364 ZOOLOGY FOR MEDICAL STUDENTS chap. 



eastward with the ocean currents and on nearing the coasts of the old 

 world they metamorphose into young eels or " elvers." These work 

 their way along the coasts and crowd up into fresh waters, there to spend 

 their lives until the approach of sexual maturity. 



The Gadidae include a number of the most important marine food- 

 fishes such as the Cod, Haddock, Whiting, Pollack or Lythe, Coal-fish 

 or Saithe — all species of Gadus ; the Hake (Merluccius) and Ling (Molva). 

 The Burbot {Lota) is a fresh-water representative of the family. 



The Percidae (Perch), the Mulhdae (Red Mullet), the Labridae 

 (Wrasse), the Scombridae'(Mackerel, Tunny), the Xiphiidae (Sword-Fish), 

 the Zeidae (John Dory) all include well-known fish. 



The Pleuronectidae or Flat-fishes include a number of important 

 food-fish such as the Halibut, Plaice, Flounder, Dalp, Lemon Sole, Turbot 

 and Sole. They are of special morphological interest from the asymmetry 

 which they develop during their growth. In the young stage they are 

 symmetrically shaped and swim in the normal position, dorsal side above. 

 The body becomes gradually more and more compressed from side to 

 side and the young fish take to swimming on one side along the bottom. 

 In some species it is the right side which is underneath, in others the 

 left, in still others either side indifferently. In the head region growth 

 takes place with- greater activity on the lower side and this causes much 

 distortion, the eye of the lower side becoming gradually displaced up- 

 wards so that both eyes come to be situated on the same side of the 

 body. Chromatophores or pigment-cells make their appearance and 

 these crowd together near the upper surface of the body giving it a 

 colouring resembling that of the background, while the under side, 

 without pigment-cells, is pure white. That the definitive position of 

 the chromatophores is a reaction to light has been demonstrated by 

 rearing young flat-fish in an aquarium with an opaque lid and lighted 

 through its glass bottom : this resulted in fish which were reversed as 

 regards their coloration, the under side being pigmented, the upper white. 



The small family Trachinidae should be mentioned as it contains the 

 Weevers of the European coasts — well known for the poisonous wounds 

 inflicted by their sharp dorsal and opercular spines. The poison is secreted 

 by special glands at the base of the spines. 



Along with the highly evolved modern teleostomes there still exist at 

 the present day remnants of earlier stages in their evolution in the form 

 of a few odd genera which have lagged behind in evolutionary progress. 

 Of these there are first to be mentioned the Crossopterygii, which 

 during the ancient times of the Old Red Sandstone and the Coal period 



