180 THE .FAUNA Ol< THE DEEP SEA 



facets, and they are directed not dorsally as in the 

 Cymothoadre, but ventrally. The cause of these 

 curious modifications of structure in Bathynomus is 

 by no means clear, but it is quite probable that they 

 are connected with the conditions of pressure and light 

 in the deep sea. It is a remarkable fact that the other 

 deep-sea Isopods do not exhibit precisely these modi- 

 fications, and it might be supposed that the same 

 causes would produce the same or similar effects on 

 the structure of animals belonging to the same order. 

 That is perfectly true, but we cannot yet determine 

 how long ago any one species has taken to a deep-sea 

 life, or what length of time, in other words, these 

 conditions have been at work in modifying the 

 structure of the organism. A recent immigrant into 

 the abyss will naturally exhibit closer affinities with 

 its shallow-water allies than those that have dwelt in 

 the region since secondary or tertiary times. If we 

 take this into consideration we should expect to find 

 considerable differences occurring between deep-sea 

 species of the same order, which is precisely what we 

 do find. 



Concerning the Cirripedia, that curious group 

 of profoundly modified Crustacea that includes the 



