430 EVOLUTIONARY MORPHOLOGY 



the tree of vertebrate evolution, for their specialisations have 

 prevented them from evolving into anything further. Although 

 some of them, such as Periophthalmus, are capable of coming 

 out on dry land and hobbling about, they cannot compete with 

 the true land- vertebrates, which are less specialised but more 

 progressive descendants of their ancestors, the pre-Osteolepids. 

 Of the adaptations which Teleostei have undergone, one of 

 the most interesting is the modification in connexion with the 

 habit of living on the sea-bottom, and which has resulted in 

 the " flat fish." When hatched, these fish, of which Solea 

 (the sole) is an example, are normal and symmetrical in form, 

 but they undergo a metamorphosis as a result of which they 

 lie on one side on the bottom. The head becomes twisted so 

 that the eye of the " underside " (right or left, according to the 

 species) moves on to the " upper side." It is interesting to 

 compare this flattened condition of the body with that of the 

 rays. The modifications in the two groups are totally different, 

 but both are adaptations to one and the same mode of life, 

 and this accounts for what similarity there is between them. 



The so-called flying fishes, of which Exocoetus is an 

 example, have enlarged pectoral fins, and are capable of 

 prolonged leaps through the air rather than of true flight. 

 Lastly, attention may be called to certain deep-sea fish 

 (Edriolychnus) which are not only of a peculiar shape, but 

 are remarkable in that the males are dwarfed and degenerate, 

 and live attached to the females on which they are parasitic. 



Literature 



Delage, Y., et Herouard, E. Zoologie Concrete, 8, les Procordes. 

 Schleicher Freres, Paris, 1898. 



Regan, C. T. Dwarfed Males parasitic on the Females in Oceanic Angler- 

 Fishes. Proceedings of the Royal Society, B, vol. 97, 1925. 



