MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 
403 
The crystalline style appears here and there in various groups. It may be present 
in one form and entirely absent in another closely related to it. In the primitive Nu- 
culidce it is represented by a mere rudiment (Pelseneer). It has been homologized 
with the radular sac of the Glossophora , but probably not correctly, on account of its 
point of origin. Its function also is unknown. It has been regarded as a store of 
reserve food material. Barrois (No. 1) and Pelseneer suppose that its purpose is to 
surround sharp particles in the digestive tract, which might injure its lining epithe- 
lium. Such a function seems to me improbable. It is generally supposed that food is 
taken into the mouth and stomach by ciliary action only. In many forms large 
quantities of sand are taken in by the same means. It would be impossible for the 
style substance to protect the stomach walls from such a mass of foreign bodies by 
covering them. Only when an extraordinarily large and sharp piece enters, could 
this function of protecting the stomach take place, which seems altogether improbable. 
The lining cilia of mouth and oesophagus could probably not pass into the stomach a 
foreign body much larger than a grain of sand. The digestive tracts of those forms 
which have no style are probably not easily injured. 
THE LIVER. 
This gland is paired, there being one-half on each side of the visceral mass. In cut- 
ting into the visceral mass, however, the dark -brown gland surrounding the stomach 
gives no appearance of being of two parts. If the stomach be injected, it will be found 
that the injecting subst ance has penetrated the liver mass through its ducts, which 
open into the stomach. The ducts are, in the main, very fine and traverse the liver in 
every direction. 
The openings of the ducts into the stomach are usually large and cause much irreg- 
ularity in its walls (Fig. 2, PI. lxxix, s). The position of this mass and its extent in 
Ostrea are shown in the figure to which reference has just been made (l). It extends in 
this region of the stomach from near the mouth dorsally, to the extreme ventral wall of 
the visceral mass. It nowhere touches the walls of the visceral mass excepting below, 
being surrounded by the sexual gland ( g ). Its extent in Venus is readily seen by 
referring to Figs. 11, 12, and 13, i, vertical sections approximately through the 
anterior, middle, and posterior parts of the stomach. In this case, in the oyster, in 
Cardita (Fig. 51, l), and in Mytilus (Figs. 33 and 34), the genital gland (g) more or less 
completely surrounds the liver. In Pecten (Fig. 44, l) the liver in the region of the 
stomach is only bounded by the sexual gland on its ventral surface. This is also the 
case in My a, shown in Fig. 25, l. The posterior end of the stomach in this form (Fig. 
26) is not surrounded by the liver, but by the sexual gland. Though varying a great 
deal in size in different lamellibranchs, the liver seldom, if ever, extends farther back- 
ward than the posterior end of the stomach. It extends forward, however, to the 
anterior end of the visceral mass, as is shown in the sections anterior to the stomach. 
The posterior end of the liver is irregular, and in vertical sections of this region por- 
tions of the sexual gland may be seen surrounded by the liver mass (Fig. 34, g). The 
boundaries of both glands are irregular and they everywhere lie closely applied to one 
another. 
As a general thing, the secreting tubules of the liver are packed together very 
closely. In the more primitive forms, however, as Nucula , Yoldia, and Solenomya , the 
