On the taxonomy and presumable evolutionary pathways of planktonic larvae of Ceriantharia (Anthozoa,Cnidaria)

After investigation of the type specimens of 11 genera of ceriantharian larvae, it was concluded that the overwhelming majority of planktonic forms should be referred to the family Arachnactidae. Despite the fact that the presence of acontioids is traditionally considered as the only characteristic feature of this family, in most of the species acontioids develop at rather late ontogenetic stages. The importance of another feature, the presence or absence of p-rhabdoids, was considered. Members of suborder Penicillaria (Arachnactidae) typically have p-rhabdoids in the endoderm of alternate mesenteries and in acontioids. Members of suborder Spirularia (Cerianthidae and Botrucnidiferidae) have no p-rhabdoids and all cnidae in their mesenteries are restricted to the region of the mesenteric filament. In Arachnactidae the number of known larval forms significantly exceeds the number of benthic ones. The planktonic forms with developing and mature gonads were described in this family. The paedogenetic origin of Arachnactidae is discussed. It seems likely that, due to the uniformity and deficiency of definite morphological characteristics of the benthic members of the family Arachnactidae, only cnidom and type of the planktonic larva can be used for reliable taxonomical identification of the polyps.