on a clear cokmration over sandy bottoms (while congers) or become dark on gravels or in 

 rorky places (black congers). The tnorays have a tiger-coloured coat. In northern regions 

 there are wolf-lishes, also without pelvic lins, gliding over the continental shelf and slope. These 

 are much less dangerous, for their powerful teeth are merely used to crush shell-fishes or the 

 carapaces of crabs. 



Lastly, very near tlie bottom, swim gadids, such as cud, haddock and iiake, fishes that may 

 be considered as semidemersal in habit. The demersal fishes can be called '• white fish ", 

 after the name given to them by trawlermen. Indeed when a well-filled trawl is hauled up, 

 liie bellies of rays and cod and the blind sides of pleuroneciids, which make uf) most of the catch, 

 are seen as a great white patch as the trawl breaks surface. In the night linuis tiiis whiteness 

 appears leaden in the rays of the moon. 



The bottom-dwelling fishes are sedentary and voracious, with diverse reproiliiclive methods. 

 The cod-like and plaice-like fishes lay pelagic eggs, while the rays produce opaque tough-shelled 

 eggs bearing horn-like lateral prolongations that anchor them to the sand. The angel-fishes 

 (Squatina), sting-rays, and torpedo-rays are viviparous. The conger-eels have a leptocephalus 

 larva which undergoes a metamorphosis very like that of freshwater eels. Dillerences between 

 the sexes are well marked in the rays and some pleuronedids but parental instincts are lacking. 

 All these fishes are eurythermal and euryhaline, submitting to changes of temperature and 

 salinity without uuk-Ii shifting of their grounds. 



Peluijic types. — Pelagic fishes are creatures of the high seas. The outlines of coasts or 

 continental shelf have little bearing on their lives, which are entirely governed by oceanic laws. 

 From time to lime the movements and changes in temperature and salinity of the water masses 

 irrevocably bring them to the same regions; and so they form part of the life of the ocean. 

 Gathered into countless shoals or into small schools, they move around, feeding and breeding 

 in the light and warm, blue, tropical waters or in the cold, heavy, grey waters of northern seas. 

 They follow the Atlantic and Pacific transgressions or retreat before them so as to keep within 

 the bounds of temperature and salinity that fix the hydrological habitat of their kind. 



Their movements are very variable in extent. Some of them, the scdsoiuil lishet!, ne\er 

 venture seaward of the continental shelf, while others, the miqralonj fishes, undertake great 

 journeys, swimming in the oj)en sea. All are designed for rapid swimming. The clujieids 

 (herrings, sardines, etc.) have long spindle-shaped bodies that are somewhat (•om])ressed from 

 side to side. The median fins are rather reduced but keep the body fui an ujiright keel, while 

 the pelvic fins are set well back so as not to impede the rapid flow of water o\ er the flanks. The 

 scombroid fishes (mackerel and tumiies) are more highly evolved with their torpedo-shaped 

 bodies and tapering rear section. Supported by the pectoral fins and forward-set pelvics the 

 powerful head (Uits a passage through the water for the rigid body. The \ery strong tail acts 

 as a rudder and series of Unlets help in smoothly leading to the rear the fier<'e fiow of water 

 stirred up during rapid locomotion. 



All pelagic fishes have much the same kind of colour i>attern and deserve the name " blue 

 fishes ". As a matter of fact, they have slaty, deep-blue upper parts, becoming lighter over 

 the flanks and merging with a silvery or iridescent lower surface. The perfectly precise body- 

 lines and the brilliantly simple colouring express power and beauty together. On the other 

 hand, their gregarious kind of life leaves no room for individuality. Each fish is but a unit 

 lost amid thousands of similar units, all forming an immense complex of dose-packed schools 

 of fishes. As if controlled by supersonic waves picked up by perfect recording devices they 

 twist and turn with precision and speed. Arrangement in the shoals varies from species to 

 species. Some, such as herring, swim in dense columns while others, like sardines, call to mind 

 the arrow-head flights of wild ducks. Such variations can be revealed thanks to moilern echo- 

 sounding gear. Dillerences between the sexes are not found in pelagic fishes, nearly all of 

 which lay pelagic eggs ferlilisc(| at randotn in the water. Parental instincts are lacking. With 

 their stenothermal and stenohaline habits these fishes are rigorously controlled by physico- 

 chemical conditions and perhaps by .some unknown, hidden discipline ordering the ' march " 

 of the shoals. 



22 



