ANATOMY AND PHYSIOLOGY OF ATRINA RIGIDA. 429 
(Fig. 19, pi. xlix.) The anterior retractors are cylindrical and composed of white 
fibers. At one end they are inserted into the anterior surface of the foot, while at the 
other they are attached to the shell just posterior to the anterior adductor muscle. (Fig. 
19, ar, pi. xlix.) The posterior retractor muscles appear large in the drawings, being 
attached at one end to the base of the foot and at the other to the shell just anterior to 
the posterior adductor muscle. (Fig. 19 pr, pi. xlix.) This structure in reality con- 
sists of two parts, the byssal apparatus and the muscle proper. These retractor muscles 
have become very much reduced. They no longer serve to retract the foot, but have 
taken on a new function, that of supporting the byssus. 
VISCERAL MASS. 
The main body of the visceral mass is approximately cone shaped, with the apex at 
the anterior end. It is slightly flattened dorso-ventrally and at the posterior end there 
is a slender horn which projects back below the pericardium between the posterior 
retractor muscles. Its hindermost extremity rests upon the adductor muscle. (Fig. 
18, pi. xlix, and fig. 9, m.) The visceral mass includes the digestive and reproductive 
organs. The liver, which surrounds the stomach, fills the anterior part. The repro- 
ductive organs fill the remainder of the space not occupied by the coils of the intestine. 
The sexes are separate and are easily distinguished by the color of the reproductive organ, 
which shows through the thin body wall. The testis is white, as shown in figure 16, 
plate xlviii, while the ovary is orange red. The main duct, which carries the repro- 
ductive elements to the exterior, opens into the kidney very near the renal aperture. 
(Fig. 6, g.) Fertilization of the eggs takes place after they are extruded into the water. 
FOOT AND BYSSUS. 
The foot is cone shaped and is attached to the anterior end of the visceral mass. 
At its base a large opening, from which the byssus protrudes, is to be found. (Fig. 16, 
pi. xlviii.) From this point a groove extends along the ventral surface of the foot to a 
point near its tip. (Fig. 15, g.) The byssal gland is situated in the floor of this groove 
and is continued back of the foot into the posterior retractor muscles. The foot can 
be protruded, and it is probably of service in attaching the byssus. 
KIDNEY. 
The kidneys are two in number, and each consists of a glandular and a nonglandular 
portion. They lie between the gills on the ventral side of the body, just anterior to the 
abductor muscle. They hang down into the central suprabranchial chamber as two 
dark colored bags and are very conspicuous organs, requiring no dissection to expose 
them. (Fig. 20, K, pi. l.) Each is in open communication with the pericardial 
chamber above and each opens below into the suprabranchial chamber by a large tube, 
which ends at the summit of a papilla. The glandular portion forms the prominent sac 
mentioned above and lies about midway between the two extremities of the kidney. 
For convenience of description the kidney may be divided into three portions: First, a 
