DISTRIBUTION AND LOCATION 285 



said that the majority of the division Scombriformes have 

 pelagic habits, all the spiny-finned fishes hitherto mentioned 

 belonging to this division. The chief Atlantic tunnies are 

 Thynnus thynnus, the common tunny, Thynnus alalonga, the 

 albacore or germon, Thynnus pelamys, the bonito, Pelamys 

 sarda, the short-finned tunny, and Auxis rochei, a smaller fish, 

 the plain bonito. Many Carangidae or horse mackerels are 

 oceanic, for example Naucrates ductor, the pilot fish, species of 

 CaranXy Trachynotus and Seriola, and Lichia glauca, which has 

 been taken on the English coast. The Trichiuridse or hair- 

 tails seem to show a transition from active pelagic to sluggish 

 abyssal habits ; some like Tliyrsites are found at the surface and 

 have well-developed tails, while others like Gempylus and Tri- 

 chiurus have a narrow slender body and tapering degenerate 

 tail ; active pelagic fishes always have a deeply forked caudal 

 fin like the tunnies and the sword-fishes. Luvarus imperialis 

 and the various species of Bramidse are short-bodied Scom- 

 broids which seem to have a great range in depth ; as they are 

 rare fishes their habits are little known. The same may be said 

 of the opah, Lampris luna> which is oceanic and widely distri- 

 buted. The flying gurnards Dactylopta-us, are Acanthoptery- 

 gians, which are adapted for short flights in the same way as 

 the true flying- fish Exocoetus ; they are chiefly found in the 

 tropical Atlantic and Indian Oceans, one species being fairly 

 common in the Mediterranean. The sargasso-fish, Pteropkryne, 

 has its home in floating weed such as the sargasso of the Gulf 

 stream and the weeds of the Japanese Kuro Siwo or Black 

 Current, and in the same conditions are found species of Hippo- 

 campus or sea-horse. Finally may be mentioned the sun- 

 fishes, the pelagic, extremely modified form of the Plectognathi ; 

 they are passive fishes which drift with the currents and seem 

 to descend to great depths or to rise to the surface at will. 



Abyssal Fishes. — The deep sea is often taken to commence 

 at a depth of 200 fathoms, as that is about the limit to which 

 the light of the sun penetrates ; it is certain that fishes which 

 live habitually beyond this depth usually show some structural 

 differences, such as the enlargement of the dermal sensory tubes, 

 from those which live in shallower water. Almost every littoral 

 species which lives on or near the bottom has a restricted range 

 of depth: we do not find the same species usually at a depth 



