ZOOLOGICAL 
AS Ran: 
METAMORPHOSIS OF THE COMMON EEL. 
ocean to breed; (2), the sexual organs cannot 
mature their products until the eels have re- 
turned to the depths of the ocean; (3), the pres- 
ence of salt water is not alone sufficient, but cer- 
tain conditions of depth and temperature are 
necessary, viz., an approximate depth of 1,000 
meters and a temperature at that depth of not 
than 7° Centigrade; (4), after hatching 
the young gradually rise toward the surface as 
leptocephali, having a greatly compressed, rib- 
bon-like form, completely colorless except for 
less 
the iris of the eye, and having the languid move- 
ments characteristic of such pelagic animals; 
(5), like many other pelagic animals they are 
negatively heliotropic, and descend to about 100 
meters during the day, only rising to the surface 
at night; (6), after reaching a length of about 
three inches, the metamorphosis, which lasts a 
year or more, begins and the leptocephalus is 
gradually transformed into the young elver or 
typical eel-like form, and (7), at the close of 
their metamorphosis the young eels gradually 
make their way shoreward and in the course of 
time ascend the streams into fresh water. 
Thus far the eggs of the eel have not been 
taken nor have the young, up to the time when 
SOCIETY 
"eae nsec oe 
BULLETIN. 743 
they assume their pelagic form of 
leptocephali, been observed; but 
from the fact that young leptoce- 
phali have been repeatedly dredged 
from a depth of 1,000 
is safe to assume that the 
and early development 
take place at this great depth. Such 
being the case, it is easy to account 
for the absence of sexually matured 
fresh or shallow waters. It 
is not so easy, however, to explain 
certain other events in the life his- 
tory, and particularly the remark- 
able metamorphosis of the eels, in- 
meters or 
more, it 
spawning 
eels in 
volving as it does, the migration to 
the surface of the Such a 
migration must be very gradual to 
ocean. 
permit of adaptation to changes in 
pressure, for animals brought sud- 
denly from such a depth are killed 
before reaching the surface. The 
migration of the adult eel to its 
breeding grounds must also be slow- 
ly accomplished for similar reasons, 
and the time thus consumed is sufhi- 
cient for the development of the sex- 
ual products. Dr. discoy- 
ered through experiments with 
marked fish, that the rate of migra- 
tion is about fifteen kilometers (ap- 
promixately eight miles) a day. 
The that many of the must 
travel in order to reach the breeding grounds are 
Not only must they descend from 
the streams and lakes to the ocean, but in order 
to reach a sufficient depth they must in nearly 
all cases make long journeys at sea. Thus, the 
living in the fresh northern 
Europe find their nearest breeding grounds in 
the Atlantic, south-west of the Faroe Islands; 
while those inhabiting the waters of the Mis- 
sissippi system must migrate out of the Gulf of 
Mexico to the eastward and northward of the 
Bermudas, before the proper temperature of not 
less than 7° Centigrade at the 1000 meter line 
is reached. Not the least remarkable feature of 
this life history is the fact, for such it seems to 
be, that a species capable of withstanding such 
changes of temperature and pressure and which 
is distributed from the tropics to the Arctic circle 
should be so inadaptive in its breeding habits. 
The life history of the American eel, (dn- 
guilla chrysypa), has not been so thoroughly 
investigated as that of its European relative, but 
the facts so far as known accord well with those 
determined by Schmidt for the European spe- 
Peterson 
distances eels 
enormous. 
eels waters of 
