ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 293 
In all these only the outer gill serves as a marsupium, and during the breeding 
season, and only during this time, each water-tube is divided into three tubes running 
from the base to the edge of the gill, the division being effected by two secondary 
septa, which grow out at right angles from the primary septa (or interlamellar 
junctions). Two of these secondary water-tubes are located toward the faces 
(lamin) of the gill, while the third one, which is much larger than the lateral 
ones, occupies the middle, and is separated from the two lamine by the lateral 
tubes. Only this central tube contains ova and embryos, and properly should be 
called an ovisac (See the cross-sections, Pl. LX X XVI, figs. 8, 9, 11b, 16). Thus 
we see that, while in the forms discussed above the whole water-tube assumes the 
function of an ovisac, in the present cases only a part of the original water-tube 
discharges this function. 
In addition, these ovisacs of the Anodonta-group are also closed above, at the 
base of the gill, by a similar tissue growing out from the septa at their lower ends 
(See Pl. LX XXVIII, figs. 3a, 11). As far as I can see, this tissue, which forms the 
ovisacs here, is furnished by, and forms part of, the epithelial layer of the septa. 
This structure apparently is an adaptation to the lengthened breeding season 
and the extreme swelling of the marsupium, which is observed in these forms, and 
serves for the aération of the mass of embryos, The swelling of the marsupium 
during the breeding season is very great here, much greater than in any of the 
forms belonging to the Quadrula-group. Thus it might become difficult to furnish 
the necessary oxygen to the developing eggs in the ordinary way, by the minute 
ostia perforating the laminz, and the cutting off of a part of each water-tube close 
to each lamina must have the effect of keeping up a lively current of water within 
the gill, between the lamine and the central ovisac. 
As has been said, this threefold division of the tubes is present only during 
the breeding season. I have examined numerous sterile females, in which this struc- 
ture is not seen (Pl. LXXXVI, fig. 15); in others it is indicated (Pl LXXXVI, 
figs. 7b, 12b) by a stronger fold (or group of folds) of the epithelial layer of the 
septa, near their insertion into the lamin. In specimens where the eggs begin to 
go into the gills, this structure is sometimes not yet developed, but it appears soon, 
and the epithelial folds, which form the secondary septa within the water-tubes, 
begin to grow into the lumen of the water-tubes, and the folds of the opposing 
faces of the two septa finally unite in the middle. The point of union (cross- 
section of the line of union) is often distinctly seen in sections. After the dis- 
charge of the glochidia these structures disappear rapidly. I have seen them still 
present in some specimens, from which the embryos had recently been discharged 
