72 RICE. [VOL. II. 



which may be grouped together as filamentous gills, we find in 

 the great majority of lamellibranchs a more or less strong 

 development of vascular connections between the filaments, 

 binding them together to form the lamellae from which the 

 class takes its name. Such gills may be termed lamellar. 



As each filament consists of two limbs, a descending and an 

 ascending, so each gill is composed of two lamellae, correspond- 

 ingly designated as descending (Fig. i, ^and d') and ascending 

 (Fig. i, c and c'}. These lamellae are usually connected with 

 one another by more or less complicated interlamellar connec- 

 tions (Fig. i,/). 



The ascending lamella of the inner gill may remain free, or 

 its upper margin may fuse with the body and, behind the body, 

 with the corresponding part of the gill of the other side. The 

 ascending lamella of the outer gill may lie free in the mantle 

 cavity or may be attached to the mantle on a level with the gill 

 axis ; or it may be continued dorsally above this line, forming 

 a dorsal appendage (Fig. i, g), which is finally attached to the 

 body, or rather to the fusion line of mantle and body. Thus 

 the dorsal appendage consists of the ascending lamella alone, 

 and may show a structure decidedly different from that of the 

 gill proper. Connections similar to the interlamellar connec- 

 tions of the gill attach the dorsal appendage to the body wall 



(Fig. I,/)- 



The lower free margin of some gills is smoothly rounded off ; 



in other cases the border is deeply notched by a groove running 

 from end to end of the gill. This may be called the marginal 

 groove. In the diagram the groove is seen in cross-section in 

 the inner gill (Fig. i, //), while the outer gill presents a smooth 

 margin with no sign of a furrow. Exactly these conditions are 

 found in a large number of lamellibranchs, e.g., Astarte, Dreis- 

 sensia, Cardium, Psammobia. It may be suggested in passing 

 that the somewhat unintelligible distinction of "double gills" 

 and "single gills," in the classical paper by Williams, 1 may 

 perhaps be based on the presence or absence of this marginal 

 groove. 



1 Williams, T., "Respiratory Organs of Invertebrates," Annals and Magazine 

 of Natural History. Vol. xiv. 1854. 



