154 PRIMITIVE FORMS OF LIFE 



We have seen that the transparency and the blue colour of 

 pelagic organisms may be considered as protective characters 

 intended to render them invisible. Such colours are not 

 encountered in deep-sea specimens, which are generally 

 white (Polycheles, Deima), pink (Peniagone), red (Phormosoma 

 and many swimming Decapod Crustaceans), violet 

 (certain swimming Decapods and Echinoderms of the 

 genera Pourtalesia, Psychropotes) . The deep-sea fish are 

 generally black and their luminous organs green. Although 

 they belong to well-known groups — most commonly to the 

 type of " abdominal " or physostomous Fish — they, none the less, 

 present special characters. They are often provided with long 

 tactile appendages placed sometimes on the head, as in 

 Melanocetes johnsoni, sometimes under the jaw as in 

 Eustomias obscurns, in front of the fins as in Bathypterois 

 longipes, or in both these places (Echiostoma micripnus). 

 Here we find the same facts as among the Decapod 

 Crustaceans, which implies that they have been produced 

 by the same causes. Often, moreover, the mouth takes 

 on enormous proportions (Malacosteus niger, Eurypharynx 

 pelecanoides) . Only a few families are represented at 

 great depths ; the Scopelidae, 1 also pelagic, the Clupeidse, of 

 which the Herring is the commonest type, 2 and the Stomiadae, 

 which resemble the pike in the arrangement of their unpaired 

 fins. These all belong to the group of physostomous Fishes. 

 The physostomous Fish are represented almost entirely by 

 Malacopterygians, such as the Gadidce, 3 typical forms of which 

 are Cod ; Macruridae, with the body terminating in a point, and 

 Ophididae, elongated like Eels. 4 The Fish fauna, unrepresented 

 by Amphioxus, Lampreys, Sharks, and almost all fish with 

 spinous dorsal fins, i.e. the most agile swimmers, is therefore 

 essentially an incomplete fauna like the others, in other 

 words an immigrant fauna, and that of the freshwater Fish 

 will prove to be similarly incomplete. 



But in this case the conditions of existence are absolutely 

 different from those of the deep sea. The pervading immobility 

 and the even temperature and consistency of the water gives 

 place here to agitation that is never ceasing in the streams, 



1 Scopelus, Saiirns, Malacoceplialus, Alepocephalus, etc. 



2 Halosanrus. 



3 Mora, Balhygadus, Coryphcenoides. 



4 Bathynectes crassus. 



