STUDIES ON BIOLOGY OF FRESHWATER MUSSELS. 225 



cilia of the sac periodically carry a stream of food from the 

 intestine through it into the stomach. In common with all 

 ciliated epithelia this organ raises the very interesting and equally 

 difficult problem of accounting for the present direction of the 

 beat of the cilia. In the simpler ancestral forms the beat most 

 probably was in the opposite direction. 



The style sac of Modiolus contains more food in winter than in 

 summer. It is difficult to see why the structure in Modiolus 

 should be (as Nelson reports) more effective in winter, when the 

 metabolism is low and the food requirement slight, than in 

 summer, when the demand for food is greater. In winter the 

 secretion of the style substance is slowed down by the tempera- 

 ture to such an extent that the organ is not promptly re-formed 

 with each feeding period (p. 229). The style sac therefore con- 

 tains no style and may be utilized to reconvey food from the 

 intestine to the stomach. In the Unionidse it is certain that the 

 return of particles through the style sac is a phenomenon which 

 takes place normally only after starvation. Of the many hun- 

 dreds of specimens examined when taken from the water, not half 

 a dozen were ever found in which the style was lacking. 



Usually the newly formed style has an abnormally large core 

 of plankton. This indicates that the first undigested or partially 

 digested material which streams into the intestine is diverted at 

 the posterior end of the style sac and carried forward again into 

 the stomach. In the meantime the glands of the typhlosole 

 (Nelson, l.c.~) begin secreting and wrapping the spiral of the 

 style substance about this core. The streaming in of materials 

 from the intestine is limited more and more as the style more and 

 more completely fills the lumen of the sac. After the style moves 

 forward and has been dissolved its entire length, the newer por- 

 tion, with a diminished core, has entirely replaced the original 

 portion with its loose structure and large core of food. This 

 takes place twice daily in Modiolus or Ostrcca, and the newly re- 

 generated style is thus oftener encountered. In Lampsilis or 

 Anodonta under normal circumstances, it is a very infrequent 

 occurrence. My observation has been to a very great extent upon 

 lake forms, which are not subject to many vicissitudes. It is 



