The terminology of larval cestodes or metacestodes

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2espoused by Joyeux & Baer (1961, p. 450) in theircomprehensive review of cestode life-cycles known atthat time, namely that ‘nous espérons gagner en clartéen les reduisant au minimum’ (literally, we hope togain clarity by reducing them to a minimum). Such ananalysis might result in a system of nomenclature thatis both functional, is based on the criterion of homologyand provides phylogenetic insights. The currentreview results from a workshop held during the 3rd InternationalWorkshop for Tapeworm Systematics, heldin Sofia in 1999 at which agreement was reached thata simpler system of nomenclature for larval cestodeswas needed, even if morphological and developmentaldetails have not yet been resolved.The review utilises the taxonomic arrangement forthe cestodes proposed by Khalil et al. (1994) butaccepts the cyclophyllidean family Gryporhynchidae,following Spasskii & Spasskaya (1973) and Hoberget al. (1999).Historical account of existing systems ofterminologyOriginsThe existing terminology for larval cestodes has developedover a considerable period of time and atits inception utilised generic names. The first wasScolex, applied by Müller (1787) for larval tetraphyllideans.Zeder (1800, 1803) divided the larvalcestodes known at his time into two groups, monocephaliccestodes to which he applied the genericname Cysticercus and polycephalic cestodes for whichthe name Polycephalus was used. Rudolphi (1819,pp. 177, 536) retained the genus Cysticercus but replacedPolycephalus with two new names, Coenurus,for polycephalic larval cestodes in which proliferationwas ‘exogenous’ and Echinococcus for cestodesin which proliferation was considered to be ‘endogenous’.These genera were placed in the order Cystica,which was separate from that of the adult tapeworms,the order Cestoidea. At the time, there was virtuallyno understanding of cestode life-cycles and littlecomprehension that the ‘cystic’ forms were the larvalstages of the Cestoidea. This was particularly thecase in the Tetraphyllidea where species of Scolexexhibited strikingly different morphology to the correspondingadults. The generic names Dithyridium andTetrathyridium were also erected by Rudolphi (1819,p. 559), the first for a cestode of uncertain identityfrom the liver of a lizard and the second from a partridge;neither was allocated to an order. The genusScolex was used by Rudolphi (1819, pp. 124, 198)to include a variety of larval tetraphyllidean cestodes,although the genus was placed in the Cestoidea ratherthan in the Cystica. Subsequently, Diesing (1850) subdividedthe genus Cysticercus into those with armedscoleces, retained in Cysticercus, and those with unarmedscoleces, placed in a new genus Pietocystis,a division subsequently suppressed by Villot (1883,p. 5). Thus the nomenclature used at this stage waspurely descriptive, utilised the most obvious charactersavailable (monocephaly versus polycephaly)and predated any detailed understanding of the basiclife-cycles of cestodes.CysticercoidsLeuckart (1876) introduced the term cysticercoid fora variety of larval cestodes known at the time, mainlyfrom invertebrates, differentiating them primarily ‘intheir minuteness and in the absence of fluid’. The termwas subsequently used as a generic name, Cysticercoides(e.g. Braun, 1894, p. 1559)Villot (1883) considered that Leuckart‘s groupof cysticercoids was highly heterogeneous and reorganisedthem such that only parasites of invertebrateswere involved. This involved moving Gryporhynchus,a larval stage without cercomer, foundin fresh-water fish and now known to be a gryporhynchid,to the ‘cysticerci’ and utilising a numberof new generic names for remaining larval forms. Thefirst group, with two genera, was characterised byendogenous development. Polycercus was proposedfor the polycephalic larva described by Metchnikov(1869) from Lumbricus terrestris, which are larvaeof a dilepidid genus. Monocercus was erected forCysticercus arionis of von Siebold (1850) found inthe mollusc Arion empiricorum, and now consideredto be the dilepidid Molluscotaenia crassiscolex (seeGulyaev & Kornienko, 1998).The second group, characterised by exogenousdevelopment included the genera Cercocystis, Cryptocystis,Urocystis and Staphylocystis. Cercocystis waserected for a cysticercoid found in Tenebrio molitorby Stein (1875), named Cysticercus tenebrionis, andthought to be the larval stage of the hymenolepididRodentolepis microstoma. Cryptocystis contained C.trichodectis, a larva found in the body-cavity of thedog louse Trichodectes canis by Melnikov (1869) andnow known to be the larval stage of Dipylidium can-
3inum (= Taenia cucumerina). Both Leuckart (1876)and Moniez (1880) noted that the scolex was invaginated,as occurs in Echinococcus, rather than retractedas in the related cysticercoids. Urocystis and Staphylocystiswere both erected for polycephalic larvae foundin a variety of invertebrates, which are now knownto be larval stages of hymenolepidids from shrews(Joyeux & Baer, 1961).The terms employed by Zeder (1800), Rudolphi(1819), Villot (1877, 1882, 1883), Braun (1883)and other authors were incorporated by Braun (1894,p. 1,567) into his synopsis of types of larval cestodes(Table 1). Subsequently, most of the names ceased tobe used as generic names but were retained in slightlymodified forms to describe types of larval cestodes.A full list of such names was provided by Freeman(1973).Terms for pseudophyllideans and trypanorhynchsOriginating from studies on the life cycles of pseudophyllideancestodes came the term procercoid todescribe the first larval stage in the crustacean intermediatehost (Janicki & Rosen, 1917) characterisedby the lack of a primary lacuna and the presence of atail-like appendage termed the cercomer. The pseudophyllideanstage in the second, vertebrate intermediatehost was termed a plerocercoid by Braun (1883) andwas also used subsequently as a generic name, Plerocercoides(e.g. Braun, 1894, p. 1,559). Plerocercoidswere characterised as having a solid body and havingthe future scolex develop externally (Figure 1).The term plerocercus was also proposed by Braun(1883) for the larval stage of trypanorhynch cestodes,in which the scolex forms retracted within a blastocyst.Dollfus (1946, p. 66) provided a detailed comparisonof the structure of plerocerci and plerocercoids; hisoriginal drawings having been utilised subsequentlyby Wardle & McLeod (1952, p. 296) and Campbell& Beveridge (1994, p. 54). Dollfus (1946) utilised forhis illustration of a plerocercoid, a species of Phyllobothrium,showing invagination of the scolex withinthe pars proliferans of the scolex. Wardle & McLeod(1952, p. 66) subsequently defined a plerocercoid ashaving an invaginated scolex. However, most plerocercoidshave an evaginated scolex, such as thoseof Diphyllobothrium, Triaenophorus and Abothrium(See Rosen 1919, p. 261, plate I, figure 3, p. 273, plateIII, figure 5) as well as the tetraphyllideans Scolexpolymorphus and S. pleuronectis. Hence, the term plerocercoidhas been used for larval stages with bothinvaginated and non- invaginated scoleces. In addition,S. bothriosimplex, with a single apical sucker, isprobably the larval stage of a tetrabothriidean (Hoberg,1987), further complicating the the analysis of theearly application of this term.An additional term, merocercoid, for the larvalstage of Triaenophorus nodulosus was attributed toJanicki by Rosen (1919, p. 298), although the publicationof the name was not cited. Rosen (1919)dismissed Janicki‘s use of a novel term, insisting thatthe larva in question was a plerocercoid. The termappears not to have been employed subsequently forpseudophyllidean larvae.In a review of types of cestode larvae, Fuhrmann(1933) used the terms procercoid and plerocercoidfor stages of pseudophyllidean life-cycles, as wellas the older terms cysticercus and cysticercoid (incorporatingcryptocystis and cercocystis), plerocercusand dithyridium (for Mesocestoides), in additionto the new term merocercus for the metacestodeof the lecanicephalidean genus Lecanicephalum (seeFuhrmann, 1933, p. 240).Particular terms for metacestodes were not necessarilyrestricted to a single taxonomic group ascysticercus was applied not only to the metacestodesof taeniids but also to tetraphyllidean metacestodes(Monorygma grimaldii). Rudolphi (1819) had previouslyapplied the term cysticercus to the metacestodeof another tetraphyllidean, Phyllobothrium delphini.An additional feature of Fuhrmann‘s (1933)description of cestode larval stages was that he referredto both Dithyridium and Gryporhynchus asplerocercoids; that is, he applied the term plerocercoidto bothriate and acetabulate metacestodes, andhe (p. 202) clearly distinguished between metacestodeswith invaginated scoleces (zurükgezogen) andwithdrawn scoleces (zurückzieht).Fuhrmann’s (1933) terms for metacestodes werewidely adopted in western Europe and were addedto by Joyeux & Baer (1945), who proposed the termmerocercoid for the metacestode of the cyclophyllideancestode Catenotaenia pusilla, characterised bythe possession of an apical sucker which is lost duringdevelopment in the adult. They were apparentlyunaware of its prior use by Rosen (1917) for pseudophyllideans.Dollfus’ (1942) classic monograph onthe Trypanorhyncha firmly established the use of theterm plerocercus for trypanorhynch metacestodes, althoughthe term was ignored by Joyeux & Baer (1961)and Wardle & McLeod (1951) in favour of plero-
Page 1: Systematic Parasitology 52: 1-33, 2
Page 5 and 6: 5Table 1. Names of larval cestodes
Page 7 and 8: Figure 2. Schematic representation
Page 9 and 10: 9Table 3. Comparison of the systems
Page 11 and 12: 11Table 5. Nomenclature employed by
Page 13 and 14: 13broad and elongate cercomer in co
Page 15 and 16: 15from which cysticercoids bud off.
Page 17 and 18: 17ing species are Choanotaenia infu
Page 19 and 20: 19characters upon which a system of
Page 21 and 22: 21Figure 4. Schematic representatio
Page 23 and 24: 23Table 6. Suggested terms for larv
Page 25 and 26: 25new difficulties for this system
Page 27 and 28: 27fimbriocercus (including armatetr
Page 29 and 30: 29Burt, M.D.B. (1962) A contributio
Page 31 and 32: 31host Branchinecta gaini Daday (Br
Page 33: 33Stunkard, H.W. (1937) The life cy

The terminology of larval cestodes or metacestodes

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