MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 403 



The crystalline style appears here and there in various groups. It may be present 

 in one form and entirely a'jsent in another closely related to it. In the primitive Xu- 

 culidcc it is represented by a mere rudiment (Pelseneer). It has been homologized 

 with the radular sac of the Qlossophora^ 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 i)lace, which seems altogether improbable. 

 The lining cilia of mouth and oesophagus could probably not j)ass into the stomach a 

 foreign body much larger tl. an 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 suiTounding the stomach 

 gives no appearance of being of two parts. If the stomach be injected, it will be found 

 that the injecting substance 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, I), and in Mytilus (Figs. 33 and 31), the genital gland (g) more or less 

 comijletely siUTOunds the liver. In Pecteu (Fig. 44, I) the liver in the region of the 

 stomach is only bounded by the sexual gland on its ventral suiface. This is also the 

 case in Mya, shown in Fig. 25, /. 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 diflerent 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 i)rimitive forms, however, as Nucula, Yoldia, and Solenomyay the 



