1908.] ASPIDOBRAXOH GASTROPOD MOLLUSCS. 831 



lateral portions are readily parted from one anotliei'. Wben a 

 single lamella is separated out, stained and examined under the 

 microscope, it has the appeai'ance shown in fig. 23, the dark 

 central tract Avith diverging horns being the expression of 

 numerous deeply stained and closely packed nuclei in this region. 

 In short, the epithelium covering the lamellae is not uniform. 

 The following arrangement can be determined in section : — The 

 lateral tracts of each face of each gill-lamella are clothed by a 

 cubical epithelium containing isolated or grouped gland-cells of 

 oval shape with clear contents. The epithelial cells of these 

 tracts (if they are ciliated at all, which 1 am inclined to doubt) 

 bear exceedingly short and fine cilia. Tlie thickened margin of 

 the lamella always bears three or four "frontal" cells at its 

 extreme edge ; these cells, as in the gills of Lamellibranchia, 

 carry a tuft of short rather stifi" cilia. External to them are a 

 few cells devoid of cilia, and at the extreme ends of the lamellfe 

 large gland-cells alternate with the epithelial cells of this region. 

 Sections through the dark median band with its two horns show 

 that this is a tract of moi'e columnar cells, closely packed together, 

 with deeply staining nuclei, each caiiying a tuft of very long 

 cilia which interlock with those of the adjacent lamella and are 

 the cause of the adherence noted above. 



There are no supporting rods or skeletal bars, such as those 

 described by M. F. Woodward in Pleurotoinaria, but, as shown in 

 fig. 21, the connective tissue underlying the epithelial cells is 

 thickened near the attachment of each gill-lamella to the axial 

 plate. There is some resemblance betAveen the arrangement of 

 the lateral cilia in Septaria and other Neritida^ and in Pleuro- 

 iomarict, and by parity of reasoning the ciliated tracts of the 

 former genus must differ from those of Lamellibranchia in the 

 same manner that Woodward has shown them to differ in the 

 case of the latter genus. It is interesting to note the structural 

 analogies of gastropod and lamellibranch gills. In the case of 

 the ISTeritidae the ciliated tracts fulfil the same functions as the 

 ciliated discs of the Filibranchia. But their arrangement is 

 different. The cell-mechanism is the same, but it cannot be 

 doubted that it has been independently evolved in the two 

 groups, afibrding a good instance of the evolution of similar but 

 not identical structure in similar organs subject to similar 

 conditions. 



Lenssen, describing the gills of Xeritina fiuviatilis, has given 

 an incorrect account of the epithelium. He figures an almost 

 uniform covering of ciliated cells, and among them a few gland- 

 cells, I have found the same arrangement in N. fliwiatiUs as in 

 Septaria^ and Lenssen would appear either to have altogether 

 overlooked the ciliated tracts, or to have confused in a single 

 drawing and description the ciliated cells of the one tract and 

 the glandular cells of the other. It is curiously difficult to obtain 

 good preparations of the gills of N.flaviatilis, and if my attention 

 had not been called to the subject by the much more obvious 



Peoc. Zool. Soc— 1908, No. LIII. 53 



