THE OSTRACODA. 



297 



Hauls 27 a and 36 I are of course omitted. The table following gives these general 

 results ; they are diagram matically expressed in figure K, p. 298. 



The table shows in the first place how admirably adapted are Ostracods for the study 

 of oceauic planktology, by their great numbers at even considerable depths. The next 

 thing obvious is the plurimum at 75 fathoms, as compared with the other epiplanktonic 

 horizons studied ; from this the distribution tapers upwards to the surface, downwards to 

 about 250 fathoms. 



Less obvious, but apparently representing a real fact, is the broadening out of the 

 distribution spindle (fig. K) between 750 and 300 fathoms, to which reference has 

 been made in the discussion of the distribution of several species ; the same thing 

 has been noticed in some earlier parts of this series of papers. This puzzling 

 phenomenon may perhaps ultimately find its explanation from Ostwald's recent work. 

 He has shown that the viscosity (inuere Reibung) of sea-water increases very rapidly 

 with diminishing temperature or increasing specific gravity ; and he and Brandt liave 

 both shown how very slight a difference in specific gravity determines Avhether an 

 organism shall sink or float. It seems possible that for each species, according to its 

 own specific gravity and contour-resistance (Form-Widerstand *), there will come a 

 point in its fall towards the bottom, when dead or dying, where the increased specific 

 gravity of the water (due to pressure) and the much diminished temperature produce 

 a viscosity which so nearly balances the excess-weight of the animal over that of water 

 (Uebergewicht) as to reduce the velocity of descent; these points, or rather levels, will 

 be different for different species, and will together form a zone where dead and dying 

 animals are more abundant than they are above that zone. 



* For the explanation of the terms and for the whole (lucstion see Oatwald : " Theoretisohe Planktonstudien," 

 Zoologische Jahrbiicher, Abthcil. Systematik, Baud xviii. S. 1-62. 



