by contractions of gill muscles bringing together 

 the filaments of the plicae, followed by constriction 

 of the ostia. The contraction of the ostia some- 

 times takes place independently of the contraction 

 of gill muscles. 



DETERMINATION OF THE RATE OF WATER 

 TRANSPORT 



The rate at which water is transported through 

 the gills can be studied by both direct and indirect 

 methods. In direct methods the volume of water 

 discharged through the gills in a given time is 

 collected and measured, or is calculated from ob- 

 servations of the velocity of the cloacal current. 

 The indirect methods are based on determinations 

 of the rate of removal of particles suspended in 

 water. The powders used for the latter purpose 

 include kaolin, natural silt, calcium carbonate, 

 colloidal carbon, isolated chloroplasts, various 

 plankton microorganisms such as Euglena, (_'hlo- 

 rella, and Nitzschia, and cultures of unicellular 

 algae that have been made radioactive. 



Direct methods 



Tliere are two groups of direct methods. First, 

 the water disciiarged by the gills may be inter- 

 cepted, its volume measured, and the sample re- 

 tained for analysis. In the second method tlie 

 velocity of the current is determined by measuring 

 the angle of deflection of a light paper cone or 

 glass plate placed in front of the current. The 

 rate of transport of water masses may be computed 

 from the cross-sectional area of the column of 

 moving water and the angular deflection of the 

 plate. Both methods have certain advantages 

 and disadvantages. 



The methods which make it possible to collect 

 and analj'ze water after it has passed through the 

 gills are particularly suitable for studies of feeding 

 and metabolism. The slight disadvantage is that 

 the mollusk must be partially enclosed in rubber 

 or plastic. In the second group of methods the 

 mollusk is not in direct contact with any foreign 

 material and is kept under normal conditions. 

 The drawback is, however, that the discharged 

 water can only be collected in very small quantities 

 directly from the stem of the current inside the 

 cloaca. Choice of method must be governed, of 

 course, by the purpose of the observations. 



Constant level tanks. — A very simple device con- 

 sisting of two connecting vessels in which tiie level 

 of sea water is kept constant was designed by 

 Galtsoff (1926, 1928) and has been adapted and 



modified by many investigators making long-term 

 observations on the feeding of the oyster. The 

 apparatus, shown in figure 168, consists of two 

 rectangular vessels, one large and one small, con- 

 nected by a semicircular trough or by glass tubing 

 of wide diameter inserted in the partition between 

 the two vessels. The large vessel should be just 

 big enough to accommodate an oyster mounted 

 on a stand, the inlet tube for sea water, and a 

 thermometer. The volume of the tank is made 

 as small as possible in order to permit rapid ex- 

 change of water. Convenient dimensions for 

 observations on adidt Crassostrea vinjrmca and ('. 

 gi'jas, are 10 by 6 by 4 inches for the large vessel, 

 and 2 inches by 1}^ by 2 inches for the small one. 

 The tanks are made of transparent plastocel, 

 lucite, or similar nontoxic material about }4-inch 

 thick. The edges are sandpapered and cemented 

 together by a solution of plastic in acetone, cello- 

 solve, or by undiluted methylene chloride, and 

 the small vessel is cemented to the side of the 

 large one. After the cement is dry the tank 

 should be washed carefully and rinsed in sea water. 

 A semicircular trough made of thin plastic bent in 

 hot water to the proper shape is mounted on the 

 wall between the two vessels. The drain pipe is 

 glass tubing, 6 to 8 mm. in diameter, inserted 

 through the bottom of the small vessel; its upper 

 end is slightly widened and adjusted to the exact 

 level of water in the large vessel, which is con- 

 trofled by a wide cut in the opposite wall of the 

 vessel (left side in figure 168). The level of water 

 is adjusted by moving the vertical overflow tube 

 up and down (right side of fig. 168) until excess 

 water running through the large vessel spills 

 tln-ough the overflow and nothing enters the 

 small vessel. Levels must be set very accu- 



Frc.uRE 168. — Constant level tanks designed by Galtsoff 

 to measure the volume of water transported by the gills 

 of the oyster. Dimensions of the large vessel are in by 

 6 by 4 inches. 



186 



FISH AND WILDLIFE SERVICE 



