406 ADAPTATIONS TO MEDIA AND SUBSTRATES 



Necturus. Closer study reveals that the retina is packed with great num- 

 bers of pseudo-stratified rods, three million of them per square millimeter. 

 This emphasis on the rods seems surprising; for though Anguilla is noc- 

 turnal in its feeding, its habits in fresh water would not appear to call for 

 such an extraordinarily sensitive retina. The eel has even had to manage, 

 somehow, to make its pupil highly contractile, something which very few 

 other teleosts have accomplished. Moreover, the chorioid is extremely 

 thick and the retina is full of capillaries, making it the only vascular 

 retina which has been found outside the mammals. 



The excessive retinal sensitivity and the potentially enormous nutritive 

 supply have been explained by Franz : they are preparations for a minor 

 miracle which takes place in a brief period of time toward the end of the 

 eel's life. The common eel begins and ends its life as a deep-sea fish. 

 Some months before her one and only breeding period, the eel's skin 

 turns silvery and her eye rapidly grows until it is relatively huge. The 

 eye is now emmetropic or possibly even myopic, and its great sensitivity 

 to light is no more than enough to make vision possible in the next phase 

 of the life-cycle. The formerly voluminous chorioid is finally justified by 

 the great ocular growth which has been so rapidly accompUshed by its aid. 



The female eels now travel down the rivers to the sea, and they and 

 the males make their way to the south Atlantic, in the vicinity of the 

 West Indies. Here the eggs are laid and fertilized, whereupon both of 

 the parent eels die. The early larvae, which live at great depths (where, 

 for all we know, the eggs may be laid), develop into the pelagic 

 'leptocephalus' stage, in which the ribbon-like, glassy-clear fishlet is quite 

 unrecognizable as an eel. After an extremely slow and largely passive 

 migration, the baby eel reaches the ancestral estuary as an elver — the 

 more eel-like stage in which the eels enter fresh water. During the mi- 

 gration, the relatively large leptocephalus eye must be converted into an 

 eel eye, thus to remain for years until its time comes to share in its 

 owner's final preparations for reproduction and death. 



Some other fishes develop through a leptocephalus stage : the tarpons, 

 ladyfishes, and ten-pounders. Some of these may breed in brackish or 

 fresh water; but none of them passes its adult existence as a small-eyed, 

 nocturnal, freshwater fish. One leptocephalus (T. mirabilis') has been 

 found which has tubular eyes. It may possibly develop into some abyssal 

 species of eel; but the adult has not been identified. The nearest ap- 

 proach to the whole ocular story of the common eel is that of some 

 lampreys. Many species of the latter pass through a silvery-bodied, 



