410 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



are connected with the ventral floor of the pericardimn on either side anteriorly, and 

 this opening on one side is vShown in Fig. 45, n, the adductor muscle here having been 

 entirely cut away. 



TheiJosition of the organ in Ostrea is seen in Fig. 97, PI. xciv, atw. Its anterior 

 end lies under the i)ericardium, and it extends backward, close under the adductor, 

 as far as the small notch between the light and dark muscle fibers. It is thus seen, 

 as in Pecten, to exfose a great part of its wall directly to the water in the epibranchial 

 chamber. The glands of either side are entirely separate from one another. The 

 secreting portion of the gland gives off a great number of brancMng diverticula, lined 

 by an excretory epithelium. 



A portion of one of these diverticula is represented in Fig. 54, PI. lxxxvii. The 

 lining cells (exc) are columnar and rest upon a thick basement membrane. The 

 cell protoplasm is mostly collected at the bases of the cells. The distal ends contain 

 large, watery vacuoles which are finally discharged into the lumen of the tube (l). 

 Many of these vacuoles contain a small, round particle which stains deeply and which 

 may be of a concretionary nature. Concretions as commonly found in other forms, 

 however, seem to be entirely wanting here. The products discharged into the lumen 

 are undoubtedly liquid, for by tapping on the cover, when fresh preparations are used, 

 they may easily be made to run into one another and form larger masses. None of 

 these cells give evidence of ciliation. 



Figs. 56 and 57 represent concretions from the secretory cells of the kidney of 

 Pecten. They are sometimes very large and their concentric structure is easily seen. 

 Figs. 58 and 59 are from macerated preparations. The former shows a basal 

 nucleus (n) and the concretion in the distal portion of the cell {con). The concretions 

 possess a more dense, deeply staining, central portion of varying shapes and sizes. 

 Scattered through the cell substance are many small, vacuole-like bodies {vac), prob- 

 ably the first appearance of the excreted products in fluid form, which afterward 

 construct the solid concretion. Other cells (Fig. 59), of a more elongated shape, 

 api)ear to contain only watery vacuoles {vac). 



The secretory cells are deeply pigmented in most lamellibranchs, but are entirely 

 devoid of coloring matter in the oyster. The gland is thus rendered very inconsx)icu- 

 ous and was for a long time overlooked. It has been described in the European 

 oyster {Ostrea ednlis) by Hoek (Joui-n. Soc. l!^eerlandaise de Zool., 1883). 



By means of this tube-like nephridium, the pericardial cavity is connected with 

 the exterior, and it has often been supposed that water entered the pericardium by 

 this means. This has, however, never been demonstrated, and there are reasons for 

 supposing that it does not occur. Among these are the facts that the narrow 

 channels leading from the pericardium and to the exterior possess, as a rule, many 

 cells with very great cilia, which have been shown, in some cases, to cause a current 

 outward; and also in Pecten there are valves guarding the opening into the pericar- 

 dium, preventing any such back flow into it. 



Garner has shown that tlie sexual gland of Pecten opened into the kidney. In 

 examining the nephridia of this form in the spring and early summer, I have almost 

 always found that they contained eggs in various stages of development, often in 

 very great numbers. They are, in all probability, the eggs of the individual exam- 

 ined, which have lodged in the kidney instead of being thrown out to the exterior, 

 and not eggs from some other mollusk taken with the surrounding water. In this 



