416 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



All examination of sections made througli the gill plates in different regions will 

 give a better idea of these points. Fig. 79 represents a section cutting the plate of the 

 gill of Yoldia in a plane represented by the line a in Fig. 81. This section thus shows 

 the structure of both the ventral and the dorsal borders of the plate as well as that of 

 its interior regions. The thick ventral border is shown at the upper end of the figure. 

 As pointed out by Mitsukuri for N'ucula, this corresponds in structure to that which 

 is usual in the outer edges of the gill filaments of other lamellibranchs. This iJortion 

 of the gill is supported by a framework of the so-called chitiu (cJi), which widens out 

 at about the middle of its extent and makes the inclosed blood space a conspicuous 

 channel at this place (6). The "chitin" is here quite thick and gradually becomes 

 thinner in either direction. 



The outer epithelium of the ventral edge is made up of closely packed columnar 

 cells. Those at the extreme edge of the plate (e) bear peculiarly long and powerful 

 cilia, which differ greatly in appearance from cilia in other regions of the ventral edge 

 of the plate, or in fact in any other gill. In cross-section they are circular. The pen- 

 cil cells, found frequently in other regions of the body in lamellibranchs, bear a stiff 

 bristle which is found, by maceration, to be made up of several fased cilia. Whether 

 or not these cilia may be of a similar structure I can not say, but it seems hardly 

 necessary to suspect such a structure merely on account of their great size. Judging 

 fi'om their position and that of the other lines of cilia on the edges of the plate, these 

 greatly developed cilia of the frontal cells are the ones which produce the rapid cur- 

 rents in the water over the ventral surface of the gill. 



At some little distance inward from these frontal cells, the epithelium rises into 

 a ridge on either side (r) and these bear a second row of cilia much shorter than the 

 first, and very fine. They protrude laterally and outward, and their ends touch those 

 of similar rows of cilia on contiguous plates. These rows, however, do not interlock 

 with one another, and I believe that they serve simply to prevent currents of water, 

 bearing food particles and other foreign bodies, from getting in between the plates, and 

 not at all as a means of connecting neighboring plates. 1 think, also, that similar lines 

 of cilia on the filaments of other forms, which will be noticed, serve the same purpose. 



A third line of cilia is borne by elongated cells, which do not, however, form a 

 ridge, and is situated near the inner edge of the thickened, ventral edge of the plate (t). 

 These cilia are longer than those of the second row and are also fine in appearance. 

 In sections they appear bent outward toward the edge of the plate. 



All these rows of ciliated cells are sharply defined and the cells between them bear 

 no cilia. Gland cells usually exist in the frontal region of all lamellibranch gill filaments 

 or plates. If they appear in this region in the gill of Yoldia, it is in very small num- 

 bers. I have frequently seen cells here which appeared much like gland cells, but I 

 have not been able to decide positively that they were such. 



Lining the blood space inclosed by the chitinous layers is a perfectly distinct 

 endothelium, represented in the figure. Not only were the slightly elongated nuclei 

 easily seen, but also the cell protoplasm flattened out over the chitin. There was no 

 possibility of confounding these cells with the nucleated blood corpuscles seen in the 

 blood space, as certain observers have been accused of doing in other forms. 



The thickened ventral edge, as seen in the section, is sharply separated from the 

 remainder of the plate. This latter portion is made up of large epithelial cells, whose 

 boundaries are not distinct and are seen but occasionally. These cells are of uniform 



