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Platynereis dumerilii
''Platynereis dumerilii'' belongs to the Annelids. It was originally placed into the genus Nereis〔 and later reassigned to Platynereis. ''Platynereis dumerilii'' lives in coastal marine waters from temperate to tropical zones. It can be found in a wide range from the Azores, the Mediteranian, in the North Sea, the English Channel, and the Atlantic down to the Cape of Good Hope, in the Black Sea, the Red Sea, the Persian Gulf, the Sea of Japan, the Pacific, and the Kerguelen Islands.〔 ''Platynereis dumerilii'' is today an important lab animal, it is considered as a living fossil, and it is used in many phylogenetic studies as a model organism. In the lab, ''Platynereis dumerilii'' reaches a length of 3 to 4 cm, and an average age of 7 month.
==Reproduction and Development==
''Platynereis dumerilii'' is dioecious, that means it has two separate sexes: During mating, the male swims around the female while the female is swimming in small circles. Both release eggs and sperm into the water. This is triggered by sexual pheromones. The eggs are then fertilized outside of the body in the water. ''Platynereis dumerilii'' has like other Nereidids no segmental gonades, the gametes mature freely swimming in the body cavity (coelom).
''Platynereis dumerilii'' develops very stereotypically between batches and therefore time can be used to stage ''Platynereis dumerilii'' larvae. However, the temperate influences the speed of development greatly. Therefore, the following developmental times are given with 18°C as reference temperature:
After 24 hours, a fertilized egg gives rise to a trochophore larva. At 48 hours, the trochophore larva becomes a metatrochophore larvae.〔 Both trochophore and metatrochophore swim with a ring of cilia in the water and are positively phototactic. A day later, at 72 hours after fertilization, the metatrochophore larvae becomes a nectochaete larva. The nectochaete larva, already has three segments, each with a pair of parapodia bearing chaetae, which serve for locomotion.〔 The nectochaete larva can switch from positive to negative phototaxis. After five to seven days, the larvae start feeding and develop on their own speed, depending on food supply. After three to four weeks, when six segments have formed, the head is formed.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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