43 



At the same time it must be remembered that in order to get precise results from the 

 statistical treatment of distributional questions, percentages should be struck on every haul of 

 the nets made, and every specimen should be identified. Neither of these was practicable in 

 this case, because the whole collection was not sent to me, and because the specimens sent 

 were already too numerous to admit of every one being identified, even had the material been 

 in perfect condition. 



2. Diurnal and nocturnal oscillation. 



The epiplankton ') hauls were then arranged as far as possible into two groups, those 

 of 'dayHght' and 'darkness', for which Prof. Weber was kind enough to give me such data as 

 had been recorded in this connection. The number of occurrences of each epiplanktonic species 

 in each group was then calculated as a percentage of the total hauls in that group. That the 

 resulting table should have yielded no special indications is not really remarkable, because the 

 question of the vertical movements of epiplanktonic species in the twenty four hours can only 

 be attacked by systematised day and night work, in which the numbers of specimens captured 

 are taken into account, and not merely the presence or absence of the species; and this is so, 

 because, if a vertical oscillation really occurs, the most that can be expected is, that the 

 species shall rise or sink in general ; in other words, that its centre of distribution shall rise 

 or sink. It is not to be supposed that every single specimen must be found at a fi.xed depth 

 at a fixed hour of the day or night. The zoological side of the Siboga Expedition was planned 

 for general faunistic collection, and not for the study of such special problems as vertical 

 o.scillation. 



For the same reasons it was not to be expected that the hauls made during rain would 

 show any marked alteration in the surface fauna; this again is a point which can only be 

 brought out by means of systematic hauls planned for the purpose. 



3. Vertical distributi(3n. 



In this connexion it is at once noticeable that certain species occurred only in nets which 

 were lowered to a greater depth than 200 metres (109 fathoms), the suggested boundary between 

 the epiplankton and the mesoplankton. 



Some misunderstanding appears to exist with regard to the application of certain terms, meant to 

 express briefly the vertical distribution of oceanic plankton, introduced by the present writer (Proc. Zool. 

 Soc. London, 1898, p. 545), and since adopted to some extent by other naturalists. It is generally 

 admitted to be the case that, at any particular locality, the main part of the superficial fauna (owing to 

 its dependence on light, warmth, food, and so forth) may reach to, but does not descend much beyond, 

 100 fathoms; for this fauna the term Epiplankton was intended. Those floating or swimming organisms, 

 which are closely dependent on the bottom of the sea, for food-supply and for other reasons, were classcd 

 as Hypoplankton: to this category, for example, belong the remarkable series of Mysids recently described 

 by Messrs Holt and Tatter,s.\li, (Aniiual Report on the Sea and Inland Fisheries of Ireland, 1902 — 1903. 



i) That is to say from 200 111. oi- less, to the suiface. They were almost ontirely surface hauls. 



