fungi and bacteria inside the rubber connections. 

 To avoid this growth all rubber tubings and joints 

 should be periodically cleaned, dried, and steri- 

 lized. With these precautions duplicate deter- 

 minations in the Bureau's shellfish laboratory 

 gave consistent results, the error not exceeding 

 ±0.01 nig. of oxygen in the 100-nil. sample. 



Oysters were kept in the respiratory chamber 

 from 4 to 9 hours, and samples of water were 

 taken at half-hour intervals. The duration of the 

 test was limited by the available quantity of 

 filtered and aerated sea water. For o.xygen 

 determination the Winkler titration was used; in 

 some tests the Van Slyke volumetric method was 

 employed for determining the carbon dioxide and 

 the oxygen content of the water. 



To obtain data corresponding to the level of 

 basic metabolism, the oysters were starved for 

 24 hours by placing them in filtered sea water. 

 This period was found to be long enough to 

 cleanse the intestinal tracts and to discard the 

 feces and pseudofeces. Throughout all the tests 

 the temperature of water was maintained at 

 25°C.±0.1°C. 



MICRODETERMINATION OF OXYGEN 



Oxygen content in very small volumes of water 

 can be found by using one of the microdetermi- 

 nation methods developed by Lund, 1921; Thomp- 

 son and Miller, 1928; Kawaguti, 1933; Krogh, 

 1935b; and Van Dam, 1935a. The volume of 

 water used for analysis in these methods varies 

 from a few ml. to a fraction of 1 ml. Samples 

 can be taken simultaneously or nearly simul- 

 taneously from the inhalant and exhalant currents 

 of a bivalve. It is obvious that such a procedure 

 requires great precision of sampling. This is 

 made possible by Krogh's syringe pipette (Krogh 

 and Keys, 1931) or its modification nuide by Van 

 Dam (1935a). The syringe pipette designed by 

 Krogh and suitable for delivery of small quantities 



of fluid with a high degree of accuracy is shown in 

 figure 185. It is a glass cylinder with a carefully 

 ground plunger and a heavy-walled glass capil- 

 lary welded to the tip instead of the conventioiuil 

 metal injection needle of a hypodermic syringe. 

 Tuberculin syringes with blue plungers are suit- 

 able for this purpose. The glass capillary is 

 about 5-cm. long with a small bore of inner 

 diameter of about 0.15 to 0.20 mm. The syringe 

 is mounted on a frame of two steel rods set in a 

 bakelite or ebonite base. The notched bar, N, 

 determines the highest position of the plunger. 

 The volume delivered by the syringe is adjusted 

 to any desired fraction of its capacity by a metal 

 collar which may be pushed into one of the 

 notches and set in a fixed position by set screw S. 

 The pipette with the special tip answering Van 

 Dam's specification is not available from stock at 

 any scientific supply store in this country and 

 has to be made to order by an experienced glass 

 blower. 



For taking samples two syringes are mounted on 

 an adjustable screw stand that allows fine and 

 independent movement in the vertical and hori- 

 zontal planes. The syringes are attached to the 

 arms of the stand by ball bearing holders H, so 

 that they can be set at any angle to the horizontal 

 plane. The stand must be heavy and must have 

 adjustments fine enough so that the tips of the 

 collecting sjTinges can be introduced into the 

 cloacal region of the oyster or the branchial and 

 anal siphons of clams without touching or disturb- 

 ing the sensitive tissues. The type of stand 

 suitable for this purpose is shown in figure 1 of 

 Van Dam's paper (1935a). 



Samples of water for microanalysis also can be 

 taken by means of a siphon; the tip is introduced 

 deep into the cloacal chamber of a bivalve mol- 

 lusk, as shown diagrammatically in figure 186. 

 This device was used by Van Dam (1954) in his 



K 



^Illlllllllll.lllllWItWJttlhlw 



V-2. 2CC 



50 



Centimeters 



FiGi'RE 185. — Van Dam's modification of the syringe pipette for taking .small samples of fluid with a high degree of accu- 

 racy. H — clamp holder; N — notched bar; S — set screw. 



204 



FISH AND WILDLIFE SERVICE 



