GENERAL DISCUSSION 



MATERIAL AND METHODS 



During the Carnegie expedition 162 "oceanographic 

 stations" and 69 "surface plankton stations" were oc- 

 cupied. The collecting program at each "oceanograph- 

 ic station' ' included the towing of a 1 meter plankton net 

 at the surface, and of 1/2 meter nets at 50- and 100- 

 meter depths. The upper half of the nets was construct- 

 ed of no. 10, the lower half of no. 15 silk bolting cloth. 

 The nets were towed for 1 hour. At the "oceanographic 

 stations," from station 11 on, duplicate quantitative 

 samples were taken with a Pettersson plankton pump. 

 At the "surface plankton stations" various types of silk 

 nets were used depending on the speed of the vessel, and 

 they were towed for different lengths of time. 



Only open nets were used. This introduced a pos- 

 sibility of error in the census of the subsurface sam- 

 ples. This error, however, was not great as the length 

 of time the nets were towed at their respective depths 

 (1 hour) was so much greater than the length of time 

 required for raising the net (about 2 minutes). Thus, 

 unless there was a very pronounced density of popula- 

 tion in an upper stratum, the proportion of upper-level 

 organisms to the standard- level organisms would be 

 very low. The comparison of the hauls from the three 

 levels would reveal this. 



It should be noted that all collections outside the 

 tropics were made in the summer time. The cruise was 

 planned so that the vessel would be in the lilgher lati- 

 tudes of each hemisphere in its respective summer in 

 order to avoid too rough weather. For that reason, the 

 distributional records show no winter records outside 

 the tropics. 



The plankton samples were preserved and stored 

 in formalin. In the laboratory the peridlnian material 

 was treated with a solution of hypochlorite, from 10 to 

 50 per cent, according to the nature of the material as 

 established by experience. This reagent was applied in 

 order to dissolve the cell contents and to weaken the 

 thecal sutures so that the plates could be readily sepa- 

 rated. In some instances it caused a swelling of the 

 protoplast followed by premature rupture of the theca. 

 For this reason, drawings were made of the specimen 

 in various views with as much detail as could be made 

 out before the hypochlorite treatment. In cases where 

 there was ample material, portions of unsorted sample 

 were given the hypochlorite treatment, but where the 

 material was rare, single specimens were transferred 

 with a micropipette. 



After the treatment with hypochlorite for a varying 

 length of time, depending on the material, the specimens 

 were washed in water and, in some cases, stained with 

 trypan blue. Specimens with oil droplets, such as Perl - 

 dlnlum depressum. were cleared with xylol in order to 

 dissolve the oil, which interfered with proper staining. 

 The specimen thus treated for study was transferred to 

 a warm flat drop of glycerin jelly by means of a micro- 

 pipette and examined without a cover glass. In this me 

 dlum the specimen could be oriented to any desired po- 

 sition, and, after the jelly cooled to room temperature 



and hardened, the specimen was in a solid medium and 

 a camera lucida drawing could be made. After obser- 

 vation and drawing in one view, the jelly could be melt- 

 ed and the specimen reoriented any number of times. 

 The glycerin jelly was softened by placing it on a warm- 

 ing table heated to a temperature of 50° C. During 

 periods of warming, the drop was covered with a small 

 damp chamber to prevent undue evaporation of water 

 and consequent troublesome thickening of the jelly. The 

 specimen was manipulated by means of a micro-needle 

 prepared by mounting a piece of spun glass from a 

 glass-wool filter on the end of an ordinary dissecting 

 needle. The glass hair projected about 3 millimeters 

 beyond the point of the steel needle. As it became 

 clogged with jelly, it was trimmed with scissors until 

 about 1 millimeter long, when it was replaced. 



The melting point of the glycerin jelly can be con- 

 trolled by varying the water content. All the water was 

 never allowed to evaporate on the warming table, how- 

 ever, as the jelly then had a tendency to become sticky 

 even though more water was added. The solution of jel- 

 ly kept on the warming table for the preparation of 

 slides was at all times covered, and water was added 

 occasionally to compensate for evaporation. 



This method proved satisfactory for studying the 

 specimens in various positions without injuring them. 

 It is impossible to orient some material, for instance 

 Peridinium depressum. by rolling a specimen in a water 

 mount under a cover glass, because of the unbalanced 

 nature of the theca in all but four positions, none of 

 which is particularly desirable for descriptive purposes. 

 The above method also afforded a satisfactory means of 

 dissecting. A specimen which has been properly cleaned 

 in hypochlorite will begin to break apart in a glycerin- 

 jelly mount when pressure is applied with the glass hair 

 or when it is gently touched with a fine steel needle. 

 When the plates have beg^un to separate, an individual 

 plate or group of plates may be selected and oriented to 

 any desired position by stroking the surrounding jelly 

 with the glass hair, without danger of crushing or losing 

 the rest of the specimen. For the examination of details 

 Uke pores, when it is necessary to use an oil- immersion 

 objective, it is usually advisable to mount imderacover 

 glass in glycerin jelly, water, or balsam. 



All drawings were made with the aid of a camera 

 lucida. In some cases they were later enlarged to facil- 

 itate shading. 



Owing to the pronounced asymmetry of the body of 

 some of the forms, they presented quite different pro- 

 files in different positions. Thus, the measurement of 

 any contour may vary considerably even though the po- 

 sition of the body is changed but slightly. For this rea- 

 son it is necessary to adopt standard positions for mak- 

 ing measurements. In the present work the ventral view 

 has been adopted for the principal standard position. 

 There is some difficulty in determining the "ventral 

 view" in some of the more asymmetrical forms, partic- 

 ularly in species of Peridlniimi with long horns. In such 



