420 BULLETIN OF THE UNITED STATES FLSH COMMISSION. 



The large ciliated cells, theu, occui' either in single rowf^ or, 1 believe, in the 

 iiKijority of cases, in comi^ouncl rows, showing several closely packed cells in cross- 

 section. They do not always occui)y a latero- frontal i)osition. 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 ttl anient is com])osed of low cells whose boundaries are 

 indistinct, excepting where an interfilamentar 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 cdlium moving independently of the others. This motion is of 

 service, probably, in reuniting patches which have been separated fi'omone another. 



The chitinous lining {ch) is thin and at no place shows a marked thickening. It 

 is slightly thicker just interior to the straining-line cells (at ch). 



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 waUs, though in 

 section they are, as in Solenomya, closely applied to each other. Here and there ( i's) 

 this space may be recognized. 



THE (JILL UF PKCTEX I H RADIANS. 



The gillsof J->c«, just described, and those of many other lamellibranchs, possess 

 a smooth outer surface. In many forms, however, the lamellie, both outer and inner, 

 arethrown 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 lamelhie of the gills of Peeten (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 

 <;ases {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 dift'erent folds. All of tiie 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 



