214 DEVELOPMENT OF FISHES 
larval life may be said to begin when the following con- 
ditions have been fulfilled: the outward form of the larva 
must be well defined, separating it from the mass of yolk, 
its motions must be active, it must possess a continuous 
vertical fin fold passing dorsally from the head region 
to the body terminal, and thence ventrally as far as the 
yolk region; and the following structures, characteristic 
in outward appearance, must also be established, the sense 
organs, —eye, ear and nose, — mouth and anus, and one 
or more gill clefts. : 
Among the different groups of fishes the larval changes 
are brought about in widely different ways. These larval 
peculiarities appear at first of far-reaching significance, 
but may ultimately be attributed, the writer believes, 
to changed environmental conditions, wherein one proc- 
ess may be lengthened, another shortened. So too the 
changes from one stage to another may occur with sur- 
prising abruptness. As a rule, it may be said the larval — 
stage is of longest duration in (I) the Cyclostomes, and 
thence diminished in length in (II) Sharks, (III) Lung- 
fishes, (IV) Ganoids, and (V) Teleosts; in the last-named 
group, a very much curtailed (z.e. precocious) larval life 
‘many often occur. 
I. Larval Cyclostomes 
The Cyclostome larva is represented in a stage as 
early as that of Fig. 212: its form is here retort-shaped ; 
the yolk material is concentrated in the ventral region 
immediately in front of the blastopore (the anus?), but 
is distributed in addition in the cells of other body regions. 
In the section of a slightly older larva (Fig. 215), in which 
the mouth is all but established, the form outline has 
become regular, the bulk of the yolk, Y, restricted to the 
