208
THE BIOLOGY OF FISHES
,
mysteriOuS Læmargus has so far eluded observation. If we
,
regard Cladoselache, the fossil
“
shark,” dispassionately
without any predisposition to call it a shark, we nd that it
posSesses Teleostean features of almost the same level as
_
_
those of the Salmonidæ.
The larval Dipnoan and Ganoid
'
are just as specialised in their own way as the Elasmobrarich.
The simplest larval forms are found among the Teleosts,
especially these Which later acquire an air—bladder With an
open communication to the mouth (Physostomi). Among
these also the Simplest larva is that of the Clupeids, With
Which those of the Eels and Albula stand in close relation.
'
Here we_nd—the same shape of the body, single dorsal
n
the heterocercal tail, no anal and no ventral ns, just
_
as in Cephälaspis—(Fig. 50) and Birkenia. A membranous
'
pectdral has not yet been traced in these early forms, but the
_
.. )
'
_,
,
FIG. so…—Cephalaspis' murchisoni from the Upper Silurian beds of
Scotland.
(From Bridge, after Smith Woodward.)
.
basal
Clupeid pectora‘l is acknOWledgedto
*
be primitiVe, similar to that of the Arthrodira and perhaps
'
,
Cephalaspis. A'Characteristic of the
also is the
deVelopment of a ventral row of:{Scutes,i sometimes also—a
*
dorsal rOW‘, similar to those in the Anasp1da(B1rkema)
-Z_
'
?
theChlpeld 1äWä: are,,
,
èar1y: indications of “the pr1marymouth,the _1aœ* opening of
j
_
the
é,1äçäé,a'àppearaçs‘f—çf deñite;vascular and renal :
:
…,
? SyStémsf,
Of
organs, , '
:
,
"the œntinuôuS notochord With its
: :
,
‘
brous,
='Fù_rther, the alimentary canal
\
Î
_
runs bacl{Wafrdto thefäc0otof the
tail (cf. pp. 85, 88, and 96).
:
_
f
'
No other larva, Whether of Teleost or other sub-order,
—
posesses such primitive features, and the signicance of these - '
is
way dinished
fact, that the Clupeid dUririg
,
thecourse of its
into a
_