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Iments" will contain imaginal discs, too because the unpaired rudiments Iments" will involve imaginal discs, also as the unpaired rudiments not formed by invaginations [10]. These terms indicate tissue origin and formation, so it truly is important to distinguish amongst them. Beyond alterations in morphology, it's also increasingly clear that pilidiophorans have transitioned from a planktotrophic pilidium to a lecithotrophic pilidium repeatedly [2, 15, 22]. Considering that 2005, the number of pilidiophoran species known (or suspected) to possess a nonfeeding larva has enhanced from 3 (i.e. Desor's larva, Schmidt's larva and Iwata's larva) to twenty [2, 15, 22?4]. Some of these are uniformly ciliated, while others, in addition to a complete covering of quick cilia, have a single or two circumferential ciliary bands of longer cilia which superficially resemble the prototroch and telotroch of trochophore larvae of other spiralians, e.g. annelids and molluscs [2, 15, 23, 24]. The topic of this study, a trochophore-like pilidium with an anterior "prototroch" and posterior "telotroch," was dubbed pilidium nielseni [24] in honor of Claus Nielsen, for his provocative theories on the evolution of marine larval types, in which the trochophore is viewed as the ancestral larva of spiralians [25?0]. Pilidium nielseni, which resembles a trochophore, is a lecithotrophic larva of an undescribed lineiform species (Lineidae, Heteronemertea, Pilidiophora) provisionally referred to as Micrura sp. "dark" [24]. Its mere existence prompts a central question inside the trochophore debate -- may be the widespread occurrence of the trochophore morphology amongst spiralians due to the retention of an ancestral larval form, as Nielsen suggests, or did this larval physique strategy evolve numerous times independently [31?6]? Convincing evidence to get a nemertean trochophore was conspicuously absent until 2004, when a vestigial prototroch was discovered inside the palaeonemertean Carinoma tremaphoros [37, 38]. This discovery supplied support for the view that a trochophore-like larva might have been ancestral to nemerteans. Having said that, all palaeonemertean larvae (including Carinoma's), and all hoplonemertean larvae are uniformly ciliated, and lack distinct ciliary bands (Fig. 1). Distinct ciliary bands are only present inside the Pilidiophoran lineage. In light of the current transcriptomic molecular phylogeny of your phylum [8]: (Palaeonemertea (Hoplonemertea; Pilidiophora)), it really is most parsimonious to assume that a uniformly ciliated larva was ancestral to the Nemertea, and thus the ciliary bands of pilidiophoran larvae (planktotrophic pilidia and lecithotrophic larvae, such as pilidium nielseni, alike) evolved secondarily, and are unlikely to be homologous to the trochophore's prototroch. This view is additional supported by the differences in cell lineage [39], cell fateHunt and Maslakova Frontiers in Zoology (2017) 14:Web page three ofFig. 1 Evolution of larval improvement in nemerteans. The pilidium larva, defined right here as a complicated character including improvement through imaginal discs and juvenile rudiments, a larval physique distinct from the juvenile physique, an inflated blastocoel, catastrophic metamorphosis and distinct ciliary bands (black), is only identified inside the Pilidiophora. Hubrechtids possess a helmet-like planktotrophic pilidium with all of the listed characteristics. Heteronemerteans show an incredible diversity of pilidia, which includes lecithotrophic forms, including pilidium nielseni. Lecithotrophic pilidia lack inflated blastocoel; several also lack distinct ciliary bands. Nevertheless, all pilidiopho.