gh. vii THE EXTEKNAL FEATUBES OF THE' BODY 433 



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Ganoids Amia and Lepidosteus the main mass of yolk retains its 

 form for a considerable period, causing a great bulging of the ventral 

 body-wall anteriorly. In the Teleost this is still further accent- 

 uated, the bulging forming the yolk-sac which remains prominent 

 even in larvae sufficiently developed to be able to swim actively. 

 An extreme case of the prominence of the yolk-sac is afforded by 

 Gymnarchus (Fig. 199) where it shows a peculiarly elongated form 

 for a certain period. Cement organs are as a rule absent in Teleostei : 

 so also are external gills though in rare cases the latter have physio- 

 logical representatives in filamentous prolongations of the gill 

 lamellae (Fig. 199, B). 



Great variety of form exists amongst the larvae of Teleostean 

 fishes, more especially amongst those of pelagic habit. Familiar 

 examples are seen in the pelagic larvae of the Eels — much com- 

 pressed from side to side, transparent and colourless — even the blood 

 being free from haemoglobin — and much greater in bulk than the 

 immediately succeeding phase in the life-history. The larvae of the 

 Flat-fishes (Pleuronectidae — Flounder, Plaice, Sole, etc.) are again of 

 special interest owing to the extraordinary asymmetry which they 

 develop. They are at first quite symmetrical and in no way abnormal. 

 The larva, swims at this time with its laterally compressed body 

 vertical after the manner of a Bream but later develops the habit 

 of swimming on its side. The side of the head-region which is below 

 now grows more actively than the other so that the head becomes 

 strongly asymmetrical and the eye of the lower side becomes gradu- 

 ally transferred to the upper, the right and left eyes being now both 

 on the same side of the head. Correlated with this asymmetry in 

 form there comes about a corresponding asymmetry in colour, the 

 chromatophores being collected together on the upper side and 

 giving it its characteristic obliterative colouring. In some genera it 

 is the right side of the body which is above, in others the left — while 

 in a few species it appears to be indifferently the one or the other. 



Dipnoi. — Both of the dipneumonic Lung-fishes — Lepidosiren and 

 Frotopterus — have been investigated (Graham Kerr, 1900 and 1909 ; 

 Budgett, 1901). They closely resemble one another and Zepidodren 

 will be chosen here for description (Fig. 200). 



During the early stages of the modelling of the embryonic body 

 (Fig. 200, A) the latter is curved round the egg, occupying about 

 290° in angular extent. The head-region becomes demarcated as a 

 slight, somewhat lance-shaped protuberance above the general surface 

 of the e^g due to the neural rudiment. The branchial region becomes 

 marked at an early stage by a slight elevation of the surface which 

 soon becomes divided by shallow oblique grooves into the series of 

 branchial arch rudiments. About stage 25 (Fig. 200, B) the tip of the 

 head and the tip of the tail project sharply above the general surface: the 

 external gills (e.g) are now in the form of four distinct little knobs on 

 ' each side, and the cement organ (c.o) has made its appearance as a 

 crescentic structure on the ventral side curving round the tip of the 



vol. II 2e 



