MORPHOI.OGV DF LAMELLIBRAXCIIIATK MOLLUSKS. 417 



si/<' MiroujilHtnt, mikI tluoiigh them is cveuly distributed a lij-ht yellow pigjiieut, which 

 is quite abuudaut aud probably gives the red tint to the liviug ojn. Iiuuiediately 

 above the ventral edge, these sides expand laterally, leaving a large space (6-), in which 

 are numerous blood corpuscles. At the dorsal edge of the plate (d) there is also a 

 marked wideuing of the bounding avails. The walls in the center of tlie plai"e are 

 found in sections to be greatly convoluted (/'), probably due to the contraction of mus- 

 cular fibers (m/) contained within the sj)ace between the walls, to be described later. 



In these three regions described, the walls are connected wdth one another by 

 uumerous, branching, lacunar cells (/</(^), though they are less numerous in the central 

 region (/). 



The entire interior of the ])late is seen to be of one continuous blood space, 

 with deftnitely constricted areas and enlarged channels. While blood corpuscles are 

 naturall>' more abundant in these channels, they are found in all parts of the interior 

 of the gill jilate. 



Fig. 82 represents a section cut horizontally through the gill plates of one side 

 in a plane indicated by tlie line h in Fig. 81. In this region, above the thickened 

 ventral border, no cliitinous fi-amework appears. The outer edge of each plate is 

 rounded and composed of short columnar cells, bearing very short, fine cilia on the 

 extreme outer or fi-(»ntal surface {e); but on the lateral sides of the rounded edges are 

 narrow lines of cilia interlocking ch)sely with each other [cj). At times, spaces (s) 

 are found where the ciliation is absent, though thesemayperhai)s be due to a mechan- 

 ical tearing of one plate fiom anotlier. The extent of these ciliated lines on the sides 

 of the plates has been referred to abov«\ The remaining parts of the i)late walls are 

 the same as aheady described, though less convoluted. In this section a part of the 

 sui)porting membrane of the gill has been cut across. Its surface epithelium (e/?) is 

 columnar, and nniny of tlie cells bear tliick spines, perhaps bundles of cilia. The 

 inner part of the membrane is made up chiefly of muscle bundles {m). 



Fig. 80 represents another horizontal section, ijassing close to the dorsal end of 

 the plate (in the plane of ^, Fig 81). Here the ciliated connecting lines or rows have 

 disappeared and the e])ithelium is everywhere the same pigmented, indistinct kind 

 above described for the main body of the plate. In this dorsal region, the lacunar 

 cells (lac) are very numerous, and their processes, extending ti-om one wall to another, 

 are generally fine and thread-like. 



That the plates of one side of the gill structure are not continuous with those of 

 the opposite side is evident from a section passing horizontally in the plane indicated by 

 tl in Fig. 81. Such a section is represented by Fig. 48, PI. lxxxvi. When the ]>lates 

 come together from either side «^n the median line [ml) they are not opi)osite each 

 other, but on the contrary, break spaces. They are figured by Mitsukuri as being- 

 opposite each other in Nncula. Tlu' space (t represents the intericn^ of a }>late, and h 

 the s])ace between t^vo plates. The wall of one plate, then, runs over <jn the median 

 line of the gills and bec/)mes continuous with the wall of the next on the same side. 



Fibers have been described in this gill by Mitsukuri as running down into the i^lates 

 from the supporting membrane aboA'c. He regarded them as cliitinous structures, 

 serving to kee]) the plate expanded for purposes of alH*ation. They are shown in Fig. 

 70 and Fig. 48, mt\ as being cut across more or less transversely. They are always 

 closely applied to the inner face of one of the walls of the plate, but whether always 

 to the anterior oi' i)osterior surface I do not know, A longitudinal, nearly vertical, 

 F. V. K. 1890—27 



