The taxonomy, ecology and host-specificity of some Phyllobothriidae (Cestoda: Tetraphyllidea), a critical revision ofPhyllobothriumBeneden, 1849 and comments on some allied genera

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Abstract
Two new and twelve little known species ofPhyllobothriumor of allied genera, mainly from elasmobranchs and teleosts caught off the British Isles, have been investigated. As some of these could not be identified to species and the others presented difficulties, material from various other sources was obtained and examined for comparison. In addition, the literature on about 100 species allocated toPhyllobothriumwas consulted and brought together. A critical appraisal of this literature is given and the genus is revised for the first time. Information on most of the species was found to be inadequate to provide a key and, therefore, a host-parasite list was compiled. As only about fifty species ofPhyllobothrium sensu latohave been found in about 100 of the 3000 species of elasmobranchs known to exist, it is estimated that a very large number ofPhyllobothriumspp. remains to be discovered and described. The possible significance of this fact is discussed. Brief descriptions of the most well-known species ofPhyllobothriumare given and reasons against listing synonyms for these are emphasized. Of the 100 species already allocated to the genus only twenty-two are accepted at present; further studies may show that only three of these, viz.P. lactuca,P. dagnalliumandP. serratum, show the typical features of the genus, as originally described. Fourteen of the twenty-two may, eventually, be placed inAnthocephalumLinton, 1890, if this genus is revived; the erection of a new genus or genera may be necessary for the remainder.P. britannicumsp.nov., fromRaja montagui, is provisionally placed nearP. lactucabut the bothridia are only slightly bifid, their margins are not so folded and the species is euapolytic.P. minutumsp.nov. fromR. fullonicaclosely resemblesP. auricularia, P. foliatumandP. loculatumand may, eventually, fall as a synonym of one of these; at present they are all little-known forms. Reasons are given for provisionally retainingCrossobothriumandMonorygma, with about ten species in each. It is suggested that three species originally placed inPhyllobothriummay be allied toSphaerobothrium lubetiEuzet, 1959. An examination of the type material ofP. ketaeGanavan, 1928, previously regarded as unique or as a neotenic form, has shown that the original description was partly based on a pseudophyllidean and possibly on the larvae ofP. caudatum. A number of larvae ofPhyllobothriumin invertebrates, teleosts and marine mammals, fourteen little-known species of the genus and invalid members are discussed. A detailed discussion is given of the ecology, host specificity and attachment ofPhyllobothriumand allied genera to the gut mucosa of elasmobranchs. In a general discussion brief comments are made on life-history, the identification and classification ofPhyllobothrium, self-fertilization and on ‘segmentation’. Almost 200 references are cited.