On the Ciliary Mechanisms and Interrelationships of Lamellibranchs.: PART VII: Latero-frontal Cilia of the Gill Filaments and their Phylogenetic Value

Abstract

Certain Lamellibranchs have the latero-frontal tract composed of large complex ‘cilia’, here called eu-latero-frontal cilia , together with subsidiary ones, termed pro-latero-frontal cilia. This type of latero-frontal tract occurs in some or all of the three families of Protobranchs (there is some doubt as to the presence of pro-latero-frontal cilia in all the families), and in the Mytilidae and probably the Trigoniidae (fixation too imperfect for the identification of pro-latero-frontal cilia) among the Filibranchs, and in all the marine families of Eulamellibranchs obtainable at Plymouth, and in the fresh-water families, Dreissensiidae, Sphaeriidae, Unionidae, Mutelidae, and Aetheriidae. A list of the species investigated is given. Other Lamellibranchs, which were previously considered as lacking latero-frontal cilia, have been found to possess small ones only, difficult of observation, termed micro-latero-frontal cilia. These occur in the Arcidae, Anomiidae, Pteriidae, Pectinidae, Spondylidae, Limidae, Pinnidae, and are inferred to be present in the Amussiidae, Vulsellidae, and Isognomonidae, in which eu-latero-frontal cilia are certainly absent. A list of the species examined is given. In one family, the Ostreidae, moderate-sized latero-frontal cilia, termed anomalous latero-frontal cilia, together with subsidiary ones, termed para-latero-frontal cilia are present. In bivalves having eu-latero-frontal cilia the arrangement of the various ciliary tracts, frontal, latero-frontal, and lateral is fairly constant, notable exceptions being a Protobraneh, Nuculana, and a Filibranch, Trigonia . In bivalves having micro-latero-frontal cilia the arrangement of the various tracts seems more or less constant. The homology of the various types of latero-frontal cilia is discussed. The composition of the latero-frontal ciliated tracts has been found to be a stable character, and, as it is correlated with other characters, has taxonomic value. It is suggested that the variations in the constitution of the latero-frontal tracts tend to show that Ridewood's (1903) classification does not express genetic affinities, as he himself conceded, nor does Pelseneer's (1911) entirely, and that Pelseneer's order Filibranchia, and Ridewood's orders Bleutherorhabda and Synaptorhabda are not monophyletic. Families possessing micro-latero-frontal cilia appear to be closely related, and form a group, which, with certain modifications, corresponds to ‘the Aviculidae and their allies’, or the ‘sedentary’ branch of Lamellibranchs, previously established by the palaeontologists, Jackson and Douvillé respectively, largely on shell characters. Thus the constitution of the laterofrontal tracts of the gill filaments supports the findings of palaeontologists with regard to this group. Unfortunately neither Jackson nor Douville proposed a formal name for the group. The relationship of forms with micro-latero-frontal cilia, and the evolution within the group of the eulamellibranchiate or synaptorhabdic gill are discussed. One family, the Ostreidae, which must be included on account of its relationship with either the Pteriacea or Pectinacea (based on other evidence) has moderatesized, or anomalous latero-frontal cilia together with paralatero-frontal cilia. The anomalous latero-frontal cilia differ in certain respects from the large cilia characteristic of the majority of the Laraellibranchia, and are presumed to have arisen independently. Common characters of the group characterized by the possession of micro-latero-frontal cilia, in addition to the form of the latero-frontal cilia, are: (1) shell characters of the prodissoconch, Arcidae excepted; (2) byssal fixation; (3) considerable free posterior region to the gill axes; (4) considerable development of muscles in the gill axes, Ostreidae excepted; (5) method of division of the pallial cavity, Ostreidae excepted; (6) gills without a supra-axial extension to the outer demibranch; (7) presence of longitudinal currents at the free ventral edge of both inner and outer demibranchs; and of opposed frontal currents on all lamellae and frequently on the same filament, Pinnidae excepted; (8) absence of pallial sutures, Pinnidae and Ostreidae excepted; (9) inner fold of the mantle margin characteristically well developed, especially in swimming forms; (10) insertion of the retractor muscles of the mantle margin at a considerable distance from the shell edge, Arcidae excepted; (11) tendency for members, except the Ostreidae, to lie on the right valve; (12) abdominal sense organs on the posterior adductor muscle; and (13) intercommunication of the auricles, Anomiidae excepted. Two groups of the Lamellibranchia are proposed provisionally, namely Group I, Macrociliobranchia, including the orders Protobranchia (Pelseneer), Filibranchia (emended to include only the Mytilacea and Trigoniacea), Eulamellibranchia (Pelseneer, 1911), and Septibranchia (Pelseneer); and Group II, Microciliobranchia, with the order Pseudolamellibranchia, emended to include the sub-orders Arcacea (excluding the Trigoniidae), Anomiacea, Pteriacea, Pectinacea, and Ostreacea. The Macrociliobranchia will need revision, for it is very probable that the Filibranchia (emended), if not the Eulamellibranchia, are still not monophyletic.