300 FISHES. 



expand, so that, if the withdrawal from a depth is not an 

 extremely slow and gradual process, the various tissues must 

 be distended, loosened, ruptured ; and what is a vigorous fish 

 at a depth of 500 or more fathoms, appears at the surface as 

 a loosely -jointed body which, if the skin is not of sufficient 

 toughness, can only be kept together with difficulty. At 

 great deptlis a fibrous osseous structure and a thin layer of 

 muscles suffices to obtain the same results for which, at the 

 surface, thickness of muscle and firm osseous or cartilaginous 

 tissue are necessary. 



The muciferous system of many Deep-sea fishes is deve- 

 loped in an extraordinary degree. "We find already in fishes 

 which are comparatively little removed from the surface 

 (that is to depths of 100-200 fathoms), the lateral line much 

 wider than in their congeners or nearest allies which live on 

 the surface, as in TracMchthys, Hoplostethus, liismj Scorpcenidce. 

 But in fishes inhabiting depths of 1000 and more fathoms, 

 the whole muciferous system is dilated ; it is especially the 

 surface of the skull which is occupied by large cavities 

 {Macruridce, deep-sea OjjJiidiida^), and the whole body seems 

 to be covered with a layer of mucus. These cavities collapse 

 and slnink in specimens which have been preserved in spirit 

 for some time, but a re-immersion in water for a short time 

 generally suffices to show the immense quantity of mucus 

 secreted by them. The physiological use of this secretion is 

 unknown ; it has been observed to have phosphorescent 

 properties in perfectly fresh specimens. 



The colours of Deep-sea fishes are extremely simple, their 

 bodies being either black or silvery ; in a few only are some 

 filaments or the fin-rays of a bright scarlet colour. Among 

 the black forms albinoes are not scarce. 



The organ of sight is the first to be affected by a sojourn in 

 deep water. Even in fishes which habitually live at a depth 

 of only 80 fathoms, we find the eye of a proportionally larger 



