8 BULLETIN OF THE BUREAU OF FISHERIES 



three groups of cells, frontal, {jr. c.) latero-frontal, and lateral (Z. c), the relative 

 positions of which are shown in Figure 1. It has been fully established by the work 

 of Orton (1912) and Gray (1922) that the ingoing current of water, passing through 

 the water pores into epibranchial chambers and cloaca, is produced by the activity 

 of the lateral cilia, which beat inward, i. e., across the surface of the gill. The frontal 

 cilia beat parallel to the surface of the gill and are concerned solely with the transport 

 of the particles that settle on then' surface. The latero-frontal cilia, lying at the edges 

 of the filaments so that those of adjacent filaments interlock, are not well developed 

 in the oyster, yet they can be distinguished from frontal and lateral cilia. Their 

 function consists in keeping the adjacent filaments apart and in stopping and throwing 

 on the frontal surface the particles carried in by the current of water. 



Besides ciliary cells, the epithelium of the filaments contains unicellular mucus 

 glands, which are stimulated easily by contact with solid particles and secrete mucus. 

 Interfilamental and interlamellar junctions contain a number of vertical and hori- 

 zontal muscle fibers, which may cause the opening and contraction of the plicae. 

 No peristaltic motion has been observed either in the epibranchial chambers or in the 

 cloaca. The current produced by the gill epithelium rims with a constant speed as 

 long as temperature and other external factors remain constant. 



The gill of the oyster can be compared to a very fine and complex sieve, the holes 

 of which are represented by the water pores; the water is taken in by the whole sur- 

 face of the gills and is driven through a system of tubes into one exhalant chamber. 

 It leaves the gills as a single outgoing stream, which can be observed easily when 

 the oyster is feeding. Through the water pores and tubes there is direct communi- 

 cation between the inside and outside of the gill, and the flow of water in one direc- 

 tion is due exclusively to the rhythmical beats of the lateral cUia. These facts have 

 an important bearing on the discussion of the experimental data. 



REVIEW OF THE METHODS FOR MEASURING THE STRENGTH OF CUR- 

 RENT PRODUCED BY PLANKTON-FEEDING ORGANISMS 



Many attempts were made by various investigators to determine the rate of 

 flow of water produced by the plankton-feeding organisms. The methods they 

 employed can be grouped into two classes — indirect and direct. 



INDIRECT METHODS 



These methods are based either on the determination of the number of micro- 

 scopic organisms and other small particles suspended in water and ingested in a given 

 period of time by the animal, or on the determination of the rate of respiratory 

 exchange of a given organism. One of the first estimations of the rate of flow of 

 water through the gills of an oyster was made by Grave (1905). He kept the oysters 

 in filtered sea water for three days until their digestive tract became devoid of any 

 plankton organisms. Then the oysters were placed in the natm-al sea water and at 

 convenient intervals were taken up, contents of their stomachs were removed, and 

 the number of diatoms in them counted. Knowing the average number of diatoms 

 per liter during the three consecutive summers, and having obtained the number of 

 diatoms collected by the oyster in a given period of time, Grave estimated the rate of 



