1 68 BULLETIN OF THE BUREAU OF FISHERIES. 



accelerated by the formation of reproductive elements causing the production of large 

 quantities of carbon dioxide. The respiratory function is greatly increased in the 

 plicate canals at this time. They become enlarged and well filled with blood which 

 flows through them from the mantle to the kidney and longitudinal vein. Being 

 composed of thin convoluted membranes and covered externally with cilia which keep 

 up a constant circulation of water, these organs are able to bring the blood into intimate 

 relation with a rich supply of oxygen. They might well be termed the accessory gills. 



EXCRETORY SYSTEM. 



ANATOMY. 



In the sea mussel the excretory system consists of two sets of organs, the kidney, 

 or so-called organ of Bojanus, and a pair of pericardial glands which invest the outer 

 walls of the auricles. If the posterior adductor muscle of the shellfish be cut and the 

 valves laid open so that the gills lie flat on the mantle wall as is shown in figure 1 18 (opp. 

 p. 138), the kidney may be seen as a dark brown band of tissue along the ventral body wall 

 at the base of the gills, extending from the posterior border of the labial palps to the pos- 

 terior adductor muscle (fig. 135, K, p. 153). Sabatier (1874) divided the kidney into two 

 parts, (1) that which is independent of the blood vessels and (2) that which covers the 

 walls of the great veins. The former type lies anteriorly on the lateral walls of the liver, 

 where it is thrown into conical folds which extend across to the main canal of the kidney 

 like the teeth of a comb. These folds are designated by Sabatier as the "fusiform 

 pillars of the kidney" (fig. 153, FuP, opp. p. 168). 



The second type of kidney tissue, or that dependent on the blood vessels, consists 

 of plates of tissue which cover the walls of the great veins. This portion of the organ 

 forms a long sac with numerous diverticula which cover the walls of the large longi- 

 tudinal vein or line its cavity. As the central cavity of the kidney extends backward 

 it increases both in size and in number of diverticula, as may be seen in figures 153, 154 

 (opp. p. 168), and 157 (opp. p. 169), which are cross sections taken at the anterior, mid- 

 dle and posterior regions of the body, respectively. The kidney discharges exteriorly 

 through a small excretory pore located on a slight elevation at the base of the genital 

 papilla on its posterior side (fig. 158, EO, opp. p. 169). It was first discovered by Lacaze- 

 Duthiers (1854). 



The pericardial glands, so named by Grobben (1888), but previously described by 

 Sabatier (1877), are an extensive part of the kidney tissue which invests the outer walls 

 of the auricles. The enveloping tissue consists of numerous small folds of various sizes 

 which are dark brown in color similar to that of the kidney proper. Posteriorly the 

 glands are attached to the wall of the pericardial cavity; while anteriorly each gland 

 extends downward into the afferent oblique vein and becomes intimately attached to 

 its walls. 



The pericardial cavity, just anterior to the oblique vein, is in open communication 

 with the collector canal of the kidney by what Sabatier (1874) termed a " couloir place, " 

 which consists of a spongy partition of kidney tissue through which are many fine 

 openings that allow passage of the contents of the pericardial chamber into the kidney 

 canal but render difficult any return backward (fig. 138, RC, p. 156). 



