MOiU'JlOL(J(jiY OF LAMELLIBKANCHIATE M0LLU8KS. 423 



Very fie(|ueiitly, however, openings appear between the tihiiiieiits. In fold 3 this 

 is the ease with the iiiajority oi the tihimeuts in the plane of this section. Here water 

 may enter between the filaments into the interior of the fold and thence into the water 

 tube of the gill [ict). 



A filament has much the same structure as in Pecten, excepting that the chitinous 

 layer is abruptly thickened a short distance from its outer edge (ch). There is no con- 

 stant septum across the l)loo(l channel of the filament. I believe that an endothelium 

 is i>resent, but can not be positive. 



The surface epithelium is of columnar, ciliated cells in the frontal region (/,). At 

 its lateral edges a])])ear gland cells in the usual position (glc). They are seldom seen 

 on the inner edge of the filament in 0. virgminiiu^ described by Lankester (No. 9) as 

 being as numerous here as at the side of the frontal region in 0. edulis. The gland 

 cells are not spherical, but nuich elongated in the specimens Avhich I have examined. 



xVs in Pecteuj certain cells crowd together a little distance inward from the gland 

 cells, to form the compound straining line. The ciliated lines ((/7) do not interlock, but 

 touch each other and form a barrier to foreign particles which might otherwise be 

 carried into the water tube. 



The filament at the reentering angle {ra) is very greatly modified. It is essen- 

 tially similar, whether or not it is extended across to the opposite lamella of the gill 

 to form the thick partitions between the water tubes. The structures showing it to be 

 morphologically a filament are the ciliated frontal epithelium (Z^), the chitinous layer, 

 here modified into two extremely large and thick rods (c/^2), and, in most cases, a 

 blood channel interior to these. 



The filaments next these at the angle of the fold are broadened and shortened; 

 tlieir chitinous layer is much enlarged and has become rod-like on either side. In 

 fact, they offer a transitional stage between the ordinary filament and the extremely 

 modified filament at the reentering angle of the folds. Such a transitional filament 

 is that rei)resented at tf. 



The partitions connecting the gill lamellse are lined by an epithelium of cuboidal 

 cells (ep), continuous with that which lines the remaining w^alls of the water tube. 

 The interior of the partition consists of many longitudinal and cross muscle fibers 

 {mus), and between these often lie masses of large circular cells (pc), which I have 

 only seen in the partition. A considerable portion of the ])artition is often occupied 

 by blood spaces {bv). 



Muscle fibers also occur in masses interior to those reentering angle filaments not 

 connected with a partition. They seem to be extended in the long axis of the gill. 



