Download - 66 - The vertebral polumn of fishes is composed of two portions

- 66 VERTEBRAL COLUMN
The vertebral polumn of fishes is composed of two
portions, namely the precaudal and caudal, the line of
separation between the two being marked by the position of
the anus.
The precaudal vertebrae are characterised by. the
presence of transverse processes or parapophyses and the
caudal vertebrae by the presence of haemal arches. Though
many variations are met with in these two types of vertebrae among fishes, the basic pattern of structure is never
lost.
Compared with a typical teleostean fish, the
number of caudal and pre caudal vertebrae in the eels and
eel-like fishes is much greater, the 'number in each species
depending on its total length, as may be seen from the
following table:
TABLE III
Name of species
Pre caudal
vertebrae
Caudal
vertebrae
Total
A.bicolor
41-43
62-64
103~107
P.boro
80-87
123-128
203-215
S.bengalensis
76-80
51-57
127-137
A.fossorius
73-78
32-41.
105-119
In A.bicolor (Fig. 133), the centrum of all the
vertebrae is hour-glass shaped and is not pierced by the
notochordal canal.
minent
The neural arch, which is more pro-
tha~the cent~
is more or less tUbular, but
- 67 incomplete in
f~ont
of the neural spine.
The neural
spine is a slightly flattened, backwardly directed process formed by the convergence and fusion of the two
lateral processes of the vertebrae.
This gives attach-
ment to the mesio-dorsal portions of the lateral muscles.
The spines of the anterior vertebrae are comparatively
short, but they increase in length progressively towards
the hind end, where they appear as long, thin processes
with a small aperture at the base.
This basal aperture,
is, however, absent in the spines of the hindmost caudal
vertebrae.
The prezygapophysis of each side is
~ort,
stout
and directed outwards and forwards with a slightly concave articulating facet.
The postzygapophysis is a short,
blunt and slightly concave process behind the neural arCh,
which fits into the angle between the two prezygapophyses
of the succeeding vertebrae.
flexibility between the
This arrangement gives some
vertebr~e
but nevertheless res-
tricts the latitude of movement.
The term parapophysis has been applied to the
transverse process of the precaudal vertebrae
1937).
(Form~
In fishes, these are prominent structures on the
lateral sides of the centra.
In A.bicolor it is divided
into a long anterior and a short posterior blade.
The
hind margin of the anterior blade is strengthened by a
ridge and its distal end gives attachment to the corresponding pleural rib (Fig. 133).
The haemal arches (Fig. 134) are well developed
and form a continuous canal extending the entire length
- 68 -
of the oaudal region.
The haemal spine of eaoh vertebra
is oonfined to 'the anterior half of the arch and carries
a forwardly direoted spur-like prooess, whioh beoomes
more and more prominent towards the hind end of the tail,
exoluding the last two caudal vertebrae, in whioh these
processes are very rUdimentary.
The first s x preoaudal vertebrae (Fig. 132)
~form
a rigid unit, incapable of any movement. The centrum
of the first vertebra is cylindrical and
opi~thoooelous,
while the centra of the suooeeding five vertebrae are
more or less similar to those of the other preoaudal
vertebrae.
The neural arch of each of these six verte-
brae is laterally flattened and oarries a number of
spine-like processes all along its dorsal margin and
this gives the neural arch a oocks-comb-like appearance.
~
The neural aroh of the first carries two knobs for artioulation with the corresponding faoets of the exoooipital bones and each of the
suo~eeding
four arches carry
.-
a backwardly direoted spine on each side.
In P.boro
(Fig.137) the centrum of each
vertebra is very thin and biooncave.
The neural arch is
a complete tube with,only a small, median notch at the
anterior margin and a small median prooess on the
posterior margin, which fits into the median notch of
the succeeding vertebra.
This backwardly directed median
process represents the reduoed neural spine.
The notoh
is bounded on each side by the prezygapophyses which are
small, triangular processes direoted forwards and touohing the postzygapophyses of the preceding vertebra.
An
aperture on eaoh side of the neural aroh provides passage
for the spinal nerve.
The parapophysis is a thin, flat,
- 69 -
wing-like expansion on either side of the oentrum,each
with a small median opening close to the oentrum for the
passage of a blood vessel.·
. The lateral prooesses of the first pre caudal
vertebra is very short (Fig. 136).
ing
f~ur
Those of the succeed-
vertebrae become progressively longer.
The
lateral processes of the remaining vertebrae are of uniform length.' The hind margin of the neural arch of each
of these fiv& vertebrae overlaps the anterior margin of
the succeeding arch.
The centrum of the first vertebra(
articulates with the hind end of the basioccipital and
the lateral margins of its neural arch carries two small
processes for articulation with the exoccipita1s.
The haemal arch is
~ncomplete,
since the ventro-
lateral expansion of the centrum do not meet ventrally
(Fig. 138).
However, the postero-ventral angle of these
two lateral plates are connected by a horizontal, backwardly directed semi-circular ring.
The haemal spine
is absent.
In addition to the haemal arch, each caudal
vertebra carries a lateral process, the apophysis, on
either side of the centrum (Fig. 138).
The apophyses
extend horizontally and support the lateral musculature
of the caudal region.
Each apophysis is subdivided into
an anterior and a posterior blade.
in certain fishes.
Apophyses are present
For example, in the gar-fish, Belone
and Conger, the apophyses are known to function as
skeletal
suppor~s
for the lateral muscles.
In Pleuro-
nectidae and Bothidae, the apophyses are confined to the
- 70 caudal vertebrae only, but in Arnoglossus and Soleidae,
they are present on all the vertebrae (Ford, 1937).
The centrum of a typical vertebra of S.bengalen~
is disim1larly biconcave, the concavity being shallow
in front but deep and conical behind (Fig. 141).
The
neural arch is thin and it extends the entire length qf
the centrum.
The two sides meet together to form a com-
plete arch only at the base of the neural spine. Anteriorly, though the arch is incomplete, the prezygapophys
which extends upwards and forwards from the
antero~dorsal
margin of each side of the neural arch, meet together at
their tips,thus completing the arch in front.
Thus the
roof of the neural arch is complete only in front and
behind. The postzygapophyses are small, triangular lateral
processes.
The postzygapophyses of any particular verte-
bra do not meet the
prezygapophy~es
of the succeeding
vertebra, when the vertebral column is straight, but if
it is bent to the right or left, the postzygapophysis of
the corresponding side overlaps the prezygapophysis of
the succeeding vertebra.
This arrangement restricts the
degree of lateral movement between the vertebra.
The
parapophysis is a strong, backwardly directed process,
to the distal end of which the
pae~al
rib is attached.
The haemal arch (Fig.142) is cut obliquely from behind
forwards so that it appears like two triangUlar plates,
the apices of which alone meet to complete the arch.
From this point of union arises a short, thin, backwardlydirected haemal spine.
- 71 The first pre caudal vertebra (Fig.l40) is comparatively short
~~
opisthocoelouso
It is articulated
with the exoccipitals of the skull by a pair of knobs on
the neural arch.
The neural' arch is similar to that of
other precaudals, but the neural spine is short, flat and
pointed.
The parapophyses, though similar to those of a
typical precaudal vertebra, are smaller.
The second is
similar to the other pre caudal vertebrae except that the
prezygapophyses do not meet in the middle line.
In the vertebrae of A.fossorius (Fig.145), the
centrum is similar to that in S.bengalensis.
The neural
spine, is much reduced and is more or less of the same size
as the prezygapophysis.
The
postzygapop~yses
prominent tha~hose in S.bengalensis.
directed processes which overlap the
of the succeeding vertebra.
are more
They are backwardly
s~des
of the centrum
The parapophyses are more or
less triangular, wing-like expansions, originating from
the ventro-Iateral part of each centrum.
The ha~mal~rch
is thin and confined to the anterior half of the vertebra
(Fig. 146).
The haemal spines are short, flat and vary in
size and shape, becoming, progressively shorter and narrower
towards the hind end, the haemal spines of the last two
vertebrae being extremely rudimentary.
~e
first precaudal vertebra (Fig.144) is much
smaller than a typical vertebra and is immovably attached
to the skull.
The centrum is opisthocoelous and its
convex anterior surface fits into a median depression in
the basioccipital.
On either side of the centrum there
72 is a small process with a semi circular articulating
surface, which also fits into a corresponding depression
in the basioccipital.
The neural arch is low and the
neural spine is thin and short.
Prezygapophyses and
parapophyses are absent, but the postzygapophyses are present as small, triangular plate-like structure on the
posterior margin of the neural arch.
The second vertebra (Fig. 144) is also opisthocoelous and the centrum is long and thin.
The neural
arch is similar to that of the first, but the neural
spine is stout and slightly longer than the first.
The
prezygapophyses are short, plate-like transverse processes which do not articulate with any part of the
first vertebra.
The postzygapophyses are short process-
es, more or less like the
prezygapophyses, but they are
directed backwards and overlap the sides of the neural
arch of the third vertebra.
Parapophyses are small,
backwardly directed processes arising from the lateral
sides of the centrum.
The third vertebra almost resembles the second,
except in the disposition of the zygapophyses.
The pre-
zygapophyses are directed forwards and upwards and meet
together in the median' line to complete the neural arch
and its tip touches the posterior margin of the neural
arch of the second vertebra.
The postzygapophyses are
slightly larger than those of the second.
The fourth vertebra is similar to the third,
but slightly shorter. The centrum is opisthocoalous,
but the convex anterior surface has a small depression
in the centre.
The prezygapophyses are directed up-
wards and meet toget
~
n the mid-dorsal line.
The
- 73 postzygapophyses and parapophyses are larger than those
of the third.
The fifth
ver~ebra
is similar to the fourth,
except tha~the central depression of, the convex anterior
face of the centrum is more conspicuous..
But the outer
anterior margin of the centrum is thickened into a prominent rj,dge.
In a typical teleost there are two ,types of ribs,
namely the epipleura1s attached by ligaments to the sides
of the centra and the pleural ribs attached to the
pophyses.
In S.bengalensis and A.fossorius
pleural ribs are present.
para~
only the
They are short and fairly thick
and attached by ligaments .to the distal ends of the parapophyses of the. precaudal vertebrae.
On the other hand,
in A.bicolor and P.boro both the pleurals and epipleurals
are present. The epipleura1s are slender rods attached by
ligaments to the anterior sides of the neural arches,
while the pleura1s are similar to those of S.bengalensis.
The characteristic feature of the vertebral
column of teleosts is that in any particular region,all
the vertebrae are not identical.
Differences in size and
shape may be noticed even in any two adjacent vertebrae.
Such differences are most conspicuous in the caudal region,
where there is progressive diminution in size towards the
hind end.
The first five or six vertebrae of the pre-
caudal region or post-cranial vertebrae are less flexible
than those in the
re~t
of the body.
In A.bicolor, these
post-cranial vertebrae are more or less firmly attached
- 74 to each other so as to form an inflexible unit.
In
P.boro, the post-cranial vertebrae are capable of
limited flexion in a horizontal plane and this is facilitated by the nature of the articulation of the pre
and post zygapophyses, already referred to.
S.bengalensis and in A.fossorius
In
the first vertebra is
immovably attached to the occipital region and flexion
is possible only behind the first vertebra.
Though all these four species ,make use of lateral undulations of the body in locomotion and burrowing, .
the articulatory surface of the vertebrae are not similar
in all cases.
In A.bicolor and P.boro, the
post zyga-
pophyses of one vertebra overlap the prezygapophyses of
the succeeding vertebra.
Both these arrangements facili-
tate the same degree of lateral flexion.
Another remarkable feature is the reduction and
the modification of the neural and haemal arches.
The
90ndition of the neural arch in P.boro may be regarded as
an, extreme case of modification, where the neural spine
is reduced into a knob-like process for articulation with
a corresponding gap in the neural arch of the succeeding
vertebra.
In A.fossorius, the neural arch is very small
and is hardly more prominent than the prezygapophysis.
The haemal arch is flat and plate-like in all these
fishes and one noteworthy peculiarity of P.boro is that
the haemal
spine is absent and the haemal arch is com-
plete only posteriorly by a thin, semi-circular ring.
The 'neck region' of the vertebral column is
variously modified in different fishes.
In Gadus, the
- 75 first three vertebrae constitute a rigid region and movement is possible only behind the third vertebra (Ford,
1937).
In the
sand~l,
Ammodytes, the first post-cranial
vertebra is opisthocoelous and the head can be moved on
the first vertebra.
A similar arrangement is seen in
Conger and Engraulis (Ford, 1937).'
Ford observes that
"this type of articulation of the vertebral column to the
head is associated with the habit of burrowing into the
soil".
In S.bengalensis and A.fossorius the head is capa-
ble of lateral movement, but in A.bicolor and P.boro the·
head is incapable of lateral flexion.
When A.bicolor and
P.boro remain burried in mud, the head is lifted vertically upwards into the water and when they rise to the
surface of water to take in atmospheric air, the body remains suspended in a vertical position, while the head is
bent forward almost at right angles.'
These observations
indicate that in both these species the neck region is
capable of forward flexion.
The pre- and post zygapophyses of S.bengalensis
and A.fossorius are more or less
sim~lar.
In both these
species, the fused tips of prezygapophyses touch the hind
margin of the neural spine of the preceding vertebra and
the postzygapophyses overlap the base of the prezygapophyses.of the succeeding vertebra, though the two do not
touch each other.
Describing the vertebral variations of
Xiphias, Gadow (1895) presents an interesting analogy of a
similar condition in whales, where the prezygapophyses of
the anterior~rtebra are overlapped by the postzygapo-
- 76 physes of the preceding vertebra and in the mid-trunk
and tail regions, the prezygapophyses are modified into
vertebral plates that reach the spinous processes of
the vertebra in front.
The neural arch in P.boro is unique in forming
a continuous tube along the entire length of the vertebral column.
This fish also presents the singular
,
instance of the modification of the neural spine into a
posteriorly directed articulatory process which fits into
the space between the prezygapophyses of the succeeding
vertebra.
This condition also presents a strange
resem-
blance to the vertebral articulation in Dinousaurs.
According to Broili
~cited
by Gadow, 1933)
!
in Dlnousurs, •
there is a wedge or process extending backwards from the
neural arch in between the postzygapophyses and fitting
into a notch between the prezygapophyses of the succeeding vertebra.
This wedge is termed the hypo sphene and
the notch, the hypantrum.
Since the former term was used
earlier for an entirely different structure in Gymnophiona
by Weidersheim, the terms have been changed to metasphene
and metantrum by Gadow (loc. sit).
This condition is
just the opposite of the zygapophene and zygantrum which
•
are present in snakes.
In both the Ophidia and the GYmnophiona there is
an increase in the number of vertebrae.
The vertebrae are"
amphicoelous in Gymnophiona and proceolous in Ophidia. The
lateral processes of the vertebrae are greatly reduced in
the Opll.idia. and the GYmnaphiona..
In both these groups,
- 77 vertebrae show different lines of modifications by which
great mobility of the vertebral column is attained.
One interesting fact noticed in the four species
selected for the present study is the progressive reduction
of the caudal region in relation to the total length of
the body.
TABLE IV
Name of species
Total No. Precauof.,verte- dal verbrae
tebrae
A.bicolor
103-107
41-43
P.boro
203-205
80-87
Sobengalensis
127-139
76-80
51-57
40.2 %-41.0
A.fossorius
103-119
73-78
32-41
30.5 %-31.1 %
Caudal
vertebrae.
62-64
Percentage of
caudal vertebrae.
60.19%-59.81%
123-128 60.6 %-59.5 %
%
From the above table it may be noticed that,1h-:::
A. bicolor and P.boro, the caudal vertebrae forms nearly
60% of the total number, while in S.bengalensis and
A.fossorius the caudal vertebra form only 40% and 30%
resp~ctively.
Both the latter are burrowing fishes, but
A.fosiorius is a more efficient burrower than S.bengalensis.
From this it may be inferred that there is a pro-
gressive shortening of the caudal region corresponding
to greater adaptation to burrowing mode of life.
A
similar reduction 'of the caudal region is also noticed
in Gymnophiona and Ophidia.
According to Norman (1931) the eel-like bodyform
as assumed by fishes as a means of hiding in cre-
vices of submerged rocks or creeping through beds of
aquatic weeds.
This adaptation is eVidently responsible
- 78 for the reduction of the size of fins.
In such fishes,
the only method of locomotion through water is by the
lateral undulation of the body aided by the caudal fin.
A long tail is therefore necessary for such fishes.
But,
in the adaptation to burrowing habit; locomotion through
water is progressively minimised.
In such fishes, there-
fore, there is a progressive reduction in length of the
caudal region, as noticed in S.bengalensis and A.fossorius.
However, there is an apparent inconsistency in this argument when we consider the case of P.boro.
This fish is
a good burrower and samples have been collected at a
depth of two to three feet in mud.
But in this species,
the caudal region has the same proportion as in A.bicolor.
But the caudal fin is completely reduced, so that the
tail almost tapers to a point.
This difference, however,
can be explained by the difference in the mode of burrowing.
While A.fossorius and S.bengalensis burrow forwards
with their head, P.boro burrows backwards with its tail.
The use of tail as an organ of burrowing is probably
responsible for its greater length compared with A.fossorius and S.bengalensis.