ORTMANN!: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 291 
In the sterile female the septa separating the water-tubes in the marsupial gill 
(See Pl. LXXXVI, figs. 2, 4, 7b, 100, 140; Pl. LXX XVII, figs. 2, 6, 7, 9) are much 
more crowded than in the male. There are, on an average, only from about five 
to ten filaments between two septa, although in some cases this number may increase 
to twenty. But in every species this number is considerably smaller in the female 
than in the male. This difference is very striking, and may be detected by the 
naked eye on holding up the gill toward the light. When I was hunting for sterile 
females among my material I always used this method in discovering the sex, and 
the much more crowded septa in the gill examined always indicated the female sex. 
I never made a mistake, and a specimen selected as a sterile female according to 
this test always proved to belong to this sex, when a closer microscopical examina- 
tion was made. 
In consequence of the more crowded condition of the septa the water-tubes 
have a different cross-section and are not elongated in the longitudinal direc- 
tion of the gills, as those of the male, but are either more rounded or square (See 
Pl. LXXXVI, figs. 10, 14; Pl. LXX XVII, figs. 2, 9), or even elongated in the 
transverse direction (See Pl. LX X XVI, fig. 7). 
But not only does the more crowded character of the septa indicate a marsupial 
gill, but the septa themselves have a different structure. While in the male and the 
non-marsupial gill of the female the septa are not much different from the inter- 
lamellar outgrowth, in the marsupial gill they are considerably altered. They 
are more strongly developed, appear more as independent structures, and their tissue 
is more compact, without lacunz or blood-vessels. Generally, they are thicker, 
and also longer in the direction from lamina to lamina, and, what is their most 
striking character, their epithelial layer is quite different. While in the male type, 
described above, the epithelium, which faces the water-tubes, is a simple layer, in 
the marsupial gill of the sterile female it is folded and thrown up into wrinkles, 
which often assume considerable proportions (Compare Pl. LXXXVI, figs. 1 and 
2, 7a and 7b; and Pl. LXXXVII, figs. 1 and 2, 8 and 9). 
There is no question that this peculiar structure of the septa of the marsupial 
gills is an adaptation to their function. When the marsupium in the breeding 
season becomes charged with eggs, these gills naturally must swell, and they 
actually do so, as is well known. The swelling can be only in one direction, namely 
transversely to the longitudinal plane of the gill, that is, in the direction from 
lamina to lamina. This means, that the two lamine of the gill are forced apart, 
and since they are connected by the septa, the latter, if they are not to be torn, 
must be built accordingly. The description and the figures given here of the 
