432 Birge — Vertical Distribution of Pelagic Crustacea. 



and it was desirable to pass from a region containing fewer 

 Crustacea to one containing a greater number, rather than the 

 reverse. Any accident in washing out the dredge, by which a 

 few Crustacea remained adherent to it would introduce a consi- 

 derable error in passing from the higher to the lower levels, 

 while in passing in the opposite direction the error would prac- 

 tically amount to nothing. G-reat care was taken in washing 

 the dredge, and it is not probable that any appreciable number 

 remained adherent to it, but this method was followed to avoid 

 any possible error from this source. 



METHODS OF COUNTING AND COMPUTING THE CRUSTACEA. 



The counting and computing were done almost entirely by- 

 Messrs. Harder and Olson. 



The apparatus employed for counting consisted of a 25 cc 

 glass cylinder graduated to -J- cc. , a 2 cc. pipette, graduated to 

 iV cc. , microscope, shallow crystallizing dishes, troughs, and 

 registering cards. 



The trough was made of a microscope slide, to which were 

 cemented four strips of glass by means of marine glue. Thus 

 there was formed a narrow and shallow trough, 60 mm. X 6. 5 mm. 

 X 2 mm., holding about .8cc. The width of the trough was a. 

 little less than the diameter of the field of the microscope when 

 a low power objective and eye piece were used. In our counting 

 a Leitz objective No. 1 and ocular I were employed. A card 

 like the accompanying cut was used in recording the kinds and 

 number of Crustacea, as well as the level in which they were 

 obtained, together with other important observations. The 

 method used in counting was simple, and with practice became 

 somewhat rapid. The catch to be counted was placed in the' 

 graduated cylinder, and sufficient alcohol added to make 24 cc. 

 This quantity was then thoroughly stirred and shaken, so that 

 the animals and plants were evenly distributed through it. From 

 this mixture two quantities of 2 cc. each were taken and trans- 

 ferred to watch glasses. One of these quantities was counted 

 by each of the observers who were at work. From the watch glass 

 there was taken and placed in the trough a sufficient amount of 

 material to give about 60 or 70 animals of the most numerous. 



