listing of all genera that have ever been included in Annelida which have been found ...
Polychaeta (/ ˌ p ɒ l ɪ ˈ k iː t ə /) is a paraphyletic [1] class of generally marine annelid worms, commonly called bristle worms or polychaetes (/ ˈ p ɒ l ɪ ˌ k iː t s /).Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin.