THE OSTEACODA. 283 



indeed, has such evidence been brought forward. The presumption is always against an 

 animal living under two such totally opposed environments as the epiplankton and 

 deeper mesoplankton, until the contrary lias been shown to be probable. Such cases are 

 lielieved to exist, and three are given below in which apparently the younger stages 

 are epiplanktonic, the older mesoplanktonic ; but they are comparatively rare. 



Nor can the condition of the tissues be admitted as evidence : — Firstly, because the 

 tissues of indisputably mesoplanktonic animals are sometimes damaged, either by the 

 diminished pressure, or by the chafing together of the sides of a closed net, when being 

 hauled. Secondly, because salt water in bulk at low temperatures is an excellent 

 preservative, and at great depths bacteria are few ; the tissues may apparently remain 

 unaltered for a long time after death *. 



Brandt and Ostwald have shown how little is the difference between the specific 

 gravities of an organism and of the water, which determines whether an animal floats 

 or sinks ; from this it would seem to follow that, as soon as an ori^anism is sicklv enouo-h 

 to be unable any longer to keep up the fight against gravity, it will begin to sink, 

 although by no means necessarily dead. The only evidence available, therefore, at 

 present is from the comparative abundance or scarcity of the species at different depths. 

 Hence, when an animal is plentiful in the epiplankton, and shows fairly high averages 

 in the upper mesoplankton, but below that shows only fractional averages per 100 

 fathoms, it is assumed in the following pages that these fractional averages represent 

 only dead or dying .specimens sinking towards the bottom. It is not the actual 

 numerical value of the average that is here significant, — for many of the rarer meso- 

 planktonic forms show eqvially small averages, — but the ratio of this average to those 

 obtained at higher levels. As an example may be taken the sudden drop in the averages 

 for cnrtn (young stages) below 250-150 fathoms, when compared with the figures for 

 200-100 and 150-50 fathoms, a drop which is maintained fairly steadily down to 

 1250-1000 fathoms (p. 281, PL 27). 



In this part of the memoir, except where otherwise mentioned, all numbers of epi- 

 plankton specimens have been treated with the usual time-factors to bring them as far 

 as possible to the standard one hour haul. 



HALOCYPRID^. 



CONCHffiCIA AMETRA, Miillcr. 



This species was obviously mesoplanktonic ; it occurred at the following depths 

 only :— 



Between 400-300 fathoms in 1 of 3 hauls, withOGG 



„ 500-400 „ 3 „ 3 „ 3-GG 



750-500 „ 3 „ 4 „ 0-80 



1000-750 „ 1 „ 5 „ 0-08 



specimens per 



100 fathom 



haul. 



* H. N. Moseley : Notes by a Naturalist on H.M.S. ' Challenger.' Ed. 1892, p. .505. 

 SECOND SERIES. — ZOOLOGV, VOT.. X. 46 



