418 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



sectioUj wliicli piisses thiuiigb the .supporting membrane, shows these libers to be con- 

 tinuous with its inusculatur«\ I believe them to be muscles and the nieans employed 

 to bring about the numerous contractions described above as taking place in the living 

 gill. They do not penetrate the thickeiuMl ventral portion of the gill plates, nor do 

 they seem to be present in their extreme dorsal portions; their insertion seems to be 

 upon one wall of the plate and at all points throughout their length. 



THE GILLS OK Si )LEN< ).MYA. 



The external appearances of this gill have already been mentioned. The morjjlu*- 

 logically ventral surfaces (here lateral in position) are slightly enlarged, but not 

 nearly so much so as in Yoldia. These specialized edges extend from the median xeu- 

 tral groove of the gill to the pointed outer ends of the jdates. 



The ventral edge of the plate is composed of entirely similar cells throughout, rest- 

 ing upon the usual chitinous layer (Fig. 77, ve). In the preparation from which the 

 section represented was made, these cells had shrunk somewhat and separated from 

 • one another so as plainly to show their structure. They are columnar, have a finely 

 granular jDrotoplasm, the outer edge of which hi deeply stained, and each cell bears 

 several cilia. The cilia of all the cells are of the same length, excei)ting those on the 

 extreme ventral border, where they are slightly shorter. At the lateral edges of the 

 ventral border — never in the midline of the ventral surface — oi^en certain gland cells 

 {gl c), which are constantly and generally easily seen in a corresponding ])()sition 

 throughout the gills of lamellibranchs. 



Pelseneer (No. 17) figures two cells in the section of eacli i)late (in the region Just 

 below the gland cells in Fig. 77), which are larger than the others and bear mncli 

 longer cilia. These he probably means to compare with the " latero- frontal" cells ttf 

 Peck, not so widely found among lamellibranchs, 1 believe, as seems to be so gener- 

 ally supposed. I am confident that tliese large cells do not exist in Solenoniya rrliini. 

 Pelseneer possibly mistook a gland cell in this region t<)r a large nucleus and sup- 

 posed it to indicate a large cell. 



It is possible, though I think not probable, that the cilia on the sides of this \'en- 

 tral edge are used for joining the plates to one another, as described by Pelseneer. If 

 such a junction exists it is a very slight one, for the plates, unlike those of Yoldia, are 

 very easily separated. No direct interlocking of cilia is anywhere seen in sections, 

 as figured for Yoldia in Fig. 82. The ends of the cilia of neighboring plates merely 

 touch each other (Fig. 77), and, as in other cases, I believe that they function princi- 

 ])ally in keeping food particles from entering between the plates, thus confining them 

 to the ventral surface, where they maybe rapidly swept to the median ventral groove 

 of the gill and onward toward the mouth. 



The chitin of the gill i)late is thickest at some distance from the extreme \'entral 

 edge (c/t), and it is here relatively thicker tliaii in Yoldia. It extends, in a long, thin 

 layer, entirely to the ventral edge, and also in the opposite direction, toward the cen- 

 ter of the plate. This latter extension rapidly thins out and disappears. No \ery 

 marked expansion of the blood space takes place l)etwee]i the chitin plates. 



The epithelium of the walls of the plates is much like that of Yoldia, though the 

 cells are distinctly marked off from each other {tv). They are more elongated and 

 their outer ends are rounded. These cells also contain a fine, ecjually distributed pig- 



