GILLS or LAMliLLIBRANCH MOLLUSCA. 55 



closely contracted condition. The power of contraction and 

 expansion which obviously resides to a large degree in these 

 iiiterfilamentar junctions suggest the possibility of their con- 

 tracting rhythmically ; on the other hand, it is exceedingly 

 likely that they are simply reservoirs capable of turgescence 

 under certain physiological conditions. 



Fig. 7 is drawn from a transverse section passing through 

 the descending and ascending limbs of two filaments. The 

 section coincides with the position of the interfilamentar 

 (ciliated) and interlamellar junctions. The structure of the 

 section of an individual filament is seen from this figure and 

 from fig. 6 to be similar to that of Area, the main difference 

 being that the long cilia of the latero-frontal epithelium are 

 carried on two rows of cells {Ife, Ife", in figs. 6 and 7), and 

 further the lacunar tissue forming trabeculae within the cavity 

 of the filament is not limited to a single septum, but consists 

 of 7iutnerous irregular bands. Professor Kollmann has quite 

 recently (' Archiv fiir Mikrosk. Anat.,' 1876) shown that by 

 means of the usual " silver process " a pattern of cell-out- 

 lines similar to those of the vertebrate lymphatics may be 

 brought into view on the limiting surface of the filamentar 

 cavity. 



The form of the epithelial prominences (ep.) carrying the 

 cilia of the ciliated junctions is shown in figs. 6 and 7. 



The cessation of the chitinous deposit in that part of the 

 filaments which grows out to form the interlamellar junc- 

 tions is demonstrated by fig. 7, as is also the continuity of 

 the cavity of the filaments of each lamella with the cavity of 

 the interlamellar junctions. A blood-corpuscle (marked be.) 

 is seen in fig. 7, lying in the cavity of the interlamellar 

 junction. 



The filaments, as seen from the surface of the gill-lamella 

 in specimens stained and clarified with Canada balsam, may 

 be focussed at various depths, so as to give a series of longi- 

 tudinal optical sections. In fig. 4 four filaments are repre- 

 sented so focussed as to show two rows of ciliated junctions 

 (cj.). In fig. 5 three views are given of a portion of a fila- 

 ment taken at a part between two ciliated junctions, and 

 focussed at three different depths. That to the left is the most 

 superficial, giving the surface of the outer or lateral border 

 of the filament, that is to say, the surface which forms part 

 of the general outer surface of the lamella. In the centre 

 are seen the nuclei of the ''frontal epithelial cell^ " (fe., and 

 in figs. 6 and 7) ; on each side of these is a row of latero- 

 frontal epithelial cells [Ife.) with long cilia, the bundles of 

 cilia appearing each like a single and coarse cilium ; seen 



