108 MESSES. E. \V. L. HOLT AND "W. M. TATTEESALL — BISCAYAN PLAXKTON : 



Changes in Vertical Distribution. 



Tlie great interest of the ' Research ' collection is found in the light which is thrown 

 upon the changes in vertical distribution during the times of light and darkness. These 

 changes are sufficiently explained by the tables which follow. 



The first table (j)j). 112, 113) simply lists the species, with numbers of individuals taken 

 at each station. The sizes of individuals are detailed in the second list (pp. 114-117), and 

 will, we suppose, be considered as material for the ultimate perfection of knowledge 

 of life-history and growth, and as of some importance in regard to the weight which 

 should be attached to the capture of an individual in such and such a net at such 

 and such a depth in daylight or darkness. 



The succeeding tables * deal with a period of twenty-four hours, from midnight to 

 midnight. The ujiper strata were continuously examined by open nets towed horizontally 

 at 100, 75, 50, and 25 fathoms, and thence to the surface; and by similar nets worked 

 actually at the surface. The strata from 400 to 100 fathoms were examined by nets only 

 in hours of daylight (in July), viz. between 10 a.m. and 7 p.m. An arm-chair critic 

 would say that the exemption of these strata from examination at night vitiates the 

 whole observation, but our own experience at sea leads us to wonder how Dr. Fowler 

 achieved so much in the time. Even under the White Ensign night-work must be 

 immensely more difficult than work by daylight. 



We are dealing with a single group of organisms, and until all groups have been dealt 

 with it appears prudent to refrain from insisting too strongly on the apparent evidence of 

 one. If among organisms too feeble to escape any net ihere be found definite diurnal 

 vertical movements, the apparent results of the Schizopod gatherings will receive support. 

 If none such be found, then the absence of nio-ht-searchins' of the strata below 100 

 fathoms will be considered as requiring serious attention. 



We select as worthy of special consideration the gatherings of Eiiphausia Ilulleri, 

 Meganyctlphanes norcegica, and Stylocheiron Suhmii, because these three Euphausians 

 w ere very frequently taken during the twenty-four hours. 



So far as one can judge from structural characters, Unphausia and Meganyctlphanes arc 

 about equally active when of the same size, but Meganyctiphanes is much the larger 

 (maximum size 40 mm; to 26 mm.) and therefore the more powerful. Apart from its 

 smaller size Stylocheiron is obviously hampered by the excessive development of some of 

 its appendages. 



Taking the feeblest form, S. Suhmii i, first, and neglecting gatherings below 100 

 fathoms, we find that it was taken at the surface thrice during the hours of darkness — 

 10 P.M., 2 A.M., 3 A.M., — and never in daylight. Between 25 fathoms and the surface it 



* In these tables the hauls in which the species occurred are indicated by heavy type. 



t It must be noted that our record of S. Suhmii includes all the larvae as well as the adults, which is not 

 the case with any other species (except >S. abbreviatum). Tbe list of sizes shows that larvae greatly preponderated 

 in the upper hauls, though not to the entire exclusion of adults. S. abbreviatum, G. 0. Sars, occurred only in 

 thirteen hauls, of which eleven were made by night. The captures are not sufficiently frequent for safe deduction, 

 but if, like S. Suhmii, S. abbreviatum is a creature of the upper strata, the much greater size and consequent greater 

 activity of the latter may explain its immunity from da) light nets. 



