ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 285 
as septa, dividing the inner cavity of the gill. They run about parallel to the gill- 
filaments, that is to say, vertically toward the edge of the gill, and consist of a 
tissue similar to that of the interlamellar outgrowth, with differences which will 
be described below. Most of these septa run continuously all the way from the 
base to the edge of the gill, while others are shorter, and are intercalated at various 
distances from the base, but each septum is always continued to the edge. Thus 
the interlamellar cavity is divided into a number of canals, running from base to 
edge, which are called water-tubes. The size of these water-tubes is variable, and 
differs considerably according to sex (See below). 
There is one exception among the Najades known to me from North America, 
which does not show this arrangement, Margaritana margaritifera.' Here the gills 
are also composed of filaments and possess an interlamellar outgrowth, which, 
however, is rather slightly and irregularly developed. But there are no septa dividing 
the cavity of the gills. Instead of this, the two 
lamine are connected by more or less cylindrical 
or irregular thickenings of the interlamellar out- 
growth, which are rather heavy and solid, and are 
placed quite irregularly. Sometimes these con- 
nections are a little elongated in the direction 
of the filaments, but they never are continuous 
septa, but rather patches of tissue, and if any reg- 
ularity is observable, it is a somewhat diagonal 
or oblique arrangement. Toward the edge of the 
gill, and in its hindmost part a vertical arrange- 
ment is indicated, but even here there are no com- 
Fie. 2. Margaritana margaritifera (L.); 
plete septa, and consequently no water-tubes (See right gills spread out and seen from below, 
fig 2) showing arrangement of interlamellar connec- 
tions. 
This peculiar development of the interlam- 
ellar junctions in Margaritana is in my opinion very important. While the pres- 
ence of water-tubes is a character common to all our other Najades, the absence 
of this feature in Margaritana is very remarkable, and I have no doubt that this 
character, together with the otherwise slight development of the interlamellar out- 
growth, which gives to the primary gill-filaments a greater significance in the 
whole make-up of the gill, points to a more primitive stage in the development 
of the gill in Margaritana, than in any other mussel. 
The secondary limb of each gill (outer in the outer, inner in the inner gill) 
1In quoting species, I shall, for the present, use Simpson’s nomenclature. 
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