xxxii THE VOYAGE OF H.M.S. CHALLENGEE. 



and secretion ; but we cannot hazard a conjecture as to the manner and degree in which 

 those physiological functions may differ from those of surface fishes. 



I may here also shortly refer to a subject in regard to which very few facts are 

 known, and which therefore offers wide scope for speculation. It is the fact that the 

 spawn of some deep-sea fishes (as for instance Polyjjrion cernium), is developed at the 

 surface, whilst mature individuals of the species reside at more or less considerable 

 depths. It is not probable that these fishes rise to the surface at the season of 

 propagation ;■ we may rather suppose that the spawn is deposited at a depth of several 

 hundred fathoms and gradually rises to the surface, the young fish, after a short 

 pelagic surface existence, returning to the depths inhabited by their progenitors. 

 This, however, is certainly not the case with all deep-sea fishes ; of many the spawn will 

 not only be deposited at the bottom of the ocean, l)ut also remain there throughout 

 the period of its development. But when we consider the immense diff"erenee of the 

 conditions under which the development of the ova of these two kinds of fishes pro- 

 ceeds — the one under the accelerating influences of light, warmth, and a constant 

 supply of oxygen, the other under the retarding conditions of darkness, cold, and a 

 minimum amount of oxygen — we cannot help thinking that the one series supplies the 

 deep sea with the forms which retain the organisation of the surface fishes, whilst 

 the other develops into those degraded forms, of which the families Ophidiidfe and 

 Mursenidse offer the most striking examples. 



The colours of deep-sea fishes are extremely simple, their bodies being either black, 

 pink, or silvery ; however, some of the fishes which now are black are described as 

 having been of a bluish colour when they were brought to the surface. In a few only 

 are some filaments or the fin-rays of a bright scarlet colour ; black spots on the fins 

 or dark cross-bars on the body are of very rare occuiTcnce. An extremely common, 

 almost general characteristic of deep-sea fishes is the black coloration of some of the 

 body-cavities ; this is limited to the pharynx in many of the fishes which live about 

 the 100-fjithoms limit, but the colour is more intense, and spread all over the oral, 

 branchial, and peritoneal cavities in strictly typical deep-sea forms. The highly 

 specialised luminous organs on the head of the Stomiaticte are green or pi)ik during 

 life, whilst they fade into white after the immersion of the fishes in spirit. Among the 

 black- coloured deep-sea fishes albinos are not scarce. 



