t that large eels migrated towards the sea in autumn,
and that in the spring small transparent eels of 2 in. in length and
upwards were common on the shore under stones, and ascended rivers and
streams in vast swarms. It was reasonable, therefore, to infer that the
mature eels spawned in the sea, and that there the young were developed.

[Illustration: Leptocephali. (By permission of J. & A. Churchill.)]

A group of peculiar small fishes were, however, known which were called
Leptocephali, from the small proportional size of the head. The first of
these described was captured in 1763 near Holyhead, and became the type
of _L. Morrisii_, other specimens of which have been taken either near
the shore or at the surface of the sea. Other forms placed in the same
genus had been taken by surface fishing in the Mediterranean and in
tropical ocean currents. The chief peculiarities of Leptocephali, in
addition to the smallness of the head, are their ribbon-like shape and
their glassy transparency during life. The body is flattened from side
to side, and broad from the dorsal to the ventral edge. Like the eels,
they are destitute of pelvic fins and no generative organs have been
observed in them (see fig.).

In 1864 the American naturalist, T. N. Gill, published the conclusion
that _L. Morrisii_ was the young or larva of the conger, and
Leptocephali generally the young stages of species of _Muraenidae_. In
1886 this conclusion was confirmed from direct observation by Yves
Delage, who kept alive in a tank at Roscoff a specimen of _L. Morrisii_,
and saw it gradually transformed into a young conger. From 1887 to 1892
Professor Grassi and Dr Calandruccio carried on careful and successful
researches into the development of the Leptocephali at Catania, in
Sicily. The specimens were captured in considerable numbers in the
harbour, and the transformation of _L. Morrisii_ into young conger, and
of various other forms of Leptocephalus into other genera of
_Muraenidae_, such as _Muraena_, _Congromuraena_ and _Ophichthys_, was
observed. In 1894 the same authors published the announcement that
another species of Leptocephalus, namely, _L. brevirostris_, was the
larva of the common eel. This larval form was captured in numbers with
other Leptocephali in the strong currents of the Strait of Messina. In
the metamorphosis of all Leptocephali a great reduction in size occurs.
The _L. brevirostris_ reaches a length of 8 cm., or a little more than