The terminology of larval cestodes or metacestodes

26ticercoids’ generally possess a cercomer at some stageof their development but lack a ‘bladder’. The prefixsupposedly alludes to the primary lacuna (Wardle &McLeod, 1952), but this was not known at the timethat the name (‘lacuna primitiva’ of Grassi & Rovelli,1892) was applied by Leuckart (1876). Hence the etymologyis dubious. Likewise, ‘plerocercus’ scarcelydiffers from plerocercoid in meaning (Joyeux & Baer,1961), but also indicates a metacestode lacking a primarylacuna. The argument being developed here isthat several of the current, firmly entrenched termsfor larval cestodes are, from an etymological pointof view, not completely appropriate, but are widelyaccepted. Therefore, to argue for the standardisationof nomenclature purely on etymological grounds ishazardous and could lead to increased confusion.An additional tradition is one in which metacestodenames ending in ‘–us’ (cysticercus, plerocercus)indicate the existence of a bladder, while names endingin ‘–oid’ indicate the absence of a bladder. However,no formal written presentation of this view canbe found! In the current re-arrangement of morphologicalpriorities, the presence of a bladder is of reducedsignificance, since it may be homoplasious, whilethe invagination or retraction of a scolex is accordedgreater significance than in the recent literature. Underthis system, cysticercus, a retained term, is appliedto metacestodes in which the scolex is invaginated,while cysticercoid is applied to metacestodes in whichthe scolex is retracted, although both have a primarylacuna or ‘cyst’ (sensu Wardle & McLeod, 1952).Procercoid and plerocercoid, which have either noscolex development or an everted scolex, have alsobeen retained as names for larval cestodes. Therefore,changing the name ‘plerocercus’ to a term ending in‘-oid’ to be consistent with cysticercoid, the othergroup name in which scoleces are retracted wouldeliminate one inconsistency, but would introduce anotherwith the entrenched term plerocercoid which hasan everted or invaginated scolex. Similarly, attemptingto find an alternative to ‘merocercoid’ for larvalcestodes with inverted scoleces but lacking a primarylacuna presents obstacles since none of the existingnames (tetrathyridium, cladothyridium, cercoscolex)has a ‘−us’ suffix.The most parsimonious solution to this etymologicalmaze is to retain the tradition of ‘cyst-’ indicating(usually) the presence of a primary lacuna and ‘plero-’indicating the absence of a primary lacuna. The existingterm plerocercus can be used in place of merocercus,since this adequately covers both the encystedtrypanorhynchs and lecanicephalideans, and the comparableterm ‘merocercoid’ coined for catenotaeniidsby Joyeux & Baer (1945) can be used for tetrathyridia,cladothyridia and cercoscoleces, all of which lackbladders and are invaginated. Merocercoid favours noorder over the other and while not the older term,provides the greatest etymological uniformity.In the final analysis, terminology represents findingcompromises between entrenched usage and etymologicalconsistency. The proposal suggested here isa compromise but seems to be an appropriate middlecourse between these two ideals, and will undoubtedlyattract criticism from either of the two possibleentrenched extreme positions.Based on the arguments presented above, thefollowing definitions are suggested for metacestodesidentified on the basis of morphological characteristics:Procercoid (Figure 4A) metacestode without scolexand primary lacuna, usually caudate, in which developmentcannot proceed until ingested by a secondintermediate host.Plerocercoid (Figure 4B) metacestode with (at leastpartial) scolex development (bothriate or acetabulate),scolex evaginated, non-lacunate, with or without cercomer,infective to definitive host.Merocercoid (Figure 4C) metacestode lacking a primarylacuna, in which scolex is invaginated, caudateor acaudate, usually without bladder (except inRhodobothrium, Phyllobothrium, Monorygma (Tetraphyllidea)).Plerocercus (Figure 4D) metacestode lacking a primarylacuna, in which scolex is withdrawn; caudate(disputed), with or without bladder.Cysticercoid (Figure 4E) metacestode with primarylacuna, scolex retracted,with cercomer or reduced cercomer.Cysticercus (Figure 4F) metacestode with primary lacuna,scolex invaginated, with or without cercomerand bladder.Within the two latter groups, cysticerci and cysticercoids,considerable morphological diversity exists.Some within-group diversity can be indicatedsimply by the use of prefixes based on existing names.CysticerciThe terms utilised by Hoberg et al. (2000) have beenadopted. These are:for monocephalic cysticerci -cysticercus