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The cnidome of the scleractinian cold‐water coral Lophelia pertusa (Linnaeus, 1758, syn. Lophohelia prolifera) was described by Carlgren in 1940. Due to a renewed interest in the cnidae of L. pertusa, specifically comparisons of adult and larval cnidae and their functions, we now redescribe the cnidome from material collected at the Tisler reef in Norway, close to Carlgren's collection site at Saekken (Sweden). Cnidae from column, tentacles, actinopharynx, mesenterial filaments and acontia were investigated. Fresh tissue preparations were compared to histological preparations of decalcified polyps to verify the presence of cnidocysts and secretory cells, and their composition and organization within tissues. The cnidome included microbasic b‐mastigophores, microbasic and mesobasic p‐mastigophores, holotrichous isorhizas and spirocysts. The nematocyst type cnidae (b‐, p‐mastigophores, isorhizas) appeared in different size classes with different distributions within the tissue. Spirocysts were highly variable in shape and size, without distinct size classes. In addition, developing stages of cnidae were documented, with new observations on the succession of p‐mastigophore shaft development. The present observations were in general congruent with the cnidocyst descriptions from L. prolifera made by Carlgren; however, a tiny cnida, possibly of isorhiza type, has been added. Finally, the use of the term acontia is discussed.

The cnidom was found to consist of several morphologically distinct types of nematocysts in six investigated species of Campanulariidae. Descriptions are provided for b‐rhabdoids (micro‐basic b‐mastigophores) of two sizes, occurring in the polyps of Campunulariu Integra and Clytia hemisphaerica of the johnstoni and gracilis forms, respectively. The smaller nemato‐cyst is the most frequent type and is also abundant in the polyps of Laomedea flexuosa, L. geniculata, L. longissima and L. dichotoma as well as in the medusae of Campanulariidae. Structural details of the two nematocyst types, as revealed by scanning electron microscopy, permit discussion of the classification of the nematocysts. The tube in the two types tapers almost unnoticeably from base to tip. No distinct, proximal enlargement of the tube was discernable in either type, as earlier light microscopical observations have suggested. The proximal armature consists of a right‐handed, triple helix of three separate strands of long, closely‐set, paired spines. In the small nematocyst the distal part of the tube is armed with medium sized spines and the discharged tube forms an obtuse angle to the long axis of the capsule. The discharged, wider tube of the large nematocyst roughly forms a right angle to the long axis of the capsule and is distally armed with minute spines.

We present an integrative study with molecular phylogenetic reconstructions and morphological assessment across the three Ceriantharia families: Arachnactidae, Botrucnidiferidae and Cerianthidae. The Arachnactidae specimens (Isarachnanthus spp.) form a well-supported clade, whereas Cerianthidae and Botrucnidiferidae are not recovered as monophyletic. Consequently, the validity of the suborder Spirularia is questioned. Cerianthus was recovered as polyphyletic and Ceriantheomorphe may prove to be a junior synonym of Cerianthus. The taxonomic position of Cerianthus cf. mortenseni is also discussed. All specimens identified on morphology as belonging to Pachycerianthus are recovered as a clade. Further revision of taxa within Ceriantharia is necessary. Molecular phylogenetic analyses based on six mitochondrial or nuclear loci place Ceriantharia as sister to Hexacorallia s.s., but with no significant support relative to an alternative hypothesis that it is the sister taxon to Octocorallia. Further molecular sequence data and taxon sampling will be needed to resolve the position of Ceriantharia.