THE LARVAL STAGES 115 



(2) Marr has included his actual results and this is a compensation for the percentage per level presentation in 

 both his and Eraser's text-figures. 



(3) Marr has concentrated on his positive results but has not referred to his or Eraser's negative ones. 



(4) At stations where deep Metanauplii occurred neither Marr nor Fraser record any First Calyptopes in the top 

 100 m. between 0600 and 1800 hr. These are the main hours of dayUght and from Marr's and Fraser's combined 

 observations it is reasonable to predict that very few or no First Calyptopes could be expected at this high level 

 during this period of the day. 



(5) Marr and Fraser both show a relative increase of First Calyptopes at depth in daylight and a decrease at night. 



(6) Marr's Text-fig. 8 suffers from the disadvantage already referred to in that the percentages mask the actual 

 results. Marr does not prove from this figure that ' large numbers of larvae . . . could never in fact have been anywhere 

 near the surface'. 



(7) Marr's Text-fig. 9 combines all three Calyptopis stages. Fraser doubts the validity of this. 



(8) Marr's Text-fig. 10 (free First Calyptopis) shows there is only one specimen between 100 m. and the surface 

 from 46 stations against 55 specimens between 500 and 100 m. at 1400-1800 hr., and nearly five times as many at 

 loo-o m. than at 500-100 m. between 1800 and 2200 hr. At 0600-1000 hr. the concentration is 60 times as great 

 below 50 m. as above it. The results for 1000-1400 hr. appear to support Marr's point of view but not if the 

 numbers (not the histogram figure) relative to those of other times are considered. 



(9) Fraser agrees that the First Calyptopis population is composed of newly developed larvae climbing to the 

 surface in the post-metamorphosis phase and those that have already arrived in the shallow water. What neither 

 Marr nor Fraser knows is the rate at which the 'climbers' ascend and the duration of the First Calyptopis stage. 

 Therefore there can be no assessment of the effect that the additions of First Calyptopes from below are having on 

 the migration pattern of the larvae at higher levels. On the face of it and taking into account the orthodoxy of the 

 pattern in both Marr's* and Fraser's figures it (the cUmbing factor) does not blur the impression that the diurnal 

 migration is indeed happening. It has to be borne in mind also that there is no evidence to support the assumption 

 that once the 'climbing' larvae come into the zone where the factors controlling diurnal migration are operating 

 they (the climbers) will react differently from the 'resident' population. 



(10) Marr's page 113. A minor point. As an alternative to the rafting explanation for the diminished density of 

 Furcilias 4-6, would not the expectation be that just as there are less Furcihas 1-3 than Calyptopes 1-3, so on ordinary 

 mortality factors in a population, it is to be expected that there would be fewer Furcihas 4-6 still, assuming, naturally, 

 the sampling of all stages to be adequate? 



Taking these points in turn, I reply, 



( 1 ) My presentation of the data, as Dr Fraser acknowledges in (2), does at least carry with it the actual 

 numbers on which each percentage per level histogram is based and it will be seen that in general 

 (the period 0200-0600 excepted when our data are indeed very scanty) the numbers are adequate 

 enough to be convincing. In any case they are all the data we have got and I am at a loss to think 

 what more could be done with them or how otherwise they could be presented. I would point out 

 too that in Figs. 9, 10 and 11 I show the numbers of stations, both positive and negative, made 

 every 4-hr. period during the life-span of each developmental phase in the plankton, such stations 

 being strictly confined to times and waters in which, our distributional charts show, the several 

 developmental phases would be expected to occur. 



(2) See (i). 



(3) Dr Fraser also concentrates on his positive results and I see no objection to this approach to 

 the problem. The positive results do at least show the actual disposition of the larvae at all times when 

 we strike them. Negative results at all levels, and we have many such both for day and for night, 

 merely show that the larval distribution, like that of the older stages (pp. 148-54), is very patchy (see 

 also p. 219, Tables 45-7). Consistently negative daytime results at near surface levels, with consistently 

 positive results below, would, I agree, be highly significant and would suggest strongly we were dealing 

 with rhythmic diurnal vertical migrators. However, it will be seen (Table 22) that in so far as the 



^ This refers to my Fig. 8 which I believe to be conveying a false impression. 



