MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 411) 



meut in their protoplasiu. Seetiouts .show no iiidicatiou of the fohliug of the walls, 

 such as is foiiiul in the gill of Yohlia, and I liave seen no trace of the muscle fibers 

 which Ihave described as being present in the interior of the phite in the hitter forui. 

 Endothelial cells are easily demonstrated, lining the interior of the chitinous 

 layers [ch). Lacunar tissue, if it exists, was not made out, as the walls of the plate 

 are everywhere closely applied to each other, in sections, and only here and there can 

 very narrow spaces be seen between them. 



THE GILLS <»1- AKCA PEXATA. 



If an entire filament of the gill of this form be isolated, it will be seen to be made 

 of a fully developed descending and ascending limb (Fig, oO, PL lxxxvi). The upper 

 end of the ascending limb in the filaments of both gills is free from the mantle or side 

 of the visceral mass, as in Mytilus. It bears an enlarged triangular end plate which 

 tm'ns outward. The anterior and posterior faces of this end are ciliated and form a 

 large patch which closely interlocks with those in the same position on neighboring 

 filaments. 



The most striking fact in regard to these tilamwits is that the descending and 

 ascending limbs are not separated from each other for about half their extent^ — the 

 ventral half — but are connected by the continuation of their inner walls (cp. Fig. 50, 

 PL LXXXVI, and Fig. 66, PL lxxxviii). 



There are no vascular connections between the filaments, but they are held togethei-, 

 as is usual in the genus Area, by ciliated disks (cy?, Fig. 66) arranged in a row on the 

 anterior and i)osterior faces of the filament throughout its entire extent. 



Fig. ()Q represents a cross-secticnj of tour filaments. Three of these are cut at a 

 point above the union of the ascending and descending limbs ; a single one is cut lower 

 down and passes through this connecting portion (e/>). 



The sections .show the filament to be thin fiom before backward and wide ft-om 

 side to side. The elongated cells of the frontal epithelium (/) are nniformly ciliated 

 and extend back for some distance from the end, the cilia gradually becoming shorter, 

 as do the cells which bear them. At a corresponding point on either side, there are 

 three or four cells seen in each section, which are longer than nsnal and bear long 

 cilia {If I). These are long ridges, or rows of cells, which are here cnt in transverse 

 section. They extend the whole length of the filament without any break, and then- 

 cilia do not serve to join contiguous filaments, but, as I believe, like the entirely similar 

 rows in Yoldia (where there are two), are merely to prevent the main currents, carry- 

 ing foreign particles, from entering between the filaments. Of course, currents of 

 water do penetrate between the filaments to enter the water tubes of the gills, and 

 thence proceed upward into the epibranchial chambers; but in all cases, so far as I 

 have observed, the cilia of the rows, or lines, point obliquely outward^ presenting 

 their ends to foreign particles and keeping them out on the ciliated frontal epithelium 

 while not being thick and heavy enough to prevent water from entering. 



Peck (No. 16) has shown that in a position similar to this, though nearer the outer 

 edge, in the gill filaments of Anodon, there was a single line of enlarged cells bearing 

 very long cilia. Moreover, in Dreisseiia he found tivo lines of enlarged cells lying 

 together, with no cells between, but with the tw'o lines of cilia distinct. These cells, 

 one appearing on either side in each section, he (jailed the latero-frontal cells. In the 

 latter case he distinguished a first and second latero-frontal cell, 



