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FISHERIES RESEARCH BOARD OF CANADA Translation Series No. 1011

Characteristic features of the structure of molluscs associated with controlled movements

By G. V. Zuev

Original title: Osobennosti stroeniya golovonogikh mollyuskov, svyazannye s upravleniem dvizheniem.

From: Ekologo-morfologicheskie issledovaniya nektonnykh zhivotnykh. Kiev. (Special publication), 1966.

Translated by the Translation Bureau (AT) Foreign Languages Division Department of the Secretary of State of Canada

Fisheries Research Board of Canada Biological Station, St. John's, Nfld. 1968 The devices for neutralization of the force of gravity are part of a whole array of means of movement of aquatic organisms. Yu. G. Aleev

(1963), who studied the devices of fish aimed at the neutralization of the force of gravity, established that all such devices of the fish may be divided into two following groups.

1. The devices of hydrostatic action, i.e. the devices aimed at the reduction of body's specific gravity. The hydrostatic action devices are essentially peculiar to the inhabitants of the pelagic zone and all those fish which spend the major part of their time in the main body of water, out of contact with the substrate.

2. The hydrodynamic action devices, i.e. devices, based on the creation of special supporting forces and facilitating the adoption of the suspended state.

The hydrodynamic devices are further divided into passive devices, functioning only during the movement of the fish and the active ones, functioning irrespective of the movement or immobility of the fish

(Aleev, 1963).

THE HYDROSTATIC ACTION DEVICES

The capacity of the organism to be in the state of static balance depends on the equality of the specific gravities (density) of the organism and the medium. If the equality is disturbed, the organism either sinks under the force of the residual weight, or rises. 2

The buoyancy (A) of the organism is understood as meaning /15 the difference between the specific gravity of the organism and the water:

Q — Q1 e: à where 9- specific gravity of the organism, Q i - specific gravity of the water. If the specific gravity of the organism is equal to the state of specific gravity of the water (Q.q 1), the organism is in the static balance. In this case, the force (G) of gravity applied to the centre of the gravity of the body, and the opposing force Q of the hydrostatic pressure of the water (resistance force), applied to the geometrical centre of the body volume, i.e. in the centre of hydrostatic pressure, are numerically equal (G = Q). In that case the buoyancy of the body is neutral (à = 0 ). If the specific gravity of the organism is greater than that of the water (Q>.9 1 ), then the value of G exceeds the value of Q. In that case the buoyancy of the organism is negative (à < 0). If the specific gravity of the organism is smaller than that

of the water (9 <9 ), then the force Q is greater than G and the organism 1 rises to the surface. In this case the buoyancy of the organism is positive

(à> 0). From the point of view of organism energetics the most

perfect form are those with the specific gravity near that of water. In other words, these are forms with neutral or near neutral buoyancy. Such organisms do not have to make additional movements in order to remain in the main body of water over considerable periods of time. If the buoyancy is neutral, the energy spent on keeping the balance in water is practically nil, - 3 -

The neutral buoyancy of the aquatic may be obtained by different means. Gas and fat lighter than tissue, intake of water and other means serve this purpose in the invertebrate plankton forms

(Zernov, 1949). In the vertebrates (fish, pinnipeds, whales) the specific gravity of the body is reduced either by means of swimming bladder (some fishes), the air in the lungs, or deposits of fat in different tissues and organs (mammals, fishes). But in the majority of the nektonic animals the specific gravity of the body exceeds the specific gravity of water

(Lowndes, 1955; Tomilin, 1957; Aleev, 1963; Andriyashev, 19 )44).

The cephalopod molluscs also possess different devices aimed at the reduction of the specific gravity. The pelagic forme of

Cephalopoda are known already from the Cambrian and Silurian eras; these are the representatives of Nautiloidea and Ammonoidea groups, the shells of which were filled with gas (Zittel, 193)4 ; Shimanskii, 1962, and others).

The , which connects the chambers of the cephalopod's /16 shell, was considered by the majority of the authors (Appelliif, 1893;

Abel, 1916) as the organ which conducts the gas into the chambers and regulates its quantity, i.e. as an original hydrostatic apparatus. The recent investigations on the buoyancy of the Sepia officinalis (Denton,

1961; Denton and Gilpin-Brown, 1960, 1961a, 1961b, 1961c; Denton, Gilpin-

Brown, Howarth, 1961), which has a massive inner shell (sepion) have experimentally proved that the sepion serves as a hydrostatic apparatus.

The shell of the Nautiloidea represents a tube coiled into a flat spiral which may reach about three full coils in the adult specimens.

The body cavity is subdivided into chambers by means of a number of septa.

In the last one •- the living chamber, occupying about half of the outer coil of the spiral, the body of the is situated; the other chambers - 14

are filled with a mixture of gases. Approximately in the centre of each septum is located the septic ostiole with the septic tube. Tà...e outgrowth

of the posterior part of the body, the so called siphuncle, passes through these ostioles. The soft part of the siphuncle is represented by the connecting tissue, the intermediate spaces of which are filled with blood. Furthermore, the arterial blood vessel - the siphuncle artery, passes through the connecting tissue.

The studies on the hydrostatics of the contemporary Nautiloidea (Bidder, 1962) have shown that the shell °healers contain not only gas but liquid as well and that the amount of the liquid decreases with the age of the animal. The gas, collected in the chambers, is nitrogen with its pressure near atmospheric (at atmospheric pressure). The Nautiloidea are capable of changing their buoyaney. The buoyancy is controlled by means of the siphuncle, which regulates the inflow and the discharge of the gas ("fluid" ? Translator) from the chambers. Recently the opinion was expressed (Joysey, 1961) that all Cephalopoda having shells with air filled cavities may osmotically secrete liquid from their body tissues and suck it back. This liquid plays the role of the ballast. The fact that the amount of liquid decreases with the age of Nautiloidea indicates the possibility of inflow and discharge of the liquid from the chambers.

The gas hydrostatic apparatus of Nautiloidea simplifies their sylmming and hovering in water, yet, in turn, excludes the possibility of rapid vertical displacements owing to the great compressibility of the gas. An abrupt change of pressure upsets the conditions of static balance and some time is needed to restore them. The features of the hydrostatics of the Nautiloidea, the general shape of their bodies are such that one -5 may with sufficient precision refer to them as slow-swimming megaloplanktonic/17 animals. The pattern of life of the contemporary representatives of this most ancient order of further convinces us that this judgement is correct. The Belemnoidea were essentially predators and, with a smell exception, belonged to the nektonic forms (Abel, 1916; Krymgolets, 1958; Naidin, 1965). It is worth mentioning that the functional significance of the rostrum was long a matter of controversy among scientists. The role of the inner skeleton of the Belemnoidea was studied in detail by O. Abel (Abel, 1916). As is known (Zittel', 1934; Kondakov, 1940; Krymgol'ts, 1958), the inner skeleton of the Belemnoidea consisted of three major parts: the phragmocone, the proostracum and the rostrum.

The phragmocone consisted of a more or less long core divided into chambers by closely spaced transverse septa. The dorsal edge of the phragmocone was stretched forward in form of a thin wide lamella, or the proostratum, covering the soft tissues of the body. The rear end of the phragmocone was as if embedded in a lime sheath which extended in the form of a runner, pointed at the end, i.e. the rostrum. The controversy about the functional significance of the inner skeleton of the belemnites could be solved only by mathematical calculations. At the request of O. Abel, such calculations were made by F. Gafferl'. It emerged that the chambers of the phragmocone, if filled with gas or air, created a positive buoyancy, pushing the animal to the surface of the water. So it was proved that the inner skeleton of the belemnites played the role of a hydrostatic apparatus. Apparently, the rigid rostrum was used by the belemnites as a support, a streamlined formation and a counterweight in their three dimensional orientation in

the body of water (Zuev, Makhlin, 1965; Naidin, 1965). To the best of the author's knowledge, no one studied the hydrostatics of the Octopoda. Data on specific gravity and buoyancy of octopi are absent from the literature. We have conducted measurements of the specific weight of the octopi Octopus vulgaris, Eledone moschata, Eledone cirrosa, of the Mediterranean Sea and the Octopus sp. from the Red Sea (Zuev, 1963) using the volume-weight method with the formula: ^ P (2) ' where P weight of the octopus in the air (gr.), V - volume of the octopus (cm ). The specifie gravity was determined using live specimens only. As a result of these investigations, it emerged that the specific gravity of all the above-mentioned types of octopi was 1.06, i.e. the buoyancy of these octopi wsm negative. The specific gravity of the sea /18 water was estimated to be 1,03. The change of specific gravity in the ontogenesis, beginning with the larval stages, was traced on E. moschata (Fig. 1). The larvae of E. moschata, living in the pelagic zone, have a negative buoyancy (A = -0,03) and are swimming with the help of ejecting a jet of water from the infundibulum. After some time the larvae sink to the bottom, where their development continues. According to our findings, the specific gravity of E. moschata remains unchanged throughout ontogenesis.

Fig. 1. Changes of value of specific gravity in the ontogenesis of Eledone moschata (Lamarck): - specific gravity of the animal; - ql specific gravity of the water. -

The questions of the hydrostatics of contemporary Sepioidea were studied in considerable detail by E. Denton and J. Gilpin-Brown (1959,

1960, 1961a, 1961b , 1961c) in their investigations of the buoyancy of Sepia elegans and Sepia officinalis. The squids have an inner shell which consists of approximately one hundred lite lamellae placed one above the other and supported by vertical stanchions. The spaces between the lamellae create independent chambers, divided by vertical membranes, situated longitudinally in the direction of the body. The volume of the sepion amounts to some 9% of the animals volume, whereas the volume of the swimming bladder in the fish does not exceed 5% (Denton,

1961). The specific gravity of the shell is approx. 0,6. The chambers of the sepion are filled with liquid and gas which may be shifted inside the chamber. The liquid is concentrated at the siphuncular ends of the chamber, all the remaining volume of the chambers is filled with gas at a pressure of 0.8 atm (at normal outside atmospheric pressure). When the rises from the bottom, owing to the decreasing outside pressure, the volume of the gas in the sepion increases, the amount of liquid decreases, a part of it flows out of chambers through the siphuncular wall. When the outside pressure increases (during submerging), the opposite is Observed. Anatomically, the siphuncular wall is well adapted for the penetration of the liquid, since the siphuncular membrane is fitted with a special drainage system (Denton and Gilpin-Brown, 1961b). The siphuncular wall is linked with the blood system, where the liquid from sepion is sent. A very similar system of blood vessels was found in the epithelium covering the siphuncule of Nautilus (Denton, 1961). The composition of the gas in the sepion was first studied by P. Bert (1867). It is nitrogen with a very

low aamixture of . oxygen. /19 9

The question of existence of physiological devices for actively regulating the quantity of liquid in the squid's sepion still remains debatable, but E. Denton and J. Gilpin-Brown offered the hypothesis of the possible existence of an osmotic mechanism which would allow this process to be accomplished,

We investigated the specific gravity of the larvae of

S. officinalis. The larvae directly after leaving the egg mass are swim- ming by short jerky movements, ejecting a jet of water from the infundibulum.

After a few days the larvae sink to the bottom. The specific gravity of the larvae O. officinalis is greater (1.06) than the specific gravity of the water. Apparently, the morphologically negative buoyancy of the larvae is to be explained by the insufficient development of the shell (sepion) which has not yet developed into a hydrostatic apparatus (Fig. 2).

Fig. 2, The change of buoyancy (A) in ontogenesis of octopi, squids and cuttlefish.

An article by E. Denton, T. Shaw and J. Gilpin-Brown (1958) is devoted to the hydrostatics of the Teuthoidea. The three species of planktonic squid Verriliteuthis hyperborea (Steen.), Galiteuthis armata

Joubin, pfefferi Massy which they have studied, have large body cavities, up to 2/3 of the volume of the animal, filled with a light, transparent fluid. The samples of this fluid were nearly identical in composition and qualities for all three species. The density of the cavity fluid was discovered to be intermediate between the densities of distilled and sea water, i.e. 1.010 - 1.012. The law density of the cavity fluid - 10 -

accounts for the reduction of the specific gravity of the squid. Analysis

of the chemical composition of this fluid indicates a high content of

ammonium - about 480 mM (sea water contains only traces of ammonium) and

a relatively insignificant content àf sodium - about 90 mM (sea waters contains

about 490 mM of sodium). As the authors explain, the molecular sodium is

supplied to the body cavity out of surrounding water. Subjected to an

acid reaction of the medium (the cavity fluid has a pH of 5.2) the molecules

of NH4C1 dissociate into anions N4' and cations C1- . The same takes /20

place with the molecules of NaCl. Because of a good penetrability of

live tissue for the molecular ammonium and impenetrability for its ions,

the ions of ammonium are gradually stored in the body cavity replacing

the ions of sodium. Such replacement of heavy ions of sodium by lighter

ions of ammonium leads to a lighter specific gravity of the cavity fluid.

An artificially prepared solution of NH4C1 and NaCl, in the same proportion

in which they are found in the body cavity of the squid, yields a value verY

close to the specific weight of the cavity fluid.

A similar mechanism of buoyancy is found in Noctiluca (Krogh,

1939), in which a high content of ammonium ions in the cavity fluid ensures

a sufficiently high buoyancy. It is known (Clarke, 1962) that the

planktonic squid Cranchia scabra has neutral buoyancy. The reduction of

the specific gravity of the C. scabra is also obtained by filling the body

cavity with a light liquid. The existing data allow to assume that many,

and possibly all planktonic species of squid have either a neutral, or

near neutral buoyancy.

Only very scant data are available on the hydrostatics of

nektonic species of Teuthoidea. It is only known (Denton and Gilpin-Brown,

1961b), that the specific gravity of the squid Longo forbesi is somewhat -11- greater than the specific gravity of water (by 4%). We have investigated the specific gravity and buoyancy of four species of squid - Symple.ctoteuthis oualaniensis, Loligo vulgaris, Ancistroteuthis lichtensteini, and Illex coindeti - and changes of these indices in ontogenesis. The investigations were conducted on board "Akademik Kovalevskii", an experimental ship, in the Aegean, Mediterranean and Red Seas, and in the Gulf of Aden, in 1961-1963.

The squid attracted at night by electric light were caught and immediately delivered to the laboratory.

The specific gravity of the squid was determined by the volume - weight method, according to formula (2). Before weighing the squid were placed on filter paper to dry on the surface, the rest of the water was carefully removed from the cavity. The weight of the squid of up to 0.5 gr.was determined on torsion scales with an accuracy of 0.001 gr.; the weight of squid exceeding 0.5 gr. was determined on technical scales with an accuracy of 0.01 gr. (Zuev, 1963). The volume of the body was measured in graduated cylinders with a capacity of 5 to 3 10,000 cm3 and an accuracy of 0.05 - 1.0 cm , respectively. During the immersion of squid into the water careful checks were made to ensure that no air would remain in the mantle cavity. To obtain a greater reliability of the data, the specific gravity of the squid and, accordingly, the buoyancy were calculated to the second decimal place (0.01). It should be added that the specific gravity of the larvae was determined /21 by means of a specially prepared set of solutions of NaC1, with different specific gravities.

To determine the specific weight and the buoyancy of the squid

S. oulaniensis, 145 specimens with absolute lengths of 0.3 to 54.0 cm. were used (Fig. 3), 95 specimens of L. vulgaris with absolute lengths of - 12 -

0.3 - 27.3 cm, 25 specimens of A. lichtensteini with absolute lengths of

0.7 - 18.0 cm, 41 specimens of I. coindeti with absolute lengths of

2.5 - 26.1 cm.

Fig. 3. The variation of specific gravity values (g) in ontogenesis of Symplectoteuthis oualaniensis (Lesson): the average value of Q is based on n = 10; individual values of g ; - specific gravity of water.

The difference in values of specific gravity of different specimens

(1.03 - 1.08) may be explained by individual casual deviations. The average value of q for all four species is 1.05. The bugyancy of practically all specimens under study was negative and fluctuated from 0.00 to -0.05.

The average value of A = -0.02. The individual variations of the specific weight of larvae are within considerably narrower range (1.05 - 1.06), which may be explained by the higher precision of the method of solutions.

The average value of Q for larvae of all four species equals 1.06, i.e. the larvae of squid are "heavier" than the adult specimens. The data on the specific weight and the buoyancy of the squid S. oulaniensis were processed with variational - statistical method (Pravdin, 1939). The differences between the specific gravities of the larvae and the adult forma are quite reliable, M is many times greater than three. diff In the ontogenesis of all four species which we have studied, the changes of g and à give an analagous picture: the larvae coming from the spawn have a greater specific gravity, later their specific gravity decreases, yet, throughout the life-span of the squid, it remains greater -13 - than the specific gravity of water. The similarities of hydro- static features in the ontogenesis of all four species can, probably, be explained by the similarities in their /22 ecology: they all are fast-swimming nektonic predators. Our data on the specific gravities of squid S. oulaniensis L. vulgaris, A. lichtensteini, I. coindeti are close to the data of E. Denton and J. Gilpin-Brown on the squid I. forbesi.

The hylrodynamic action devices

It is impossible to imagine the mechanism of creation of supporting forces which allow an organism to remain in water in a suspended state, without a knowledge of peculiarities of behaviour, in particular, the characteristic features of movement of the animal. The mobility of cepha- lopod molluscs does not remain constant in time. For instance, in squid the periods of active hunting or escape from more powerful enemies alternate with periods of relative rest, immobility, when the squid swims slowly or soars immobile in one spot (Zuev, 1965a). The change of activity is accompanied in each case by a change of the physical conditions of movement (including the supporting force) which entails a disturbance of balance and the descent of the squid to the bottom. Thus, the mechanism of the creation of supporting forces in cephalopod molluscs changes constantly, depending on the characteristics of the movement. Three main conditions of movement are characteristic of the squid: immobile hovering, slow swimming and fast swim- ming. Let us consider the mechanism of creation and action of the supporting (lift) forces in different conditions. - 14 -

Immobile hovering

The term "immobile hovering" indicates the complete absence of

translational movement, the squid is as if suspended in the thick of the water (Zuev, 1965a). In that moment the mantle end of the body is somewhat

raised above the head (Fig. 4), i.e. the body of the squid is not quite horizontal. In this state the twin lateral fins make synchronous undulating movements, the frequency of which attains four to five strokes per second.

The wave moves along the fins from the mantle end to the head. The propulsive force of the fins created by undulation is directed backwards

and upwards, practically along the longitudinal axis of the body. The

expansion of this force into its components, according to the rule of parallelogram, reveals the action of the horizontal component F", which

displaces the squid backwards, and of the vertical component F', striving to lift the mantle end of the body.

The free end of the infundibulum at time of immobile hovering

is almost perpendicular to the longitudinal axis of the body, as indicated /23 on Fig. 4, so that the action of the hydrojet traction force shall raise the cranial end of the squid's body. In this case the vertical component

P' is the supporting (lift) force, whereas the horizontal component P" neutralizes the action of F" and does not allow the squid to move in the horizontal direction. Under the combined action of the indicated forces the squid must float up (vertical hovering) if the combined lift is numerically greater than the force of the residual weight. Fig. 4. The plan of applied forces, acting on the squid's body in the vertical plane during immobile hovering: F - propulsive force created by the undulation of the fins; P propulsive force created by the hydrojet propelling agent; Me - moment of force F; M - moment of force P; Y - lift; G - force of weight; cP- centre of gravity; F - lift of the fins; P - lift of the hydrojet propelling agent.

The squid will be in a state of equilibrium, if the resulting force as well as the moment of the resulting couple equals zero, i.e. if:

F P 4. G = 0 (3) M +M +M =0 f (4) We have determined that after amputation of the fins,

S. oualaniensis, I. coindeti lose their ability, of immobile hovering and begin to move translationally. The direction of the translational movement of the squid with amputated fins is not linear; it swime in a sinusoid, the mantle end forward. At the moment of ejection of the jet from the infundibulum (the moment of thrust) the squid swims along the ascending part of the curve; when the mantle is filling with water, along the descending part. Such peculiar swimming may be evidence that one of the lifting forces, namely the propulsive force of the fins, disappears with the amputation, disrupting the general balance of the body. Thus, - i6 - the twin lateral fins of the squid situated at the very end of the mantle are used to maintain the conditions of immobile hovering. The remoteness of the fins from the centre of gravity ensures the creation of greater moment with a minimal muscle effort.

Thus, in the absence of translational movement, the conditions of immobile hovering of squid in the thick of the water are ensured by the undulation of twin lateral fins and the action of the hydrojet propelling agent.

Slow swimming

As indicated earlier, in slow swimming, the squid can move in any direction heading with an equal ease with its mantle or cranial end.

Characteristic is the fact that during slow swimming the front part of the body (from the point of view of flow conditions) is situated higher; in other words, at this rate of swimming the longitudinal axis of the body forms an angle with the direction of the translational movement (the angle of attack) and the body of the squid becomes a carrier surface. The angle of attack is variable and is controlled, depending on the speed of swimming. The value of the angle of attack in squid S. oulaniensis and

I. coindeti while swimming in the aquarium reaches 20-300 . The traction is created by the simultaneous action of the fins and the infundibulum.

Fig. 5, The plan of applied forces acting in vertical plane on the squid's body during slow swimming. The symbols are the same as in Fig. 4. - 17-

Since the direction of the force of traction forms an angle with the horizon level (Fig. 5) and with the direction of the squid's translational movement, this force may be expanded into twO components: /2 5 in the direction of the movement (F" - for fins and P" - for hydrojet propelling agent) and perpendicular to it (F' - for fins and P' - for infundibulum). The components and P' shall be the lift which does not permit the squid in the process of slow swimming to sink to the bottom.

Besides, the body itself creates an additional lift, if the angle of attack is positiire.

As is known from the aerodynamics (Martynov, 1958), the actual value of the lift depends, in particular, on the relative extension of the wing. To determine the value of the relative extension of the body of cephalopod molluscs we have used a formula which is utilized to determine the relative extension X of the wing span, since the body of the molluscs may be compared, by analogy, to a wing. 12 X = S— (5) where 1 - wing span (in our case, the greatest mddth of the body),

S - surface of the wing (in our case, the surface of longitudinal horizontal projection of the body).

Even at very small relative extensions of the wing, at X,-., 1/30, lift already occurs (Martynov, 1958). The greater the X, the greater the lift force. Knowing the relative extension one can definitely judge the extent of the lift created by the body. Hence, it was interesting to investigate the relative extension of the body of the

Cephalopoda belonging to different ecological groups. The following types were chosen for the investigation: E. moschata, S. officinalis, S. elegans,

0. sagittatus, 0. sloanei pacificus, 0. banksii, I. coindeti, G. fabricii,

A. media, A. lichtensteini, 0. pteropus, L. vulgaris, L. forbesi, and L. edulis. -18-

The value of the relative extension of the body of cephalopod molluscs fluctuates from 1/6 to 2/5 (0.16 - 0.40), i.e. well in excess of the minimal value of X at which the carrying force occurs. It is further known

(Plartynov, 1958) that the carrying force increases with the increase in the relative thickness of the cross-section and reaches its maximum at the thickness of the cross-section being equal to 9-13% of the chord. Studying the value of maximum relative thickness (height) of body of Teuthoidea, one may notice that H amounts to 10.8-17.6% of the absolute length; thus numerically the height of the squid's body is quite close to the indicated optimal values of cross-section thickness. It is quite possible that the functional significance of this phenomenon is to some extent related to the function of the lift, which the squidls body performs.

A significant increase among the representatives of demersal /26 pelagic Sepiidae (0.39-0.40) is achieved at the expense of a strong dorso-ventral compression of the body which ought to be regarded, primarily, as a means of camouflage on the bottom. It may be assumed that the capacity of the body of cuttlefishes to function as a carrier surface is minimal, since they can regulate their buoyancy. The octopi, having a negative buoyancy, probably also generate some lift in swimming. Some octopi are capable of increasing the lift, using to this end the thin skin web between their arms. At the moment of thrust the second and the third pairs of arms are spread widely apart, while all the other arms are extended along the longitudinal axis of the body. The web between the anus is then spread creating an additional carrier surface (Popov, 1963). It is unknown how common this phenomenon is in octopi.

The value of the lift depends on a number of other factors as well, among which is the speed of the movement. The lift is directly dependent on the speed. In the ontogenesis of the Cephalopoda the value - 1 9 - • •

of the lift should increase according to the increase in the size of the animals and the speeds they are capable of attaining. In slow swimming, the shape of the squid's body changes insigni- ficantly, since squids take up small amounts of water in their mantle cavity. Since the shape of the squid's body, owing to its thin and flexible skeleton, may change easily, in some cases their dorsal profile is more convex than the ventral, and vice-versa. In principle, in conditions of slow swimming these differences in the shape of the body are of no importance, as far as the mechanics are concerned. Thus, by means of undulation of twin lateral fins, the action of

hydrojet propelling agent as well as the whole body functioning as a peculiar carrier surface, the nektonic squid create a system of lifting forces which,

in the process of slow swimming, neutralize the force of the residual weight.

FAST SWIMMING

In fast swimming, the swift rushes of the squid are accompanied by strong periodic changes in the shape of the body, i.e. the non-stationary

character of the movement is accentuated. During one cycle of the hydrojet propelling agent's operation, the body thickness varies greatly, changing almost twofold. In fast swimming the longitudinal axis of the squid's body coincides with the direction of the translational movement, the body keeping

a constant zero angle of attack which is of utmost importance for the /27 creation of lift in the vertical plane. The core of the problem lies in the fact that at a zero angle of attack, a body creates a carrying

force only if the upper contour of its section is more convex than its lower contour. In the case of squid, this should be understood to mean that in the moments of fast swimming the dorsal contour of their section - 20 - should be more convex than the ventral contour. As indicated earlier, the form of the vertical longitudinal projection of the squid's mantle is subject to great variations owing to their very thin and flexible skeleton. In pelagic species of squid the relative weight of the skeleton does not exceed 0.2-0.5% of the total body weight (Zuev, 1965b). The aquarium observations allowed us to determine that during fast swimming in some cases the dorsal contour of the section of the squid, s body remains more convex, and in other cases, the ventral contour is more convex. It was found that the degree of vertical asymmetry of the cephalopod molluscs (fl) could be expressed by the following ratio:

.10.1•1 fi fl = 100 (6) La where f1 - distance from the median line of the contour of the body to the longitudinal axis, L a - absolute length of the body. From the study of forces acting in the vertical plane in each of the cases, it appears that in the first case (the dorsal contour of the section more convex than the ventral) the expansion of the propulsive force of the hydrojet propelling agent into its components shows that the verti- cal component P , is a lift. Besides, owing to the creation of the gradient of the pressures on dorsal and ventral sides of the body, the body itself becomes a carrier surface, i.e. creates a lift (Prandt1 1 . 1949), Fig. 8. The experimental creation of lift by the body of the squid in conditions of fast swimming was proved by a study of the movement of a model of the squid Symplectoteuthis oualaniesis (Lesson) (Zuev, 1965v, 1965g) in water. The model was shaped after a specimen caught in the Gulf of Aden, the absolute length of which was 26.0 cm. The model, made of wood, was a true copy of the original magnified 4.4 times. The length of the model was 114.4 cm. The vertical curvature of the model was the sue as - 21-22 - that of the original, i.e. 2.70. The model was given slightly negative buoyancy by means of weighting. The creation of the lift by the model was studied with the help of a specially de- signed installation offered by Yu. G. Aleev, which permits to photographing all stages of the movement. A stationary cable is tightly secured underwater, parallel to the side of the ship, at a depth of 5-10 cm. The model glides along this /28. 129 cable. Two metal rods are set into one side of the model on its longitudinal axis. The free ends of the rods are connected with an immobile metallic pipe which is parallel to the longi- tudinal axis of the model and is somewhat longer than the model. The pipe is connected to the stationary cable by three rings. For towing the model, a tow-line is used which is connected to the first ring on the pipe. The towing is accomplished by means of a winch. The speed of towing is 1.2-1.3 m/sec. . A rope graduated in meters and stretched above the water along the stationary cable helps to determine the position of the model at each stage. Mark 1 coincides with the front end of the model at its starting point (Fig. 6, a). On the photo- graph the model in starting position is seen in profile. During the towing some lift force is generated in the direction of the more convex back and begins to shift gradually the model in that direction. The shift of the model in the direction of its back increases with the distance covered (Fig. 6, b). The length of the installation reached 14 m (marks 1-15 of the graduated rope). The turning angle of the model around the axis of the system attained 70-80 0 (Fig. 6, c). But fast swimming with preservation of dorso- ventral asymmetry is only a particular case. As a rule, the squid, while swimming fast, preserve the ventrodorsal asymmetry (the ventral contour of the mantle is more convex than the dorsal). Apparently, this can be anato- mically explained by the dorsal disposition of the skeleton lamina (). In this case, the true lift is created only by the action of the hydrojet Fig. 6. Sequential moments of the movement of squid model. Point 1 of the . graduated rope coincides with the front end of the model at its starting point. propelling agent (Fig. 7), whereas Y from a supporting force turns into an opposite force that pushes downwards.

It is necessary to keep in mind that in any case, when vertical forces are created, an important role is playe d by the flexible arms; the - 2)4- modification of their position in respect of the mantle affects the absolute value of these forces. The horizontal thickening of the third pair of squidls arms, particularly pronounced among the more mobile scies, as well as the formation of arm-fins should be considered not only as a device for greater vertical dynamic stability (see belou), but also as an increase in the total surface of the horizontal longitudinal projection of the body in order to croate, in the final count, a greater lift. For the preservation of the linear direction of movement, it is required that the total downward force be compensated by an equivalent supporting force, acting in the opposite direction. There is no doubt that if the resultant of all these forces and of their moments is equal to zero ethis should be ascribed to the action of the arms.

With the ceasing of hydrojet propelling agents action, /30 the squid ought to sink under the effect of residual weight, but the rapid rate of pulsations (up to five times per second) and a strong inertia do not permit observation of this phenomenon which is otherwise clearly visible in slow swimming.

Fig. 7. The plan of forces acting during fast swimming on the body of the squid in a vertical plane. The same symbols as in Fig. 4 are used. - 25 -

PHYLOGENY OF DEVICES AIMED AT THE NEUTRALIZATION OF THE FORCE OF WEIGHT

The study of devices aimed at the neutralization of the force of gravity permits us not only to correctly understand the cephalopod molluscs' ability to move but also allows us to highlight, to some extent, the paths of their evolution. All Nautiloidea had an outer shell divided into chambers and the siphuncle. The chambers of the contemporary forms are filled with liquid and gas which can be displaced through the siphuncle. The presence of a gas hydrostatic apparatus in the Nautiloidea simplifies their swimming and hovering, yet, owing to great compressibility of the gas, excludes the possibility of their rapid vertical migrations. An abrupt change of pressure upsets the conditions of static balance and sonie time is needed to restore them. The contemporary nautiloids are slow swimming megaloplanktonic animals.

A characteristic feature of ancient cephalopods with an inner shell was a relatively homogenous development of all three parts of the shell (Krymgortz, 1958). They had a well developed straight conical /31 phragmocone, the rear part of which was surrounded by the rostrum. The proostracum was of varying length, thin and fragile. It is difficult to speak very definitely about the habits of belemnites, since all their forms are extinct. However, making certain allowances, they can be visualized by analogy with the contemporary squid which morphologically are quite similar to the belemnites. An elongated spindle-shaped mantle, existence of a sturdy inner skeleton and twin fins lead to the conclusion that the Belemnoidea were fair swimmers (Krymgol i tz, 1958; Naidin, 1965).

The hydrostatic apparatus of belemnites consisted of an inner skeleton, or, to be precise, the phragmocone, divided into chambers by transverse septa

somewhat curved backwards. The siphuncle passed through all chambers,

near ventral edge, ensuring the emission of gas in the chambers. Apparently, - 26 -

the belemnites could regulate the quantity of gas in the chambers of the

phragmocone and could regulate their buoyancy accordingly. In all likelihood,

by analogy with contemporary squids, the changes in specific gravity of the

belemnites were obtained by generation and emission of liquid, rather than gas.

On the basis of the existence of a well-developed hydrostatic

apparatus in the belemnites, it may be assumed that, unlike the contemporary

nektonic squid, they were somewhat restricted in the ability to make rapid

and big vertical displacaments. Most interesting in this respect are the

indications of D. P. Naidin (1965) that the belemnites populated shelf seas

with depths of 100-200 m. The contemporary Sepiidae, as a rule, also do not

venture beyond a depth of 200 m. •

The devices for neutralization of the force of gravity in con-

temporary Cephalopoda were developed in various ways, depending on the characteristic features of their ecology. Only demersal pelagic Sepiidae

and planctonic Spirulidae have preserved the gas hydrostatic apparatus in

a substantianymodified form. In planctonic squid it is replaced by a

fully liquid-oporated hydrostatic apparatus (Denton and Gilpin-Brown, 1959;

Clarke, 1962). As a special morphological structure the hydrostatic

apparatus simply does not exist in nektonic forms of the Teuthoidea and the demersal Octopoda..

The rather massive inner shell is preserved in the demersal

pelagic Sepiidae and pelagic Spirulidae. The main part of the squidls

shell consists of a convex-oval or rhomboido-oval proostracum lamella

situated under the integument on tho dorsal side. The dorsal part of the phragmocono is transformed into a thick porous multilayer lamella. In

squids, the volume of the shell reaches 9% of the total weight of the body.

In Spirula, the initial part of the phragmocone is coiled into a /32 - 27 -

spiral. The chambers of the shell are filled with liquid and gas which ensures

the neutral buoyancy of the animals. The liquid and the gas in the chambers

are redistributed according to the changes in the external pressure: the

liquid is forced out, or, inversely, it penetrates into the chambers through

a siphuncular membrane directly linked with the blood system. Although this has not been actually determined yet, it is assumed that the Sepiidae are

capable of actively controlling their buoyancy (Denton a. Gilpin-Brown, 1961c;

Joysey, 1961).

In contemporary planktonic Teuthoidea the liquid operated hydro-

static apparatus is in use (Denton, Shaw, Gilpin-Brown, 1958). The special

cavity of squid is filled with a light liquid with a specific gravity of

1,010 which creates a neutral buoyancy. Thanks to the existence of this

liquid, the squid are able to hover easily in water. The replacement of the

gas hydrostatic apparatus by the liquid one is a more advanced phenomenon,

since the squid have the possibility of rapid vertical migrations: the

easily compressible gas is replaced by a practically incompressible liquid,

which ensures a constant preservation of the static balance.

In the contemporary nektonic Teuthoidea the skeleton is repre-

sented only by a thin and narrow conchiolin lamella, or proostractum, which

is called a gladius. The volume of the skeleton is only 0.5% of the total

volume of the body; its weight, 0.2-0.5% of the weight of the body. The

skeleton of the squid fulfils the function of a support. The buoyancy of the

nektonic squid does not reach a neutral value, being, however, close to it

in the ontogenesis (A = -0.02). The tendency towards decrease in specific

gravity is apparently explained by a strong reduction in rigid and heavy

skeletal structures, as well as by biochemical peculiarities. Besides, the

hydrostatic action devices in nektonic squid are supplemented by hydro-

dynamic devices: in a number of cases, in slow or fast motion, the body

fulfils the role of a carrier surface generating lift. - 28 -

In demersal octopi, closely associated with the subtrate, the specific gravity of the body largely exceeds that of water = 1.06).

Nevertheless, the octopi do have some hydrostatic action devices. Among these is the practically complete reduction of the skeleton, even more pronounced than in the nektonic squid (Zuev, 1965b), which simultaneously contributes to better camouflaging at the bottom.

The tendency towards decreasing the specific weight of the representatives of all ecological groups is quite explicable from the point of view of energetics. It is unlikely that the more substantial decrease in the specific gravity of the octopi, further approximating their buoyancy to the neutral level, is caused by ecological necessities, since otherwise, when the buoyancy is neutral, the octopus may be easily separated from the /33 substrate. The existence of a negative buoyancy (A = -0.03) allows the octopi to remain on the ground without using any special morphological

devices for attaching themselves.

Devices ensuring the stabilization of and

change in the direction of movement

The nektonic way of life and predatory character of feeding of

the majority of cephalopods require, besides the devices for rapid trans- lational movement, other devices for high manoeuvrability, instantaneous

change in the direction of movement, such as turning, breaking, etc.

These qualities are necessary both in the moments of hunting and when

escaping from the enemies. The perfection of the squid as swimmers con-

sists, in particular, in the fact that they easily change the direction of

their movement, rise abruptly and dive absolutely vertically, soar immobile

in one spot, suddenly pounce sideways, stop instantly and swim backwards.

All those who have had the occasion to observe live squid at sea note their - 2 9 - amazing ability to disappear with lightning speed at signs of approaching danger (Lane, 1957, Jaeckel, 1958); the escaping squid may jump out of water.

Apparently the jumps above tfie surface of the water are common to many species.

We have observed the jumps of the squid S. oualaniensis above the water surface in the Gulf of Aden.

MORPHOLOGICAL CHARACTERISTIC FEATURES ENSURING STABILIZATION

-OF AND CHANGE IN THE DIRECTION OF MOVEMENT. DYNAMIC S TAB IL I TY AND MANOEUVRABTT I TY

The morphological characteristic features of cephalopod molluscs, functionally connected with stabilization of and change in the direction of their movement (breaking) are ensured by the constitution and the position of the propelling agent, the shape of the body, the location and the morphology of twin lateral fins and arms with tentacles. The combined utilization of all these devices guarantees a high degree of the manoeuvrability of the cephalopods, and primarily the pelagic squids.

Physically, the initial direction of the movement is changed by means of rotating moments which act around the centre of the dynamic pressure.

As was illustrated by Yu. G. Aleev (1963), when a turning fish is /34 swimming during a time interval at an angle to the longitudinal axis of the body, a force couple is at action ) i.e. the inertia and the water resistance.

The force of the inertia applied in the centre of gravity, owing to the existence of an angle between the direction of the momement of the fish and its longitudinal axis, has the component R which is perpendicular to that axis. The R is a centrifugal force. The force of the water resistance is applied to the centre of the dynamic pressure in a direction opposite to that of the movement. In the case of the fish moving at an angle to its longitudinal axis the resistance force has a component F perpendicular to -30-

that axis. The antiparallel forces R and F form a force couple with a rotat-

ing moment which may have a stabilizing or turning action (Fig. 8). The torque

will be stabilizing (negative) if the centre of the dynamic pressure is

located behind the centre of the gravity (c). In that case the action of the

force couple hinders the turn deviating the front end of the body in the

Fig. 8. The action of a stabilizing force couple (according to Yu. G. Aleev, 1963): A - force of inertia; B - force of resistance; ID, R, F, Q - their com- ponents; c - centre of gravity; o - centre of dynamic pressure; 10 - lever arm of torque of force couple FR; TT1 - trajectory; 001 - longitudinal axis of the body.

opposite direction to the turn. If the centre of the dynanic pressure (o) is

in front of the centre of gravity (c), the moment will have turning action,

i.e. will be positive, facilitating the turn by deviating the front end of the

body in the direction of the turn. Actually, the torque value depends on two

principal factors: 1) the value of the surface of the longitudinal projection

(vertical or horizontal), on which to a substantial degree depends the value

of force F, and 2) the value of the projection of the distance between the

centre of gravity (c) and the centre of the projection (o) on the longi-

tudinal axis of the body, which constitutes the lever arm of the moment.

Morphologically, the turn is accomplished mainly by the

change in direction of the jet ejected from the infundibulum, i.e. thanks

to the turning of the free end of the infundibulum. In linear translational

movement, in order to lessen the drag, the infundibulum of squid is tightly

pressed to the ventral surface of the head, where there is a special cavity

for the infundibulum, the so called foevola (Berry, 1912, 191)4). The -31- mobility of the free end of the infundibulum allows cephalopods to 135

eject the jet in any direction and at any angle to the longitudinal axis

of the body thus easily modifying the direction of movement. For a 180 0

turn, it is sufficient for the squid to swing the free end of the infundibulum

in the direction of the movement. For turning downwards the direction of the

jet forms a greater angle with the body axis; in turning upwards the angle is

mnaller. In left and right turns the free end of the infundibulum is turned

right and left.

The very build of the squidls body presupposes an equal ease of

turns in both vertical and horizontal planes: the cross-section of the

mantle of the squid is near circular so that, potentially, the squid has an

equal ability to turn in any direction. The twin laterial fins, also function-

ing as rudders, serve for changing direction. The effectiveness of the fins

as rudders depends, above all, on their distance from the centre of dynamic pressure. The further the distance of the fin from the centre, the greater

its torque. S. Jaeckel notes (1958) that S. officinalis turns with the help

of a fin wave. Directly at the moment of turning in the horizontal plane,

one of the fins ceases to undulate, whereas the undulation of the other fin

increases; moreover, in some cases the waves travel along the fins in opposite

directions speeding the turn.

The rudder function of the fins is particularly pronounced in

squid. The nektonic species have their fins at the very end of the mantle,

i.e. as far as possible from the centre of the dynamic pressure. The

terminn• fins are the most perfect rudders. The turning is further faci-

litated by the flexibility of the m.antle end. Our observations àf the

movements of the squid S. oualaniensis, A. lichtensteini, I. coindeti, and

L. vulgaris have determined that the mantle end of all squid bends during

turns; the bending angle being quite substantial. With the exception of

L. vulgaris, in all enumerated species, that angle reached 20-30 ° (vertical - 32 - bend). In the horizontal plane the angle of the mantle bending is somewhat

smaller, i.e. 15-20 0 (Fig. 9). The vertical bend angle in L. vulgaris amounts

to 10-15. horizontal one to 10e. The discovered differences are cor-

related mith the structure of the gladius of these species: in contrast to

the other species, the gladius in L. vulgaris has the shape of a mdde

feather. The arms and the fins also played an important role during the

turns. As a result of their combined simultaneous use, a force couple /36

is created which generates the necessary rotating moment (torque). Thus,

pelagic squid possess a perfect system of rudders (vertical and horizontal),

consisting of front rudders or arms and rear rudders, or fins. The front

and rear rudders of the squid are topographically located at the opposite

ends of the body, i.e. as far from the centre of the dynamic pressure as

possible, which amplifies their action.

Fig. 9. The scheme illustrating the turning of the squid in vertical plane. F1 and F2 - turning forces M1 and M2 - turning moment.

But the arme of the molluscs are much more than rudders. The

capture and holding of prey remains their main function, therefore a study

of the structure of arms of different ecological species reveals that the

mere relative length of the arms is not sufficient evidence of the swimming

capacities of the animal. - 33 -

The stabilization of the linear translational movement in the

Cephalopoda is ensured by the presence of arms the third pair of which, in

squid, carries the so-called arm fins. Already in squids the ventral pair of

arms is somewhat thicker dorso-ventrally, so that when these ar as are extended

along the longitudinal axis of the body, they form laterally protruding fins.

The arm fins, situated well behind the centre of gravity, fulfil during swim- ming the functions of stabilizers (Fig. 10). All the arms of the demersal

octopi are built alike and have no additional external structures. /37

Fig. 10. The degree of development of arm fins in squid. a - Loligo forbesi Steenstrup; b Symplectoteuthis oualaniensis (Lesson); v Ancistroteuthis lichtensteini (DiOrbigny).

The investigations of dynamic stability and manoeuvrability of fishes, conducted by Yu. G. Aleev (1956, 1957a, 1957b, 1958a, 1959, 1963),

allowed us to estimate quantitiatively the development of morphological

characteristic features, functionally related to the stabilization and change of direction. In our work, we have used the methods of Yu. G. Aleev.

The value of the dynamic stability characterizes the position of a moving body in the flow and its ability to change the direction of its movement. With the increase in the dynamic stability the manoeuvrability and the dynamic stability are related by an inverse ratio. The methods of - 3)4 -

Yu. G. Aleev are fully val id also for the Cephalopoda, since both the fish and the Cephalopoda accomplish the turns in the some way. Directly at the moment of turning, the body slightly twists. This twisting is accomplished by the flexible anus and the bending mantle end. The jet of water from the infundi- bulum also changes its direction at the moment of turning. The turning is not instantaneous, for a fraction of time the longitudinal axis of the animal does not coincide with the direction of the movement, i.e. a cross drift occurs.

In a cross drift of the body, the rotating moments may either amplify the rotation or hamper it. Our investigations have shown that, as a rule, the rotating moment in the Cephalopoda remains negative, stablizing, i.e. it contributes to the preservation of the dynamic stability during the turn. The value of the dynamic stability is small, actually near zero, whereas the horizontal dynamic stability of the fish during the turn remains rather substantial. This may be explained by the fact that fishes swim in a dis- tinctly different way from the Cephalopoda while the longitudinal axis of the body of the suinuuing fish is constantly deviated in horizontal plane, the body of the Cephalopoda maintains a constant posture in the horizontal plane and does not require an additional safeguarding of horizontal stability. /3 8 We studied the value of the dynamic stability characterized z . z Z Z . by the indices of dynamic stability min and max, max and min (Aleev,

1963) 5 and its changes in the ontogenesis of the following species:

E. moschata, E. efficinalis, I. coindeti, L. vulgaris, A. lichtensteini. •

It is necessary to note that, contrary to the fish, the value of dynamic stability in cephalopod melluscs remains constant both for the translational movement and for turning, since the surface of the longitudinal projection does not change during the turns (in fast swimming the fins are always tightly clasped to the body). Hence the index of the maximal Z horizontal dynamic stability min is equal to the index of the minimal horizontal dynamic stability Zmax, and the indices of maximal vertical -35- dynamic stability zmin and minimal vertical dynamic stability zmax are n7lso z] equal accordingly. In the future we shall refer to Zmax and max, since uo are stud7ing the dynamic stability during turning.

Cannon to nll the species under study - wore the near zero values z for max and max, which indicates that the Cephalopoda have a sufficiently high degree of manoeuvrability. The stabilizing moment decreases with an increase in the size of the animal. The demorsal octopus E. moschata is an exception; in ontogene sis his turning moment becomes a stabilizing one.

Howevor, it may be assumed that for demersal creeping forms, constantly living at the bottom and spending only insignificant periods of time in motion, the index of the dynamic stability is not characteristic. In the cuttlefish,

S. officinalis the numerical value of the horizontal dynamic stability. ( max) is substantially mnaller than the numerical value of zmax in connection with the peculiarities of morphology. In Sepia the zmax is ensured by a strong dorsoventral contraction of the body which, as indicated earlier, is.a device for bettor camouflage. A further reduction of max in squids undoubtedly would be linked with a reduction in the value of the horizontal projection of the body, which would be undesirable as destroying the camouflage of the animal.

In the ontogenesis of the Sepiidae the dynamic stability decreases, thereby increasing the manoeuvrability.

In pelagic squid the stabilizing moment is less important. Their manoeuvrability further increases in the ontogenesis, but the values Zmax and zmax remain negative. As the animal grows bigger and swims faster, the drag, too, increases, complicating the accomplishment of turns and increasing the stabilizing moment. Accordingly, the manoeuvrability of a bigger squid ought to have been worse than that of a smaller one, yet, in the ontogenesis the value of the stabilizing moment in both the Sopioidea and the Teuthoidea /39 decreases, i.e. their manoeuvrability increases. - 36 -

The increase in manoeuvrability occurs owing to a progressive

development of morphological specific features, aimed at the weakening of the stabilizing couple. Among these features are the decrease in the distance between the centre of gravity and the centre of dynamic pressure, the dis-

placement of the centre of dynamic pressure nearer to the centre of gravity.

Besides, in the ontogenesis of squids and cuttlefish, a reduction in relative

values of longitudinal projection of the body occurs. The combined result of

all this leads to an increase in the manoeuvrability of the Cephalopoda in

ontogenosis (Fig. 11).

Fig. 11. Curves for different species of cephalopds: 1 - Loligo vulgaris Lamarck; 2 - Sepia off icinalis Linne.

z: In the study of Zmax and max indices of all species without z exception, particular attention is drawn to the fact that the max is secured

better. One must assume that this phenomenon is based on the method of move-

ment of the Cephalopoda, in particular, the position of the propelling agent

in a vertical plane, below the level of the centre of gravity. The ejection

of a jet of water from the infundibulum, even at a relatively small angle to

the longitudinal axis of the body, tends to deflect the front end of the

body upwards, while there are no constantly acting moments in the horizontal

plane. Therefore, it is necessary that the vertical dynamic stability be

morphologically ensured more securely, preventing vertical deflection of the

front end of the body and the resultant deviation from the course. -37-

Morphologically, the increase of zmax in the Cophalopoda is achieved at the expense of some dorsoventral contraction of the body as well as some development of arm fins.

Breaking

Breaking, in the physical sense, means the additional creation of drag. Tho morphological devices of the organism which ensure breaking are functionally opposed to the devices which create the propulsive force.

In the Cephalopoda, breaking is achieved in different ways de- pending on the specific conditions. In the representatives of the Decapoda and the Octopoda the breaking may be accomplished by regulating the /40 force of the jet. The very structure of the infundibulum becomes in that case a regulating device: the walls of the infundibulum are provided with ring-shapod muscles which, contracting, decrease the cross-section of the foramen. In slow swimming squid the foramen is oval-shaped and wide open. With increasing the speed, the surface of its cross-section decreases, assuming an ellipsoidal shape. In fast swimming the foramen looks like a narrow slit.

Another moans of breaking used by the Cophalopoda is the instantaneous fanning out of arms and straightening of twin lateral fins with the simultaneous turning of the froc end of tho infundibulum by 180 0 , resulting in a sharp increase of drag, on account of the turbulization of the border-layer at the anterior part of the animalls body. By the sole action of the hydrojet propelling agent, a full stop or reversing of the direction of the movement may be achieved. In that case, the infundibulumls free end is turned in the direction of the movement. The breaking is caused by the reaction force of the jet. Squid often resort to this manoeuvre in escaping from their enemies. - 38 -

Usually the Cephalopoda use for breaking not just one device, but the whole range of them - changing the direction and the force of jet and simultaneously fanning out their arms and fins; all of this substantially increases the effect of breaking, improving their manoeuvrability.

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