104 THURLOW C. NELSON 



We have seen that the ribbed mussel hves under conditions 

 such that its feeding periods are usually restricted to a few hours. 

 As a result, it makes the most of every opportunity, and if one 

 examines a cluster of these molluscs after the rising tide has just 

 covered them, the surface of the water will be seen boiling like a 

 miniature spring, due to the very active siphoning. 



When removed to a dish containing sea-water with much fine 

 sand in suspension, the water is soon completely filtered by the 

 activity of these molluscs. Comparatively little discrimination is 

 made by the palps and gills, save against the larger particles; 

 everything else is taken into the stomach and sorted there. 

 Within about an hour after placing a mussel in roily water it will 

 begin to void faeces in a continuous ribbon, composed almost 

 wholly of sand grains held together by thick mucus. 



In a recent paper on the ciliary mechanism of lamellibranchs, 

 Kellogg ('15, p. 660) describes the peculiar 'sand-eating' genus 

 Macoma. He states that the entire digestive tract of this form is 

 filled with debris. Since the backwardly directed currents of 

 the palps and mantle are as well developed in this as in other 

 species, Kellogg is unable to account for the ingestion of such 

 large quantities of silt. In view of the fact that Modiolus, 

 though possessing well-marked outgoing currents, ingests much 

 sand along with its food, it is probable that Macoma will show 

 upon investigation some similar means of sorting out the food 

 materials from foreign matter in the alimentary canal. A further 

 point of similarity between these two species lies in Kellogg's 

 observation of the extraordinary power of the cilia in this form. 

 My own observations on Modiolus indicate that the outgoing 

 tracts are for the purpose of removing the larger particles which 

 do not enter the mouth. 



As we have seen above (p. 71), Anodonta possesses a divertic- 

 ulum which somewhat resembles the 'sand-sorting' caecum of 

 Modiolus, though less well defined. Space will not permit of a 

 description of analogous structures in the various forms studied. 

 Suffice it to say that Lampsilis, Ostrea, and Venus all show some 

 modification of the stomach wall in connection with the intes- 

 tinal groove and extension of the major typhlosole, by means of 



