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BULLETIN OF THE UNITED STATES FISH COMMISSION. 
The large ciliated cells, then, occur either in single rows or, I believe, in the 
majority of cases, in compound rows, showing several closely packed cells in cross- 
section. They do not always occupy a latero-frontal position. Considering these 
reasons, and the supposed function of their cells, I would designate them as the simple 
and compound straining lines. 
In the case of Dreissena there are two simple, in Yoldia two compound, lines. 
The inner part of the fil ament is composed of low cells whose boundaries are 
indistinct, excepting where an inter filamentar junction appears (cj). These are round 
patches of columnar cells bearing long cilia, arranged in the same region of the fila- 
ment, at intervals throughout its length. The patches appear opposite each other 
on neighboring filaments, their cilia closely interlocked, and form a comparatively 
firm attachment. The cilia of these connecting discs are too closely packed together 
to allow of much motion, but by a high magnification, whether locked together or 
torn apart, the cilia of these patches show in a very feeble way the characteristic 
lashing in one direction and then the slow recovery of position. This motion is very 
slow indeed, each cilium moving independently of the others. This motion is of 
service, probably, in reuniting patches which have been separated from one another. 
The cliitinous lining (eh) is thin and at no place shows a marked thickening. It 
is slightly thicker just interior to the straining-line cells (at eh). 
The membrane connecting the two limbs of the filament possesses walls, each of 
which is, as usual, made of a single layer of cells. They do not differ much in appear- 
ance from those of the inner part of the filament proper, with which they are continu- 
ous, except that they are slightly longer. These cells possess no cilia. 
There is probably a continuous, vascular space between these walls, though in 
section they are, as in Solenomya, closely applied to each other. Here and there (vs) 
this space may be recognized. 
THE GILL OF P15CTEN IRRAJDIANS. 
The gills of Area, just described, and those of many other lamellibranchs, possess 
a smooth outer surface. In many forms, however, the lamellae, both outer and inner, 
are thrown into definite folds, in order that their surface may be increased to facili- 
tate aeration of the blood or the procuring of food, or both . 
This folding appears in the lamellae of the gills of Pecten (Fig. 86, PI. xcii). The 
filaments are joined to each other only by cilia. The filament which occupies the sal- 
ient angle of the fold ( sa ) is similar to the others in this form, although in some other 
cases (Lima, etc.) it is much enlarged. The filament at the reentering angle in Pec- 
ten ( ra ) is greatly enlarged and peculiarly modified. 
Fig. 86 represents a, section passing through an entire fold of the gill. This fold 
is made up of fifteen filaments, not counting the specialized ones at the reentering 
angle. The number varies slightly in different folds. All of the descending limbs of 
the filaments, forming one lamella of the gill, are cut on one side, and all the ascending, 
forming the other lamella, appear on the opposite side. 
A section of a single filament is shown in Fig. 83. The chitinous layer is of nearly 
equal thickness, being thinner at the outer and inner edges, and thickest beneath the 
