VERTEBRAL COLUMN

Transcription

1 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 precaudal 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 Precaudal Caudal Total vertebrae vertebrae A.bicolor ~107 P.boro S.bengalensis A.fossorius In A.bicolor (Fig. 133), the centrum of all the vertebrae is hour-glass shaped and is not pierced by the notochordal canal. The neural arch, which is more prominent tha~the cent~ is more or less tubular, but

2 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 attachment 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. This arrangement gives some flexibility between the vertebr~e but nevertheless restricts the latitude of movement. The term parapophysis has been applied to the transverse process of the precaudal vertebrae (Form~ 1937). 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

3 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 vertebrae 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 arti- oulation 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. aperture on eaoh side of the neural aroh provides passage for the spinal nerve. An The parapophysis is a thin, flat,

4 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 precaudal vertebra is very short (Fig. 136). Those of the succeeding f~ur 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 ventrolateral 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. Apophyses are present in certain fishes. For example, in the gar-fish, Belone and Conger, the apophyses are known to function as skeletal suppor~s for the lateral muscles. In Pleuronectidae and Bothidae, the apophyses are confined to the

5 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 complete 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 vertebra 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.

6 The first precaudal 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 precaudal 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 are more prominent tha~hose in S.bengalensis. They are backwardly directed processes which overlap the s~des of the centrum of the succeeding vertebra. 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

7 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 processes, 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 prezygapophyses 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 upwards and meet toget ~ n the mid-dorsal line. The

8 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 para~ pophyses. In S.bengalensis and A.fossorius only the pleural ribs are present. 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 precaudal 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

9 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. In S.bengalensis and in A.fossorius 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 zygapophyses of one vertebra overlap the prezygapophyses of the succeeding vertebra. Both these arrangements facilitate 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 complete only posteriorly by a thin, semi-circular ring. The 'neck region' of the vertebral column is variously modified in different fishes. In Gadus, the

10 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 capable 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~rtebraare overlapped by the postzygapo-

11 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 resemblance 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 hyposphene 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,

12 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 spe- Total No. Precau- Caudal Percentage of cies of.,verte- dal ver- verte- caudal vertebrae tebrae brae. brae. A.bicolor %-59.81% P.boro %-59.5 % Sobengalensis %-41.0 % A.fossorius %-31.1 % 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 progressive shortening of the caudal region corresponding to greater adaptation to burrowing mode of life. 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 crevices of submerged rocks or creeping through beds of aquatic weeds. This adaptation is evidently responsible A

13 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, therefore, 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.