404 BULLETIN OF THE UlSITED STATES FISH COMMISSION. 



liver mass is not at all compact. In Yoldia the liver tubules, surrounded by the sexual 

 giar.d, are free from one another throughout most of the extent of the gland, being 

 only connected by long branching threads of a connective tissue. 



THE GENERATIVE MASS. 



The greater part of the visceral mass is made up of this organ and the liver. It 

 is of a much lighter color than the latter gland and varies much in its outward ap- 

 pearance in different forms. It is a large gland, surrounding the liver, and constitutes 

 the posterior part of the visceral mass. It grows in between bundles of muscle fibers 

 of the foot, wherever spaces may be left. In Ostrea it reaches the body wall anteriorly, 

 surrounds the liver, and closely invests the intestine throughout almost its entire 

 extent. A part of the visceral mass extends down under the pericardium, and back- 

 ward for some distance below the adductor, where it ends in a blunt, rounded point, 

 forming most of the anterior boundary of the cloacal chamber. The intestine runs 

 almost to this extreme tip and then returns toward the stomach. It is, in this region, 

 under the adductor muscle, entirely smrounded by the sexual gland (Fig. 7, PI. lxxx, 

 g), which is also seen at the extreme posterior part of the body represented in Fig. 8, 

 g. In the oyster, also, the visceral mass extends in much the same way above the peri- 

 cardial cavity and over part of the adductor. It consists here of the rectal part of 

 the intestine, sui-rounded by a layer of the generative gland, which becomes thinner 

 and thinner until it finally disappears, and the extreme tip of the rectum is continued 

 on into the cloaca without this covering. This is seen in section above the heart 

 region at g, Figs. 4, 5, 6, and 7. In Fig. 6 it becomes thinner, and in Fig. 7 posterior 

 to the pericardial chamber, and over the adductor it has almost disappeared. This 

 backward extension surrounding the rectum is an unusual one. In Venus the genera- 

 tive gland penetrates into spaces between the uppermost muscle bundles of the foot, 

 as is usual in forms with a locomotor foot. The posterior part of the visceral mass 

 has many scattered muscle bundles, generally transverse, as indicated in Figs. 14 

 and 15, m/, running from one side to the other. The sexual gland pushes down 

 among these muscles for a considerable distance. In a case like Yoldia, where the 

 organs of the visceral mass are not at all crowded, the sexual gland still occupies a 

 considerable part of the base of the foot. The definite boundaries of the gland in 

 Mya will be seen by a glance at Figs. 24 to 26, PI. lxxxiii, g. 



In Anomia, greatly modified in many ways on account of its fixed condition, the 

 sexual gland is very asymmetrical, extending out into the mantle on the right side. 

 In Mytilus the foot (not present in Anomia) is entirely muscular and contains none of 

 the sexual gland. Much of the visceral mass, also, is occupied by the large byssus 

 muscles, and, as a result, the generative gland has pushed out into the mantle lobes 

 of both sides and completely fills them, as seen in Figs. 32 to 41, m. 



The ducts leading from the sexual glands open in a variety of ways. They may 

 open directly to the exterior or into the excretory organs. If into the latter, they may 

 open near its pericardial end, its middle, or its external end. It is probable, from 

 the fact that in the most primitive forms this gland opens into the pericardial end of 

 the kidney, that the fi-ee opening to the exterior is, in some cases, a secondary condition, 

 as shown by Pelseneer (No. 17). 



