78 THURLOW C. NELSON 



branchs studied would lead too far afield. With minor modi- 

 fications the figure and description of the stomach of Anodonta 

 serves equally well for Lampsilis, Ostrea, and Venus. 



In Modiolus, however, there is an important accessory structure 

 of the stomach which is closely connected with one of the func- 

 tions of the style in this form. That part of the ventral wall 

 of the stomach which lies between the orifice of the style sac 

 and the base of the gastric shield forms a spirally shaped structure 

 suggesting a whorl of a small shell. Close examination proves 

 this to be a series of conical orifices, each connected with a deep 

 groove (fig. 12), These furrows, at first separate, reunite to 

 form a tract which crosses the ventral wall of the stomach to the 

 left of the extended major typhlosole, and enters the ventral 

 diverticulum or blind sac mentioned above (p. 71). The sur- 

 face of the conical orifices and the furrows leading from them 

 bear large and pow^erful cilia, which beat in the direction indi- 

 cated by the arrows in figure 12. The tract formed by the 

 union of these separate channels bears regular transverse furrows 

 which lead to a deep groove on the left side of the major typhlo- 

 sole. The same structures are continued down into the caecum, 

 where the transverse furrows become somewhat longer. 



Figure 15 illustrates the appearance of the caecum when cut 

 along the dorsal wall and opened out flat. The major typhlo- 

 sole, T, passes to the further end of the caecum, where it ends 

 blindly. In apposition to it under normal conditions is a simi- 

 lar ridge, T', while between them the epithelium is thrown up 

 into regular ridges and furrows. 



The cilia on the surface of the typhlosole, as well as those on 

 the crests of the transverse ridges, beat inward toward the end 

 of the caecum. This holds true for the region R'-T-R of figure 15, 

 while in the region R-T'-R' the cilia on these same structures 

 beat in the opposite direction. 



In the transverse furrows the cilia are very large and powerful 

 and beat toward the deep groove which follows the base of the 

 major typhlosole and its opposing ridge as shown by the arrows 

 in figure 15. As a result of this mechanism, a particle of debris 

 falling into the transverse furrows at any point would be car- 



