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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 ...
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
Hence annelids' chetae are structurally different from the setae ("bristles") of arthropods, which are made of the more rigid α-chitin, have a single internal cavity, and are mounted on flexible joints in shallow pits in the cuticle. [8] Nearly all polychaetes have parapodia that function as limbs, while other major annelid groups lack them.
This was backed up by studies of the anatomy and development of these animals, which showed that many of the features that supported the Articulata hypothesis showed significant differences between annelids and the earliest Panarthropods in their details, and some were hardly present at all in arthropods. This hypothesis groups annelids with ...
[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 ...
These two groups have long been considered close relatives because of the common presence of trochophore larvae, but the annelids were considered closer to the arthropods because they are both segmented. [53] Now, this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla. [54]
Likewise, the relationships between various arthropod groups are still actively debated. Today, arthropods contribute to the human food supply both directly as food, and more importantly, indirectly as pollinators of crops. Some species are known to spread severe disease to humans, livestock, and crops. (Full article...
In the most recent research, the three phyla Cycliophora, Entoprocta and Bryozoa makes up a single clade and are the first to branch off from the other lophotrochozoans. The second split is the molluscs, and the third consists of two sister phyla, annelids and nemerteans. Lastly remains the clade that consist of the phoronids and the brachiopods.