40-

BURROWING MECHANISM

The anguilliform fishes are capable of swimming

and burrowing. The simplest type A.bicolor swims about

freely and only occasionally hides in the bottom clay,

while the most advanced type A.fossorius usually·remains

in deep burrows most of the time and comes out only

oocasionally. S.bengalensis and P.boro occupies inter­

mediate positions between these two extremes. The burrows

made by them are not so deep as those of A.fossorius. At

the same time, they do not swim about so freely as

A.bicolor. So in these four species the organs that are

concerned with locomotion, namely, the skeletal and mus­

cular systems, show varying degrees of adaptive changes

depending on the extent of the burrowing habit.

The pattern of the body segments in fishes has

been observed by various authors, but only a few have

studied their functional mechanism. Owen (1866) des­

cribed the lateral muscles of fishes as an aggregate

structure formed of a series of .transverse musc+es, but

failed to recognize their morphological importance.

Humphrey (1871) described the division of the lateral

muscles into dorsal and ventral moities by a horizontal

septum pas.s.ing inwards beneath the la,terel-line.

Chevrel(19a3) noted the conical structure of the myo­

meres forming the lateral muscles. Greene and Greene

(1913) explained the mechanics of locomotion in fishes;

based on dissections of king salmon, Oncorhypchus

tachawytscha. A more complete study of the subject was

- 41 -

made by Roc~well, Evans and Pheasant (1938) and they

stressed the oblique nature of the attachment of the

myomeres to the verteb~al column. The pattern and dis­

position of the muscles were also described by Shan

(1914) and Nishi (1938). The studies of Marey (1895),

Breeder (1926), Magnan (1930), Gray (1933) and Nvrsall

(1956) have enabled a clear understanding of the mecha­

nics of locomotion of fishes in their natural aquatic

environment. But the question, how the body musculature

and the skeletal axis, which are normally designed for

swimming can also help the animal to push through mud

has not received much attention so far.

In a typical teleostean fish like the Scomber

or Mackerel, there is a thick sheet of muscle on each

side extending from the posterior region of the skull to

the hind, end of the vertebral column. This is dividedj

into as many muscle-segments or myotomes as there are

vertebrae. The myotomes are separated by the myocom-

mata (myosepta) and are innervated by the spinal serves

of the corresponding segments. A horizontal septum ex­

tending the entire length of the body divides each myo­

tome into a dorsal epaxial and a ventral hypaxial muscle.

The myocomma of the epaxial mu~cle is V-shaped with the

apex directed backwards. The two arms of the epaxial por­

tion are distinguished into the mesio-dorsal and latero­

dorsal portions. The hypaxial muscle of each segment is

a mirror image of the dorsal epaxial muscle and can be

similarly distinguished into latero-ventral and mesio-

I

- 42 -

ventral portions. The muscles of the two sides do not

cross the mid-dorsal and mid-ventral lines. The mid-

ventral line in the region of the trunk is ~emarcated by~

a median line, the linea alba.

In the regions of the pectoral and pelvic

girdles, the muscles are modified to effect the move­

ments of the fins.

Associated with the median fins, there are

groups of muscles, the carinales. The supracarinales are

seen in,the region of the dorsal fin and are placed in

between'the two sets of epaxial muscles, while the in­

fracarinales are present in the region of the anal fin

and are wedged in between ~ypaxial muscles. The cari­

na~B are paired muscles and they function as the eleva-r

tors and depressors of the fin-rays.

From Shan's account of the internal structure

of the myotome, it is seen that the individual muscle

fibres are directed parallel to the longitudinal axis of

the body only at the surface, while internally they are

directed obliquely forwards. He has also described the

various regions of a myotome as having the shape of

pyramids.

In a recent paper on the structure of the late­

ral muscles in fishes, Nursall (1956) describes the f1e-

xures of the myosepta. The flexures are sharp and the

one at the lateral line is a forward flexure and the

flexures above and below this are backward flexures. The

flexures are described as the external signs of complex

f

- 43 -

internal foldings. The flexures form cones, internally

nesting with the cones of the preceding and succeeding

segments.

The lateral muscles of the anguil1iform fishes

are similar in essential detai!s to the typical condi­

tio~escribed below. The main differences noted are the

elongation and the apparent simplicity of the lateral

muscles due to the absence of the pelvic fins and the

thoracic position of the pectoral fins (Figs. 17, 18,

23, 24, 28, 29, 33 & 34).

The myomeres which compose the lateral muscles

are similar in shape and arrangement to those of any

typical te1eostean fish (Figs. 20, 25, 30 & 35). But the

flexures are sharp and the mesio-dorsal and mesio-ventral

portions of the muscles are longer than the 1atero-dorsal

and latero-ventral portions. The mesio-dorsal portion is

long and slender and is attached to the neural spine two

or three segments in front of the one to which the myo­

mere belongs. This feature is very prominent in P.boro

and A.fossorius. The mesio-ventral portion is also

longer than the latero-ventral portion and is attached to

the corresponding muscle of the opposite side.

The angle of the backward flexure is very narrow

as compared with that of the forward flexure. In P.boro

there are two cones at the forward flexure, while in the

other three species there is only one cone. Nursal1(1956)I

observes that in the Te1eostei there are generally two

cones at· the forward flexure and in the Chondrostei and

Holostei only one cone is present at the forward flexure.

- 44 -

The arrangement of the muscle fibres in these

fishes is some what different from what has been des­

cribed in teleostean fishes by Shan (1914). All the

muscle fibres run parallel to the longitudinal axis of

the body. The muscle fibres of the eels do not show the

complicated disposition described in teleostean fishes

by Shan (1914), but there is a progressive backward dis­

placement of the internal fibres in the backward flexure

and a progressive forward displacement of the internal

fibres in the forward flexures. The individual fibres

are all of the same length and extend from one myoseptum

to the adjacent septum. The myosepta do not extend verti­

cally inwards, but are placed obliquely. This results in

a slight over-lapping of the neighbouring myomeres and

probably effects the smooth transference of waves of con­

traction to either side.

The supra and infra carinales (Figs. 22 & 27)

are associated with the fin-rays and are well developed

in A.bicolor and P.boro. In the latter species the cari­

nales are pushed inwards and hidden by the lateral

muscles, due to the presence of a groove alo~g the base

of the median fins. v

The reduction of the fin-rays in Sobengalensis

and A.fossoriuB accounts for the. reduction of the cari­

nales in these fishes (Figs. 36 & 37). In E.bicolor

the abductor muscles are present (Figs. (18 & 19). They

arise from the connective tissue over the mesio-dorsal

muscles and are attached to the fin-rays. Similar

abductor muscles are seen attached to the fin-rays of

- 45 -

. the anal fin also (Fig. 18).

An additional layer of muscle fibres running in

an oblique direction is seen in all these fishes except

P.boro. This may be termed the superficial oblique

muscle. This muscle is confined to the anterior region

of the trunk and extends obliquely from the mid-dorsal

line to the lateral line. The superficial oblique muscle

is devoid of ~~y indication of segmentation and forms a

thin investing layer over the lateral muscles of the

body wall.

The simplest condition of the' superficial

oblique muscle is found in A.bicolor (Fig. 17) where it

occupies the region between the cranial muscles and the

. base of the pectoral fin. The muscle fibres have an .

oblique course and extend from the mid-dorsal line to the

lateral line, immediately above the dorsal limit of the.

opercular chamber.

In S.benga1ensis the superficial oblique muscle

has a more elaborate arrangement (Fig. 28). It forms an

enveloping sheet ,of muscle over the dorsal moiety of about

twenty-five myomeres of the anterior region. A muscle

band from the anterior margin of the fourth myomere ex­

tends forward on either side of the dorsal aspect of the

first three segments and is attached to the dorso-Iateral

region of the skull. These probably function as nec~

muscles. The dorsal. moiety of the lateral muscles of the

segments 4-14 is almost completely covered over by the

superficial oblique muscle. This continues further back-

- 46 -

wards up to the 25th segment but gets gradually reduced

and is restricted to the dorso-median part of the body.

In addition to the superficial oblique muscle

there are also similar muscles over the lateral side of

the ventral moiety. This consists of eleven sets of

muscle bands occurring in segments 4-14 and so in these.

segments there are investing muscles on the dorsal and

lateral regions of the body muscles above and below the

lateral lines. These muec1e bands also have an oblique

course and extend from the region of the lateral line· to

the anterior margin of the preceding segment. Each

muscle band occupies two adjacent segments and the muscle

bands gradually decrease in size from the anterior to tlie

posterior region.

In A.fossorius, the muscle band over the ven­

tral moiety of the lateral muscles extends from the 4th

to the 9th segment (Fig. ~3). It forms a continuous band

of muscle from the 1ateral·line to the middle of the ven- _

tra1 moiety of lateral muscles. This muscle band is fair­

ly broad anteriorly but gradually tapers towards the hind

region.

The superficial oblique muscles occupy a position

over the lateral muscles which in the hinder region is

occupied by the abductor muscles. So it is quite possible

that these are homologous structures. It is noteworthy

that similar muscle bands over the ventral moiety, of the

lateral muscles have not so far been described in other

fishes.

- 47 -

The superficial oblique muscles are seen only in

the three species which burrow with the head and this

leads us to infer that these muscles assist in keeping the

anterior region straight and rigid in the act of burrowing.

The waves of contraction arise in the anterior region of

the body and proceed backwards and the head has to be kept

straight ~o enable it to push through mud. So the super­

ficial oblique muscles may be regarded as a special

structure developed in response to the act of burrowing.

In support of this view, it may be pointed out that this

muscle is completely absent in P.boro, a fish which

burrows with its tailo

Locomotion in a typical flattened fish ~s

effected by the waves of contraction passing along the

sides of the body and by the strokes of the caudal fin.

The movement is initiated by the contraction of the first

few muscles of the anterior region on one side, with the

. result that the anterior region is thrown into a curve.

This curve is passed backwards in a series of waves, by

the alternate contractions and relaxations of the serially

arranged myomeres. Each successive muscle segment gives

an additional momentum to the wave and as soon as the

first wave has started backwards, a second one follows;

but on the oppq~ite side. This alternating sequence of

waves follows in regular suc~ession and the forward thrust

is attained by the pressure exerted by the body on the

water which fills the troughs in the wave series. All

these fishes can swim backwards with equal ease, by revers­

ing the direction of wave motion.

- 48 -

The contraction of the individual myomere is

the result of the contraction of the muscle fibres that

compose the myomere. Since the muscle fibres are rather

short, the resulting contraction of ahy individual myo­

mere is very insignificant. But when oontraction takes

place, simultaneously, a set of successive myomeres under-

go reduction in their total length and this results in

the curvature of a section of the body and when this

curve straightens, it exer s a force on the surrounding

medium, whether it be waterOD'mud and movement of the

fish is effected. The energy required to push the body

is the energy stored up in the curve, which in turn is

produced by the contraction of a set of successive myo­

meres. So this can be taken as a case of bending and

can be treated as a physical phenomenon. Taking the

analogy of a bent bar, the bending moment is given by the

expression M = qAk:2 where M is the bending moment, q thefB.

young's modulus, A the area of cross-section of'the body,

k the radius of gyration and e the radius of curvature

of the bent bar.

The energy in the ben~ bar is given by the

expression E = t~Ak2L, where L is the length of the neu­

tral axis represented by the vertabral column. It is

this energy that is stored up as a consequence of bending

that is released as kinetic energy resulting in motion.

In all these fishes, except in P.boro, the

hind part of the tail gets progressively flattened with

the result that the cross-sec~ion o~ this region becomes

elliptical. Owing to this change in. body-form, the

- 4 9 -

energy released also varies accordingly. The energy

stored up in a bar of elliptical cross-section is given1. Atkt2

by the expression E ::::. "2q e' ,where At, k t and e t

represent the new values in the case of elliptic'cross-

section.

1)

2)

I

Area of a circle

Area of an ellipse

= 'fTr2

:=-'iT ab, where a is the semi-

major and b is the

semi-minor axes of the

ellipse.

k for a circle =~2

1) Ak2 for a circle = 'iT r 2 X r 24

k for an ellipse - a + b- ~

A t k t 2 for an ellipse ='IT ab X a2 1- b24

When the area of cross-section of the circle

and the ellipse are equal, it can be proved that since q,

L, a.nd e remain constant, the energy produced by a bar

of elliptical cross-section is more than that produced '

by a bar of circular cross-section.

Then '1T r 2 =. 'IT ab

r 2 = ab

So, in terms of a and b

I) Ak2 for a circle =11 a2b24

2) At k t 2 for an ellipse

- 50 -

If the t·W'o values are now compared,

ab

and 'TT ab (a24+ b2 )

and a2 + b 2

So a 2b2 is greater than 13.1).

So Ak2 for an ellipse is greater than that for a circle.

Since the other factors are constant, the

energy liberated by a bar of elliptical cross-section

will be greater than that of a bar of circular cross-

section.

Even when the areas of cross-sections vary, if

the rae.ius of the circle is as long as the semi-major

axis of the ellipse, A'k'2 for an ellipse will be

greater than that for a circle.

Let m be the fraction of a that gives rise to b.

b = ma,

Then ab and a2b2 can be expressed in terms of a alone.

Substi tuting ma for b in ab and a2 T b2 ,

1) ab = ama

= a2m

2) a 2 -t- b 2 = a 2 t m2a 2

a2m and a 2 (Ii- m2 )

m and 1 +- m2

Since m is a fraction less than one, 1+m2 is greater

than m. So in ~hi8 case also A'k'2 for an ellipse is

greater than that for a circle.

Therefore, it follows that the energy released

by a region of elliptical croas-section is greater than

that produced by a region of circular croBs-section.

This makes the flattened tail a very efficient organ of

propulsion as compared with a rounded tail.

- 51 -In a recent paper on the lateral musculature

of teleostean fishes, Nursall (1956) has discussed the

mechanism of bending of the body. Bending 1s explained

in terms of the contraction of the individual myomeres.

The contraction of a myomere pulls on the anterior septum

o~he forward flexure and on the posterior septum of the

backward flexure. He explained the forces exerted on", '

the axial skeleton which cause the bending and also the

forces ~hkb restore the normal position of the body.

This explanation refers to the bending of the body of

fishes in general and as such is only an elementary

treatment of the forces at work. It fails to explain the

energy associated with movement and its dependance on

various factors like the area of cross section, shape of

the body and the length of the neutral axis. It also

fails to show the importance of the tail and the caudal

fin as efficient organs of propulsion. In a more recent

paper on the mechanics of locomotion in fishes by

Elienne Ochmichen (1957), the forces required to e~fect

locomotion through water and its relation to the mass of

the fish are treated.

This explanation of the mechanics of locomo­

tion in anguilliform fishes brings to light the importance

of the tail as an organ of propulsion in water and in mud

and also the greater usefulness of a laterally compressed

tail in the liberation of more energy that is required

for burrowing.