FUSION OF FILAMENTS IN THE 

 LAMELLIBRANCH GILL. 



EDWARD L. RICE. 



IT is far from the purpose of this paper to enter fully into the 

 discussion of the morphology of the lamellibranch gill ; but a 

 brief preliminary statement of a general nature may serve to 

 define the terminology employed, and to simplify the following 

 more detailed description of a single point in the gill structure. 



When the animal is oriented in 

 the usual manner, with the hinge line 

 upward, the gills hang down on either 

 side, between the body and mantle, 

 as shown in Fig. i. Strictly con- 

 sidered, there is but one gill, or one 

 ctenidium, on each side of the body. 

 This ctenidium consists, fundamen- 

 tally, of a slight longitudinal ridge 

 along the side of the body, the gill 

 axis (Fig. i, a], and a double row of 

 ciliated filaments, which extend down- 

 ward into the mantle cavity of the 

 animal and are then reflected upward. 

 In common language each of these 

 rows of filaments, or the structure 

 arising therefrom, is termed a gill ; 

 and I shall retain this convenient, 

 though morphologically indefensible term, and designate that 

 half of the ctenidium next the body as the inner gill (Fig. i, b), 

 and that half next the mantle as the outer gill (Fig. i, c}. 



In some few forms the adjacent gill filaments remain entirely 

 free from one another, e.g., Anomia ; in other forms, as Mytilus 

 and Pecten, there is a union by means of tufts of very long inter- 

 locking cilia ciliated discs. In contrast to these two types, 



71 



FIG. i . Diagrammatic frontal section 

 of lamellibranch. Shell omitted; 

 gills shaded, a, gill axis; b, inner 

 gill; c, outer gill ; d and d' , descend- 

 ing lamellae of inner and outer gills ; 

 e and e ', ascending lamellae of inner 

 and outer gills; f and _/", interla- 

 mellar connections of gill and dorsal 

 appendage ; g, dorsal appendage of 

 outer gill. 



