284 MEMOIRS OF THE CARNEGIE MUSEUM 
There is always in the female an anatomical difference in the marsupial gills, and 
this difference may be detected even without a microscopical investigation, although 
the latter is generally advisable in order to render the structure absolutely clear. 
In addition the writer has found, chiefly along the edge of the mantle, a 
number of structural differences which are also important, and will be described 
below. 
A. GENERAL STRUCTURE OF THE GILLS. 
The primary elements of the gills are the gill-filaments (gill-tentacles of Brooks) 
—cylindrical or somewhat compressed tubes, which arise in two longitudinal rows 
from a common base (ctenidium) between the foot and the mantle of the mussel, 
and hang down into the branchial chamber. These filaments are doubled back 
upon themselves, the outer row outwardly, the inner inwardly. The filaments 
of each row being connected with each other (See below), there are thus two gills 
on each side of the animal, an inner and an outer one, each consisting of two lamin 
(lamelle) composed of filaments, the laminz including between them a space. The 
two laminz are distinguished as the primary and the secondary limb of the gill, 
the former being the descending adaxial part, the latter the reflected and ascending 
part. The latter part is the outer lamina in the outer, and the inner lamina in 
the inner gill. 
On the inside of each lamina, toward the cavity of the gill, there develops 
a peculiar tissue, called interlamellar outgrowth (Taylor, p. 177). This connects 
the gill-filaments of each lamella, leaving more or less regularly placed open holes 
(ostia), by which the inner cavity of the gill communicates with the outside. This 
interlamellar tissue forms a rather large part of the gill, in fact the lamella itself 
appears composed chiefly of this tissue, while the original gill-filaments form only 
its outer layer, separated from one another by the intertentacular (Brooks) or inter- 
filamentar grooves. The interlamellar tissue is permeated by lacunar blood-vessels. 
The filaments develop along their sides chitinous rods for the support of the gill. 
Two such rods belong to each filament, but these rods are not continuous, but 
interrupted, consisting of longer or shorter, disjointed pieces. 
The filaments and their chitinous rods run about vertically, that is to say, 
from the base toward the edge of the gill; a slightly fan-shaped arrangement is 
sometimes indicated, corresponding to the curved edge of the gill, upon which 
they stand vertically. 
The interlamellar cavity of each gill is not simple, but is divided. There 
are “interlamellar partitions” (Brooks) or “interlamellar junctions” (Taylor), which 
connect the two laminz of the gill. In most cases, these connections are developed 
