GENERAL BIOLOGY 683 



I : 5.2 to I : 7), and he imagines this to be due to differences in 

 age. In the other species of this genus at all events the 

 relation is usually 1:7 or 8. Further investigations are 

 necessary to explain these relations. 



Malacosteus also has a relatively large eye, but in this 

 genus as well as in other Stomiatidae we must suppose that 

 important vertical migrations occur. Thus we see from the 

 table (Fig. 490) that Photostomias guernei has been captured 

 at night in comparatively shallow water, and its eyes are 

 considerably larger than those of the fishes which constantly 

 live at great depths (see Fig. 67, a, p. 86). 



The pelagic decapod Crustacea show a similar correspond- 

 ence between the development of eyes and vertical distribution 

 (see table, p. 668). In the two species living above 150 metres 

 the ratio of carapace to eye is 5-7, and in the five species with 

 a maximum distribution about 500 metres the ratio is 6-1 1, 

 while in the four species living below 500 and mostly beyond 

 1000 metres the ratio is 9-20. 



Although in fact many cases as yet seem inexplicable, there 

 seems to be reason for supposing that the efficiency of the eyes 

 decreases with the decreasing intensity of light as we descend 

 into deep water. That we cannot fully explain all cases seems 

 to be a natural consequence of the fact that our knowledge of 

 the vertical distribution of pelagic fishes is still imperfect, being 

 based mainly on the closing-net hauls of the " Valdivia " and 

 the long horizontal hauls of the " Michael Sars," and both these 

 expeditions were of very short duration. Further investiga- 

 tions will probably furnish many interesting details as to 

 differences within the regions recognised by us, for we are 

 aware that various kinds of eyes occur in the region above 

 500 metres, such as stalked eyes, telescopic eyes, as well as 

 eyes built on the principles of the common type of fish eye. 



Stalked eyes seem to be peculiar to larval stages, stalked eyes. 

 and in certain cases are known to develop into normal eyes 

 even during the larval stage (Lo Bianco). They seem to 

 occur only in the uppermost layers, where all transparent fish 

 larvae live. Considering the insufficiency of our knowledge of 

 the development of pelagic fishes, I do not venture to guess 

 to what species our stalk-eyed larvae belong. 



Telescopic eyes are found only in fishes from depths less than Telescopic 

 500 metres. We have observed them in Argyropelecus. in a ^^^^' 

 new genus closely related to Dysomnia (see Fig. 540, p. 746), in 

 Opisthoproctus, and also in leptocephali. Fig. 500 represents 



