On the Ciliary Mechanisms and Interrelationships of Lamellibranchs. PAET III: Types of Lamellibranch Gills and their Food Currents. By Daphne Atkins, B.Sc. Marine Biological Laboratory, Plymouth. With 18 Text-figures. CONTENTS. PAGE INTRODUCTION .......... 375 TYPES OF GILLS AND THEIE CILIATION 379 TYPE A. Protobranchia 380 TYPE B. Filibranehia and Pseudolamellibranohia . 382 TYPE C. Many Eulamellibranohia 388 TYPE D. Unionidae 408 TYPE E. Tellinidae, Semelidae, and Anatinaeea .... 409 TYPE F. Lasaea rubra 412 TYPE G. Luoinidae, Montaoutidae, and Teredinidae . 413 DISCUSSION 415 SUMMAKY 419 INTRODUCTION. IN the course of work on living gills, those of some ninety odd species of Lamellibranchia, belonging to sixty genera and forty-one families, have been investigated. The great majority were marine, three only, Dreissensia polymorpha (Pallas), Sphaerium oorneum (L.), and Anodonta anatina (L.), being from fresh water. Separate accounts of the gills and their currents of all the species would entail much needless repetition, and, except for some forms (Nuculanidae, Arcidae, Anomiidae, Pteriidae, Solenidae, Ostreidae, Pecti- nidae) which are of special interest and have been dealt with in separate papers (see Atkins, Parts I and II), the majority 376 D. ATKINS can be grouped under more or less clearly defined types. Addi- tional notes on certain of those possessing specialized sorting devices are given in Part II. In the following notes it is quite possible that instances of frontal currents in opposite directions on the same gill filament, as in Barnea Candida (L.), Petricola pholadiformis Lamarck, Spisula sub- truncata (da Costa), and Spisula elliptica (Brown), have been overlooked and only the obvious currents recorded. A fact that has emerged from the work is the stability of form and of the direction of the currents on the inner demi- branch, and the variability of the outer one. The inner demibranch has: 1. With apparently few known exceptions, both descending (direct) and ascending (reflected) lamellae, though the depth of the ascending relative to the descending varies. The few exceptions are all found in the Filibranchia. In two of them, Heteranomia squamula (L.), and Dimya argentea (see Ridewood, pp. 193-5), both demibranchs consist of descend- ing lamellae only. Variation of the inner demibranch while the outer consists of both lamellae is found according to Pelseneer (1903, p. 41, PL vii, fig. 86; 1906, fig. 207, p. 227, and p. 228) in Adacnarca nitens Pels., a member of the Arcidae, and in Plicatula australis (1911, p. 96, PL xii, fig. 11) where the inner demibranch consists of a descending lamella only. Eidewood (1903, p. 208), however, found both demibranchs of Plicatula australis^to have descending and ascending lamellae, as did also Watson (1930, p. 26, PI. v, fig. 3).^ 2. On the inner demibranch there is always a longitudinal current along the free edge which is oral in direction, except in the Arcidae and Anomiidae where it is aboral in direction (see Part I). There is generally a more or less marked groove at the 1 Pelseneer (1911, p. 32) suggested that Ridewood had wrongly deter- mined the species, but after the confirmative work of Watson some other explanation of the differing accounts would seem to be necessary. Ciliated disks are present only at the ventral edges of the demibranchs and the upper edges of the ascending lamellae in this species, so that possibly the ascending lamellae are apt to fall down if material is not well preserved. THE CILIARY MECHANISMS OF LAMELLIBRANCHS 377 free edge,1 but in some bivalves, such as members of the Arcidae, Anomiidae, Pectinacea, Tellinidae, and Semelidae, it is extremely shallow or absent. 3. On the inner demibranch the frontal currents are toward the free edge on both lamellae. However, in Arcidae and Anomiidae, and in Cultellus pellucidus, there is a varia- tion, in that independent or separate dorsal and ventral currents occur on all gill filaments of this demibranch (see Part I); while in certain Pseudolamellibranchia and Solenidae, which have plicate gills, not only does this occur on certain of the ordinary and apical filaments, but in the troughs of the plical grooves the current is entirely dorsalward on both lamellae. In many bivalves for a short distance over the proximal ends of the descending filaments there may be a current axial in direction (e.g. in certain Mactridae, Sphaerium corneum (L.), &c). This is indicated by Orton (1912, fig. 14, p. 462) in his diagram of the general mode of feeding in Lamellibranchs. The outer demibranch on the other hand shows considerable variation both in structure and currents. Pelseneer (1911, pp. 95-6) has noted its reduction in length (extends not as far anteriorly as the inner) and in depth (dorso-ventrally) in a number of forms. The chief structural variations are as follows: 1. The outer demibraneh may be about the same depth as the inner (e.g. in Mytilus, Galeomma, Pinna, Lima): where such gills are plicate the plications are as deep in the outer as in the inner demibranch. But in most forms it is considerably, though not evenly, narrower (e.g. Sphaerium, Cardium), and, where such gills are plicate, the plications are often less marked than on the inner demibranch, with fewer filaments to a plica. 2. The outer demibranch, with few exceptions, was found to be without a supra-axial extension in the Filibranchia and Pseudolamellibranchia (as constituted by Pelseneer, 1911), 1 The free edge of the demibranohs may be regarded as normally morpho- logically ventral, but is by no means topographically ventral in relation to the shell in all bivalves. In those in which the gill axes run obliquely dorso-ventral, the free edges of the demibranchs may be actually almost anterior. 878 D. ATKINS though in some species the ascending lamellae of both demi- branchs were as deep, or nearly as deep, as the descending. In Ostrea they may be somewhat deeper than the descending, and this is especially noticeable of the outer demibranch of Ostrea angulata. A supra-axial extension was found in most of those Eulamellibranchs which have an outer demibranch consisting of both lamellae; its degree of development varies greatly, however, from narrow, Lepton squamosum, to deep, with oblique filaments and frequently smooth when the rest of the demibranch is plicate as in certain of the Veneridae; and it may not be present throughout the length of the demi- branch. Graham (1934a, p. 184) gave as characteristic of the Solenidae the absence of a supra-axial extension to the outer demibranch. In members of this family it appears very narrow in the living animal, but is nevertheless obvious in sections (see Atkins, 1936, Part I, p. 299, fig. 42). 3. The outer demibranch may be upturned and consist of both lamellae, as in the posterior third of the gill of Tellina (see Eidewood, 1903, p. 151). In the Semelidae (= Scrobi- culariidae) (Scrobicularia plana, Abra alba, Abra nitida) there also appears to be a recurved portion to the outer demibranch, but it is difficult to judge how far this is due to contraction: Eidewood (1903, p. 235) stated that it is com- posed of the direct lamella only in Scrobicularia piperata (= Scrobicularia plana). In all these forms the junction of the two lamellae of the outer demibranch is made on a gentle curve, not at an acute angle as between those of the inner demi- branch. A much rounded lower edge to the inner demibranch is known only in some Verticordiidae (Anatinacea) (Eidewood, 1903, p. 266). 4. The outer demibranch may be upturned and consist of the direct lamella only, as in the middle third of the gill of Tellina and throughout the gill of the Anatinacea (Eidewood, 1903, p. 151). Pelseneer (1911, p. 96) considered that dorsal orientation of the outer demibranch is not due to upward bending, but that what remains of this demibranch is nothing but the supra-axial extension in the Anatinacea, and the supra-axial THE CILIARY MECHANISMS OF LAMELLIBRANCHS 379 extension with a mere trifle of the demibranch itself in the Tellinidae. 5. The outer demibranch may consist of the direct lamella only but normal in direction as in Lasaea rubra (Pelseneer, 1889, fig. 2, p. 40). In Heteranomia squamula and Dimya argentea both demibranchs consist of descending lamellae alone. 6. The outer demibranch may be entirely wanting, as in Lucinidae and Montacutidae. In the Teredinidae it is but a vestige (see Eidewood, 1903, p. 260). It is wanting in the an- terior third of the gill of Tellina donacina, Tellina fabula, and of A bra. There is some variation in the Tellinidae as to the length (antero-posterior) of the outer demi- branch: in Tellina donacina and Tellina fabula it is considerably shorter than the inner, as Eidewood (1903, p. 234) found in Tellina nitida and Tellina (Arcopagia) capsoides, but in Tellina crassa and Tellina tenuis it extends as far forward as the inner, though anteriorly it is very narrow. It is tempting to connect the variability of form of the outer demibranchs in different Lamellibranchs with their later forma- tion embryologically. Unfortunately there is not a great deal of detailed information on the development of gills. Eice (1908) found in Mytilus edulis—in which the outer and inner demibranchs are very similar—that though the filaments of the outer demibranch do not begin to appear till about twenty filaments of the inner demibranch have been formed, there is a change in the method of formation of the filaments when the spat is about 1 -6 mm. in length, and the inner and outer demi- branchs then develop pari passu. TYPES OF GILLS AND THEIR CILIATION. In the Lamellibranchia a number of different types of gill structure are found, most of which Eidewood (1903, fig.