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Annelids are members of the protostomes, one of the two major superphyla of bilaterian animals – the other is the deuterostomes, which includes vertebrates. [68] Within the protostomes, annelids used to be grouped with arthropods under the super-group Articulata ("jointed animals"), as segmentation is obvious in most members of both phyla ...
Arthropod eyes Head of a wasp with three ocelli (center), and compound eyes at the left and right. Most arthropods have sophisticated visual systems that include one or more usually both of compound eyes and pigment-cup ocelli ("little eyes"). In most cases, ocelli are only capable of detecting the direction from which light is coming, using ...
[1] [2] Well-known examples of protostomes are arthropods, molluscs, annelids, flatworms and nematodes. They are also called schizocoelomates since schizocoely typically occurs in them. Together with the Deuterostomia and Xenacoelomorpha, these form the clade Bilateria, animals with bilateral symmetry, anteroposterior axis and three germ layers ...
Segmentation in animals typically falls into three types, characteristic of different arthropods, vertebrates, and annelids. Arthropods such as the fruit fly form segments from a field of equivalent cells based on transcription factor gradients. Vertebrates like the zebrafish use oscillating gene expression to define segments known as somites
Arthropods, on the other hand, display a heterogeneous mix of embryonic cleavage patterns including spiral-like cleavage and radial cleavage patterns. This led researchers to two theories: The first was that the arthropods lineage must have lost the ability to spiral cleave since differentiating from the last common ancestor between annelids ...
Well-known phyla of invertebrates include arthropods, mollusks, annelids, echinoderms, flatworms, cnidarians, and sponges. The majority of animal species are invertebrates; one estimate puts the figure at 97%. [1] Many invertebrate taxa have a greater number and diversity of species than the entire subphylum of Vertebrata. [2]
The protostomes include most of the rest, such as arthropods, annelids, mollusks, flatworms, and so forth. There are several differences, most notably in how the embryo develops. In particular, the first opening of the embryo becomes the mouth in protostomes, and the anus in deuterostomes.
It has been found that molluscs and insects have opioid binding sites or opioid general sensitivity. Certainly there are many examples of neuropeptides involved in vertebrate pain responses being found in invertebrates; for example, endorphins have been found in platyhelminthes, molluscs, annelids, crustaceans and insects.