MORPHOLOGY OP L AMELLIBRANCHI ATE MOLLUSKS. 
415 
another, and the same number on the other side, opposite these, may contract them- 
selves apparently from every direction, thus giving the appearance of a deep groove 
running entirely around the gill. How this contracted zone begins to move along the 
length of the gill and it may move in either direction. In this wave of contraction but 
three or four plates on either side are ever concerned at one time. These waves are often 
single, and at times several may follow one another in succession. 
Still another wave contraction may often be noticed on the ventral sides of the 
plates. The ventral side of a single plate, or at most two plates, is affected at one 
time. These waves occur independently on either side of the gill. A single plate 
bends a certain region of its ventral surface forward or backward, so as to separate 
this region from one of the neighboring plates and bring it close to the plate on 
the other side, either before or behind it, as the case may be. This latter plate quickly 
bends in the same way, the first one assuming its original position and then the 
succeeding plates, thus causing a wave of this bending to run along the length of 
the gill. These waves may run either forward or backward. Single plates may con- 
tract slightly independently. 
If a gill be dissected out from the body, these contractions still continue to take 
place whenever it is touched. The action of the cilia is so powerful that the entire 
gill is made to move about in the water. 
Collection of food . — Mitsukuri came to the conclusion that this primitive form of 
gill was concerned only in the aeration of the blood, and that it was probably not 
concerned in the procuring of food. I was able to observe in the case of Yoldia, how- 
ever, that not only was the function of gathering food possessed by the gills, but that 
it was performed with amazing rapidity. Carmine particles in the water once com- 
ing in contact with the ventral edges of the plates, having been swept there by the 
powerful currents which these ciliated borders set up, are at once hurried along toward 
the wide, median, ventral groove of the gill, into which they are thrown. On the 
way to this groove they have evidently become covered by mucus secreted by gland 
cells; for the separate particles of carmine are soon firmly cemented together, and 
passing along the groove anteriorly, less rapidly than on the edges of the plates, 
though still at a comparatively fast rate, they are finally piled up at its anterior end 
and gradually passed upon the surface of the palp. 
Structure . — A diagrammatic view of the ventral side of the gill is given in Fig. 78, 
PI. xoi, in which the thickened ventral edges of the plates (p) are shown on either 
side of the groove (gr). These edges in the living gill are only slightly bent down- 
ward, and are not so curved in outline as represented in Fig. 81, which was drawn 
from a hardened specimen. From the groove (gr, Fig. 78) to the point cj, the plates 
are in no way connected with one another, and one may see entirely through the gill 
between them, when examining from below. 
At the point cj, where the edge of the plate turns abruptly upward, occurs a cili- 
ated junction not before described in these forms, between adjoining plates. The cilia 
from neighboring plates interlock closely, making a comparatively substantial union. 
This ciliated junction is not confined to the point mentioned, but extends from it 
upward on the lateral sides of the gill for about two-thirds of the distance to the 
upper pointed extremities of the plates, the ciliated union being confined to the lateral 
edges of the plates. 
