228 THE LAMELLIBRANCHIA 
others, constituting a principal filament lying at the bottom of the 
furrow between two successive folds. Not all the Lamellibranchia 
possess complete gills consisting of two gill-plates each formed 
of two reflected lamellae ; in Anomia aculeata and Dimya none of the 
gill- plates have reflected lamellae; in Adacnarca the reflected 
lamella is absent from the inner plate (Fig. 207, br’); in Lasaea 
and all the Anatinacea there is no reflected lamella in the outer 
plate, and finally, the external plate is aborted in certain Lucinidae 
(Lucina, Corbis, Montacuta, Cryptodon), in Scioberetia, and the Tere- 
dinidae (Fig. 206, K). The external gill-plate, whether complete 
or not, instead of being directed ventrally with its lower border 
parallel to that of the inner plate, may be directed dorsally, and 
in such case may be without the reflected lamina, as in Solenomya 
(Fig. 231, 9), or may possess it, and be either smooth, ¢.g. Tellina 
(Fig. 206, H), or folded, ¢.g. the Anatinacea. 
The posterior, that is to say, the distal ends of the branchial 
axes are primitively free, as in the aspidobranch Gastropoda (Figs. 
82, 127), and in such case the gills 
show no concrescence wufer se, nor 
do the free extremities of the re- 
flected lamellae unite with the 
mantle. This primitive condition 
is found in the Protobranchia (Fig. 
231), the Arcidae (Fig. 208, d), the 
Trigoniidae, the Mytilidae, and the 
Pectinidae (Fig. 235, 9); and among 
the Filibranchia it is only in the 
Anomiidae that the gills are united 
by the dorsal edges of the internal 
lamellae of the inner plates. But 
in all other Lamellibranchs the 
gills, in addition to this union, are 
joined to the mantle by the upper 
edges of the external lamellae of 
Ue Ge. the outer gill-plate, and anteriorly, 
View of the ventral (pedal) aspect of the Where the visceral mass interposes 
animal of Arca noae, the mantle-flap and : 
gill-filaments having been cut away. 4, to prevent the union of the reflected 
mouth; b, anus; ¢, free curved extremity lamellae of the inner cill-plates, 
of the gill-axis of the right side ; d, do. of 5 
the left side; e, f, anterior part of these these latter are attached to the 
axes fused by conerescence to the body- : =% 
wall; g, anterior adductor muscle; h, VISCCTO - pedal mass (Fig. 209, 
postériot aavetor muscle; anterior? B, C, D). | These various unions of 
m, sole of the foot ; n, byssus cavity. (After the extremities of the reflected 
ae ig lamellae, whether with one another 
or with the mantle or with the viscero-pedal mass, may be effected 
by simple ciliary junctions—e.g. in Avicula, Pinna, Anomia, Solen, 
the Anatinacea—or by a true concrescence. 
