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 f ) ; 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, Muntacuta, Cryptodori), in Scioberetui, 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, g), or may possess it, and be either smooth, e.g. Tellina 

 (Fig. 206, H), or folded, e.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 inter 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, y) ; 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 

 the outer gill -plate, and anteriorly, 



View of the ventral (nedal) aspect of the where the visceral mass illterpOSCS 



animal of Area noac, the mantle-flap and f nrpvpn f fhp union nf tho rpflortprl 



gill-ftlaments having been cut away, a, to prevent Lf 



mouth ; b, anus ; c, free curved extremity lamellae of the inner gill-plates, 



of the gill-axis of the ri'^ht side ; ((, do. of 



the left side; e, /, anterior part of these these latter are attached to the 



^ilT^r'irS"Sr t 1 11 uc,e b ; 0d J; viscero - pedal mass (Fig. 209, 



rttoWlTS'iif SttVSj B ' c - D >- These v f io s Ullions f 



m, sole of tho foot ; n, byssus cavity. (After the extremities of the reflected 



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. 



J 



Fio. 208. 



