222 WILLIAM RAY ALLEN. 



water. However, it was possible to accumulate a considerable 

 mass of living and fragmented infusoria. Starved mussels fed 

 upon this concentrate did not renew the style. This negative 

 result must not be trusted too implicitly, considering that, as re- 

 ported above, ciliate material is sometimes found in the stomach. 

 Yet the present experiment points toward a greater utilization of 

 the smaller organisms of infusions than of relatively large ciliates. 

 Aside from the ciliated protozoa the infusorian population con- 

 sisted mostly of extremely minute forms. The maceration ex- 

 periments described elsewhere (p. 228) show conclusively that the 

 Unionid gill is capable of intercepting very small material in- 

 deed even the pyrenoids of algae. 



It may be questioned whether the formation of a crystalline 

 style is a reliable index of the feeding activity. In my opinion it 

 is as reliable as a direct examination of the alimentary tract. On 

 some occasions food may be found in the intestine before regen- 

 eration of the style is perceptible. But, on the contrary, there 

 are as many occasions when a starved mussel recently fed is seen 

 to have the beginnings of a style before anything is readily recov- 

 erable from the intestine. In far the greater number of cases 

 examined the synchronism between style renewal and the pres- 

 ence of food is exact. The bearings of this upon the significance 

 of the style are discussed below (p. 229). 



Resume. The more minute plankton organisms are of as great 

 nutrient value as the more conspicuous, often undigested matter 

 commonly listed from rectal or fecal examinations. However 

 the net plankton is shown to have a food value as well. Experi- 

 ments which more or less perfectly separated the net- from nan- 

 noplankton show that both are capable of re-forming the crystal- 

 line styles of starved mussels. The minute flagellates sometimes 

 exceed the volume of net plankton in lakes many fold. Since it 

 is certain that they can be entrapped by the gills of the mussel 

 and can be ingested, it is likely that their rarity in the rectum is 

 due to the fact that most of them have been digested. Infusions 

 which have nothing of food value except ciliates and minute or- 

 ganisms renew the style. 



