Trochozoans are a large phylogenetic group that also includes molluscs and plathelminths. They are the third major branch of the bilaterians, along with the deuterostomians (including vertebrates or echinoderms) and ecdysozoans (including arthropods or nematodes). Annelids belong to the trochozoa family. They show anatomical and developmental features that are thought to be already present in the Urbilaterian, the last common ancestor of the bilaterians. The ease of laboratory rearing of the marine annelid Platynereis dumerilii (P. dumerilii) allows the study of all aspects of its life cycle. As of today, the first investigations of the P. dumerilii genome show, contrary to what was previously thought, that annelids are genetically closer to humans than are insects or nematodes.
Eggs develop in a thick layer of jelly and drift in the water. After about 18 hours of development, the eggs emerge from their shells as larvae called trochophores. The trochophores develop into young worms composed of three segments and a terminal pole bearing the anus, named pygidium.
Young worms have a rounded head and three pairs of legs, and are constantly moving. They feed for about ten days. New segments proliferate from a sub-terminal area of the posterior pole.
After a life span of four to twelve months in the laboratory, the worms become sexually mature. Individuals then undergo metamorphosis: females appear yellow and males have a white anterior side and a red posterior side. Sexually mature individuals perform a courtship dance and die quickly after fertilization. It should be noted that in the laboratory, it is possible to obtain individuals at several stages of development by controlling the photoperiod.
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