271

THE RATE OF MOVEMENT OF PLANARIA ALPINA

BY R. S. A. BEAUCHAMP.

(Naturalist at the Laboratory of the Freshwater Biological Association.)

{Received 23rd January, 1935.)

(With Eleven Text-figures.)

PLANARJA ALPINA normally moves by gliding, that is to say the ventral surfacesecretes mucus over which the animal travels by means of waves created in theventral musculature. These waves travel from behind forwards.

Pearl (1903) observed in PL dorotocephala that mechanical and weak chemicalstimuli might cause the animal to move faster. He showed that after median in-cision of the brain the animal moved more slowly. He supposed the movements ofthe animal were brought about by the action of the ventral cilia and considered themaintenance of the tonus of the ciliary system to be the chief function of the brain.

Stringer (1917) demonstrated that the cilia of planarians are stopped by theaction of weak solutions of lithium chloride, but that the normal gliding movementcontinues.

Voigt (1904), Doflein (1925) and Koehler (1932) all recorded that chemicalsubstances in solution caused planarians to become "alarmed" or "excited", butdid not state definitely whether the speed of the animals was affected.

Cole (1926) investigated the rate of movement of PI. maculata at temperaturesbetween 7 and 30° C. He noted that the speed increases with increase in tempera-ture, and by plotting the logarithm of the rate of movement against the reciprocalof the absolute temperature he obtained a series of points through which he wasable to fit three straight lines. The slope of these lines expresses the value of /x, thetemperature coefficient; this constant increases in value with increase in tempera-ture.

Robertson (1930) observed the rate of movement of heads, tails and middleportions of Polycelis nigra. His results were expressed as the average rate of move-ment of numerous pieces, whose speeds had been estimated from the time taken totravel a distance of 1 mm.

This paper discusses the effect of temperature and the condition of the centralnervous system on the rate of movement of Planaria alpina.

METHODS AND APPARATUS.

During all the experiments recorded in this paper the animal was free to moveabout a large enamel tray, 50 cm. long and 40 cm. wide. The water in this tray wasa little less than a centimetre deep. Only filtered stream water was used. The tray,covered by a sheet of glass, rested on four rubber bungs in a large tank containing

JEB-XIliii 18

272 R. S. A. BEAUCHAMP

water. The whole was enclosed in a stout wooden box with a plate-glass window inthe top, the window being the same size as the enamel tray. The wooden box andthe two sheets of glass served to insulate the tank and the contained tray againstexternal temperature changes.

The water in the large tank was circulated through a refrigerator and its tem-perature controlled in this way. By slightly modifying the apparatus the temperatureof the water could be kept either above or below that of the laboratory. The tem-perature of the water could be kept within a quarter of a degree of any requiredvalue. The circulating mechanism kept the water in the large tank adequatelystirred.

The tray containing the animal was illuminated by a single light hung centrally1 metre above it. Screens prevented light from any other source falling on theapparatus. The source of light consisted of an illuminated disc of opal glass 3 cm.in diameter. This gave a weak and rather yellow light. This strength and quality oflight was used as PI. alpina was found to be insensitive to weak light from whichrays of short wavelength have been excluded. This arrangement made it impossibleto stimulate the animal by shading it accidentally.

Records of the animal's movements were made on a sheet of glass, resting ontop of the plate glass window. An ordinary pen and indian ink were used. Thepoint of the pen casts a shadow on the bottom of the enamel tray, and this shadowwas made to follow immediately behind the tail of the moving animal. Accidentalshading of the animal did not affect it for the reason stated in the last paragraph. Inthis way an almost exact record of the movements of the animal is made on the sheetof glass. The length of the line on the glass is very slightly shorter than the actualdistance travelled by the animal, but since the difference is very small and the pointto be investigated was the relative speeds of the animal under different conditions,this slight error was ignored. While the animal was being observed signals werereceived from a clock, striking a small bell once every minute. At each signal theobserver made a transverse mark on the record of the animal's path. The track onthe glass was then printed on to light sensitive paper, and from this reproductionthe distances moved each minute by the animal were measured by means of a mapmeasurer.

At each particular temperature, records were taken at intervals of about 20 min.throughout the period during which the animal was moving. Readings were nottaken when the animal was moving along or near the extreme edge, as it was foundthat under these circumstances the speed was slightly irregular.

Each time a record was made, the track of the animal was followed for 7 or8 min. After the distances had been measured from these records the values wereplotted on a graph to show the maximum speed, the minimum speed and the averagespeed. On the graph the points showing maximum and minimum speed are joinedby a vertical line and the average speed is shown by a short horizontal line crossingthe vertical line. Since the average point represents the mean of seven or moresuccessive readings the standard deviation from the mean can be calculated. Thisdid not as a rule exceed 2 per cent, and was frequently below 1 per cent. Fig. 1 A

The Rate of Movement of Planaria alpina 273

shows values taken from a record at 4-5° C. In this case the animal started to moveof its own accord. The speed gradually increases until a particular value is reached.This speed is then steadily maintained for a varying period of time. As will beshown later, the length of time during which an animal may be active continuouslyis affected by the temperature, and may vary from 2 hours to as long as 30 hours.A short time before the animal comes to rest, the rate of movement may be slightlyreduced, and in addition to a slight reduction in the average speed the variations inspeed may become slightly greater.

It was not always convenient to wait to take readings till the animal startedmoving on its own accord. Once a number of readings had been taken after theanimal had started moving on its own accord and the normal behaviour of the

Hours

Fig. 1. A shows the normal increase in rate of movement when an individual starts moving on itsown accord. B shows the effect of slight mechanical stimulation. The vertical arrows indicate whenthe animal stopped moving.

animal had been determined, the resting animal was disturbed by gently rockingthe tray so that a little wave travelled over it. This was sufficient to start the animalmoving, provided it was not fatigued. The animal was never disturbed unless ithad been at rest for at least 12 hours. Fig. 1 B shows a record taken at 70 C. afterthe animal had been disturbed by gently rocking the tray. The speed increasesrapidly to a value above the normal for that particular temperature, but in about20 min. has dropped to the normal.

In graphs such as those shown in Fig. 1 the range of values between the hori-zontal lines, which were drawn so as to include all the points marking the averagespeed of the animal, was taken as indicating the representative speed of the animalat that particular temperature. It will be seen that all the points marking theaverage speed do not vary by more than 0-25 cm. per min.; this range of valuesgreatly exceeds the probable error as estimated from the standard deviation.

18-2

274 R. S. A. BEAUCHAMP

A further check on the accuracy of these estimates of the speed at particulartemperatures was made by keeping an animal at the same temperature for a longperiod of time and taking a series of readings at intervals of 2 or 3 days. Just beforethe experiment was started the animal was fed on a piece of Gammarus pulex. Afterthis meal the animal was very inactive and would not move even when the tray wasrocked. This is the usual behaviour of animals after feeding and may last for aweek or 10 days. Then there is a period during which the rate of movement is in-clined to be somewhat irregular, but after 2 or 3 days the speed is maintained atapproximately the same value (within 0-25 cm. per min.) for about 6 weeks. At theend of this period there may be a slight increase in the rate of movement, and this isusually followed by a period of indefinite length during which the animal may bevery inactive. During the period between 2 and 8 weeks after feeding the fluctua-tions in speed do not as a rule exceed 0-25 cm. per min. even at the higher tempera-tures when the animal is moving relatively fast. Therefore only during this periodwere experiments carried out on the effect of temperature on the rate of movement.

All experiments were done on immature animals about 7 mm. long. A smallpiece of Gammarus about every 8 weeks maintained the animal in good condition,but was not sufficient to enable it to develop sexually. In this way it was possibleto keep individual animals in approximately the same state of development almostindefinitely. In one case experimental observations were made on one individualfor over a year.

EXPERIMENTAL DATA.

(a) Temperature.

Observations on the rate of movement of an animal were only taken after it hadbeen at least 24 hours at any particular temperature. The temperature was raisedor lowered irregularly by 2 or 30 C. at a time so as to avoid any errors that mightarise if the temperature was changed continuously in one direction.

Fin- 2. A shows the normal rate of movement of an individual at temperatures between 05 and120 C B shows the rate of movement of the same animal when stimulated with Gammarus extract.

The Rate of Movement of Planaria alpina 275The general form of the temperature-rate graph was determined from a number

of individuals. The individual variations in speed for animals of the same size(7 mm.) did not exceed 1 cm. per min., but the slopes of the curves were almostidentical. A composite curve is shown in Fig. 5, but Fig. 2 A shows a large numberof values obtained from a single individual. This curve is quite typical, and it isconsidered advantageous to consider values obtained from one individual ratherthan the average values from a number of animals. The separate points given onthis curve represent average values obtained from a series similar to those in Fig. 1.These averages have a high order of accuracy, since they are means of from 40 to100 min. rates. Their standard deviation is usually less than 2 per cent. Since theform of this curve is unusual, as compared with temperature curves obtained byother workers on other animals, a table of the actual values recorded is given (seeTable I). It will be seen that the minimum speed occurs at about 1-5 ' C , and thatbelow this temperature there is an increase of speed with increasing temperature.All the points recorded between 2 and i2u C. fall on a straight line. Below 120 C.there was no difficulty in determining the speed at particular temperatures. Theseries of readings obtained at each temperature were always similar to one or otherof those shown in Fig. 1. But above 120 C. this was not always the case.

Table I.

Temperature

r <-'•)o-51 0

i ' 52-252 7 54-254'55"56-257-08-o9 2 5

U ' 5

Normal rate(cm. per min.)

3-152-92-93'253"254 03-854'354-64-85'45756-4

Rate of stimu-lated animal

(cm. per min.)

—

5-o———

6-65———

7758-3—

1 0 0

Above 120 C. satisfactory readings, that is to say readings in which the speedremained constant over 2 or 3 hours, might be obtained for a few days immediatelyafter the animal had been raised to the particular temperature. But after a few daysat any temperature above 120 C, during any one period of activity, the speed wouldnot remain steady for long but would gradually increase to a maximum. The maxi-mum speed would not as a rule be maintained for more than a few minutes, andthen, before the animal came to rest, the speed would fall off steadily to a low value.The length of time that the speed remained at a steady value before increasing de-pended on the number of days that the animal had been kept at that particulartemperature. The longer the time the sooner after the beginning of a period ofactivity would the speed show an increase, until eventually the speed would increase

276 R. S. A. BEAUCHAMP

continuously from the moment the animal started to move till a maximal value hadbeen reached, and would then decrease. Also the maximum speed reached dependedon the length of time the animal had been kept at that particular temperature. Thelonger the time the greater the maximum speed attained. Both these effects areshown in Fig. 3. All four series were taken at 140 C , the first, 4 days after thetemperature had been raised from 90 C , the second, 11 days, and the third, 21 daysand the fourth 29 days afterwards. It will be seen that in the first series the speedremains steady. The whole length of the series is not shown, as it continued forseveral hours longer. In the second series the speed is only maintained at a steady

After 4 days at i4°C

fa 7 -

B\fter 11 diys it i4°C

Kf \

A

cAfter 21 diva at I4°C After 2q Ja\ s

at 14°C

15 0

Hours5

Hours5

Hours Ho

Fig. 3. Shows the rate of movement of an individual during four periods of activity, 4, 11, 21,and 29 days after the temperature had been raised to and kept at 140 C.

value for 1 hour and then shows an increase from 6-25 cm. per min. to 7-25 cm. permin., followed by a gradual fall off in speed. The third series shows no periodduring which the speed is steady, but from the start the speed increases continu-ously to 8-o cm. per min. and then falls off to a low value before the animal comesto rest. Finally the fourth series again shows no period during which the speed issteady, but the condition of the animal would seem to be deteriorating; there is ageneral reduction in speed and the period of activity is short.

The initial steady speeds and the maximum speeds attained by the animal keptat 140 C. are shown in Fig. 4. This set of readings includes the four readings shownin Fig. 2; they are respectively the second, fifth, eighth and eleventh points on thegraph. It will be seen that the initial steady speed can only be observed for the first

The Rate of Movement of Planaria alpina 277

11 days. After the first 9 days and for the following 12 days, the maximum speedsshow a steady increase, afterwards a steady decrease. Finally the animal becomesvery inactive. At higher temperatures the same sequence of events occurs, butoccupies a shorter period of time. Above about 160 C, however, the speed is atfirst very great and then steadily decreases from day to day. Finally the animal maybecome moribund and divide.

Clearly it is not possible to plot an ordinary temperature-rate graph over thesehigher temperatures, since the rates of movement are continuously changing with

. 5

i4o°C.

5 10 IS 20 15 Mi 35 40 45

Days

Fig. 4. The lower values, joined by a continuous line, indicate the initial steady pace of the animalwhen first brought to 14° C. The points on the dotted line represent the greatest speed attained duringa period of activity. The two points, marked by squares, are values obtained following stimulationwith Gammarus extract.

time and, in addition, readings cannot be taken with the same individual at thedifferent temperatures, since by the end of an experiment at one particular tem-perature the normal processes of the animal would seem to be damaged. Con-sequently observations from a number of different individuals have to be compared.In order to do this it is necessary to plot the results on a relative and not on anabsolute scale. In Fig. 5 the unit of speed is the speed of the individual at io° C.The points marked by black squares represent the normal speed of the animals, butabove 120 C. the normal speed, as already explained, is not maintained at a steadyvalue after the first few days. The points marked by outline squares are the final

278 R. S. A. BEAUCHAMP

maximum rates attained after several days at the particular temperatures. It willbe seen that the values for the initial speeds between 12 and 160 C. lie on the con-tinuation of the straight line that goes through the values for the normal speeds ofanimals below 120 C. The points representing the final maximum rates lie on a lineroughly parallel with the line through the initial speeds. Above 160 C, as alreadyexplained, there is no increase in speed with time, but, from the first the speed isfast and falls off with time. The values for the initial speed above 160 C. continuethe line through the points representing the maximum speeds finally attained by theindividuals kept at temperatures between 12 and 160 C. But with increase in tem-perature the slope of the curve decreases until at about 200 C. no further increasewill lead to even a temporary increase in speed.

The increase in speed which occurs when individuals are kept at temperaturesbetween 12 and 160 C. is of the order of 30 per cent, of the initial speed.

2-0 -

1-5

I - O -

0-5

5 10 15 20"C

Fig. 5. The unit on the vertical acale is the speed of the individual at io° C.Values from eight individuals are included in this figure.

If an animal has been kept at temperatures above 120 C. long enough for theincrease in the rate of movement to have occurred, and if the temperature is nowlowered to below 120 C, the animal's rate of movement is not the same as it was atthat temperature before the temperature was raised to above 120 C, nor do thereadings taken throughout a period of activity remain steady. Instead the animalbehaves to some extent as though it were still at the higher temperatures. The onlydifference being that the actual rates of movement are not quite as high as theywould have been at the higher temperatures. The speed increases during the firsthour or so of a period of activity and then declines to a low value before the animalcomes to rest. This effect may be observed 10 or more days after the temperaturehas been lowered. Finally the animal recovers its normal behaviour. Fig. 6 showsa series of readings taken during a single period of activity; this animal had pre-viously been kept for 3 weeks at 150 C, and then the temperature was lowered to90 C. The series shown in the figure was taken after the temperature had been at90 C. for 5 days.

The Rate of Movement of Planaria alpina 279

Temperature affects not only the rate of movement but also the length of timean animal may move continuously at any one time. There are two reasons why thereare not as many observations to support this statement as there are readings forrates of movement. Firstly, in many cases, when observations of the animal's rateof movement had been taken, it was impracticable to watch the animal till it finallycame to rest. And secondly, there were occasions when the animal appeared to bestopped by particular circumstances; this was especially the case with one in-dividual which nearly always came to rest at one particular place in the tray. Evenwhen the tray was turned through 1800, it came to rest in the same spot on the tray.This suggests that the animal is affected by the mucus secretions left by itself onthe tray. In Fig. 7 only the longest periods of activity recorded at the various tem-

2 -

At 9-0° C after a period of three wccki at l5-o°C

Fig. 7. Illustrates the variations with temperatures inthe length of the periods of activity of three individuals.

HoursFig. 6. Previous to the taking of this record theanimal had been kept for 3 weeks at i5°C., afterwhich time the temperature had been lowered to9" C. and had been kept at g° C. for 5 days.

peratures are plotted. This figure includes data from three individuals. The shorterperiods occur at about 40 C, and with increase in temperature the periods becomeincreasingly longer till at about u ° C. the animal may be active continuously forover 15 hours and may travel in that time about 60 metres. At still higher tem-peratures, that is to say at those temperatures where irregularities in the speed occur,the animal may be active continuously for much longer periods, but only when firstraised to these temperatures. As was indicated earlier, the animal later becomesmore and more inactive at high temperatures.

(b) Starvation.It was stated that during the period between 2 and 8 weeks after feeding the

rate of movement of an animal kept at any temperature below 120 C. remained thesame to within 0-25 cm. per min., but that after that time the rate might go up some-

280 R. S. A. BEAUCHAMP

what. This increase in speed occurs gradually and in a similar way to the increasein speed shown by an animal after a few days at temperatures slightly above 120 C.;that is to say the speed is not maintained steadily throughout the periods of activity,but increases gradually towards the end of a period of activity and then falls off.The form of the rate of movement curve is similar to that shown in Fig. 3 B, butthe increase in speed is not usually so marked. This increase in rate of movementfollowing starvation has so far only been observed at temperatures above 8° C.

(c) The central nervous system.

As was noticed at the beginning of this paper (see Fig. 1 B), a wave of watertravelling over the animal causes an increase in the rate of movement. Othermechanical stimuli and some thermal and chemical stimuli have a similar effect.For the present purpose it was desirable to discover a stimulus which, when appliedto the animal, would produce the greatest increase in its rate of movement. A cold-water extract of Gammarus pulex when added to the water caused the animal tomove very rapidly. Stimuli of a similar nature (other food extracts) had the sameeffect and caused an exactly comparable increase in the rate of movement, and sinceno other kind of stimulation caused the animal to move so fast, it was assumed thatthis was the maximal effect.

The extract of Gammarus was made by grinding up a Gammarus in a glassmortar, water was added and the resulting suspension filtered. At first quite strongsolutions were used, but weaker solutions were tested to find the minimal strengthwhich would still give the maximal effect. This was done as it was obviously un-desirable to add more of the extract than was necessary to the water in which theanimal was living. It was found that a cubic centimetre of a solution made bydiluting the extract from one Gammarus by 2 litres of water was sufficient to givethe full response. In the following experiments a stronger solution was used, namelyone Gammarus in a 100 c.c. of water. The extract was applied by means of a fineglass pipette. The pipette contained approximately J c.c. of the extract. The tip ofthe pipette was placed under the water and just in front of the animal, and theextract was then allowed to escape slowly. This stimulus causes a very considerableincrease in the subsequent rate of movement of the animal. The first part of Fig. 8shows the normal rate of movement of an animal at 30 C. The rate is 3-25 cm. permin. At the moment marked by a vertical arrow, extract was added. The nextreading between 3 and 8 min. after shows the rate of movement to have increasedto 5-8 cm. per min. During the next 40 min. the rate decreased to the original value.Thus it will be seen that external stimuli can have a considerable effect on the rateof movement; in this case as a result of stimulation the speed increased by nearly80 per cent.; we can only suppose that this result is achieved through the C.N.S.

Further experiments were done to determine the maximal rate of movement ofan animal when stimulated at different temperatures. In Fig. 2, the top line (B)shows values obtained for the maximal rate of movement following stimulation withGammarus extract. These values were obtained from the individual whose normalrate is recorded below by curve A.

The Rate of Movement of Planaria alpina 281

The chemo-receptors responsible for affecting this increase in rate of movementare localised in the very front of the animal's head. Application of Gammarus ex-tract ceases to have any effect on the animal after the extreme anterior border of thehead has been removed. Unilateral section of the head may have a similar effect;this can be explained most easily by supposing that lateral damage to the brain leadsto the disorganisation of more central tissues.

The central nervous system of PI. alpina consists of a complex of nervous ele-ments in the head. This complex receives sensory nerves from the whole of thefrontal area of the head. The cell bodies of these sensory nerves are situated in orimmediately beneath the epithelium. Two main nerve trunks, which arise from the

7 r

0-3 c c of Gammenu extract idded at 3-o°C

2 J

Hours

At ioo°C

20 25

Fig. 8. Shows the increase in speed followingstimulation with Gammarut extract.

Days after operation

Fig. 9. A shows the speed of an animal during the8 days following the removal of the extreme anteriorborder of the head. B shows the speed of another ani-mal during the 23 days following the removal of thewhole of one tentacle.

central nervous mass and which extend to the posterior end of the animal, give offa large number of branches which contain many nerves, most of which appear tomake contact with the ventral nerve net. The nerve net lies immediately dorsal tothe ventral muscular "mat". The central mass of the head complex is dumb-bell-shaped and consists of nothing but closely packed fibres; surrounding this mass isan enormous number of nerve cells. These cell bodies with their attached fibresform about three-quarters of all the tissues in the head.

Experiments were performed to determine the part played by the C.N.S. incontrolling the normal rate of movement. It was already known that headlessanimals move very much more slowly than intact animals, but the removal of thehead besides removing the brain must to some extent damage directly both the

282 R. S. A. BEAUCHAMP

most anterior part of the ventral muscular "mat" and the anterior mucus glands.It was hoped that superficial damage to the anterior border of the animal woulddestroy the peripheral nerve cells and that the dying back of these nerves might leadto damage to the central mechanism. Accordingly the extreme anterior border ofone animal was removed. The effect on the rate of movement is shown in Fig. 9 A.At first there was a slight increase in speed; then the speed decreased gradually butonly to a slight extent, and in 8 days the speed of the animal was restored to itsnormal value. Apparently no real damage had been done to the central motormechanism.

The whole of one tentacle was removed from another animal; this operationremoved about one-quarter of the head, and must have damaged a considerableamount of the tissue of nerve cells which surround the central mass of fibres. Afterthis operation the rate decreased progressively during the next 3 weeks until a valueof about half the original speed was reached (see Fig. 9 B). This gradual decrease inspeed may possibly be explained by a progressive destruction of the central mechan-ism following the dying back of the damaged cells. That it is the C.N.S. which wasdamaged and caused the speed to decrease is suggested by the time taken for theoperation to take full effect. If the result had been due to damage done directly tothe locomotor apparatus it would have shown the full effect immediately.

Median section of the head causes a sudden fall in speed to about half the originalvalue.

Regeneration soon restored the outward appearance of the animal whose tentaclehad been removed; a new tentacle was formed in about 8 days and could only bedistinguished from the other tentacle by the lighter pigmentation. But, though theoutward appearance of the animal was normal, the rate of movement still remainedat about half the original value, and remained low for about 3 weeks. The rate thenbegan to increase gradually, so that from day to day higher rates were recorded. Atthe same time as the speed began to increase the animal made the first response toGammarus extract since the operation. At first the response led to only a slightincrease in speed, but as the normal speed increased so did the increase in speedfollowing stimulation (see Fig. 10). This experiment indicates clearly that theremust be a close connection between the mechanism which maintains the normalspeed of an intact animal at a value above that of an animal with a damaged C.N.S.,and the mechanism which leads to an increase in the rate of movement followingstimulation with Gammarus extract.

It has been shown that above 12 C. the rate of movement does not remainconstant, but that after a time at these higher temperatures the rate of movementshows an increase during the first part of a period of activity, but after reaching amaximal value falls off again. The maximal values attained become gradually higherfrom day to day until an extreme value is reached, after which the maximum speedfalls off. This extreme value is closely similar to the maximal rate recorded followingstimulation with Gammarus extract. At the top of Fig. 4 there are two points show-ing the rate of movement after stimulation with Gammarus extract. It will be seen

The Rate of Movement of Planaria alpina 283

that the value for these readings is similar to the extreme value for the rate of move-ment of the animal after being kept at 140 C. for 21 days. The two upper curves inFig. 5 show a similar relation between the speed of stimulated animals and themaximal speed finally attained by animals kept at temperatures above 12° C.

0 l—105

170V-> 40 4') 50 55 60

Days since operation

Fitf- io. Shows the gradual recovery of the normal speed and the speed followingstimulation of the animal whose head tentacle was removed.

FIELD OBSERVATIONS.PL alpina is limited by temperature to the upper reaches of the streams in which

it occurs. Though observations have been made all over Europe, PL alpina has neverbeen found at temperatures above 150 C. Numerous continental workers (forreferences see Beauchamp and Ullyott (1932), have also stated that the upper tem-perature limit for PL alpina is in the neighbourhood of 150 C. This fact is mentionedsince it lends significance to some of the experimental results given in this paper.

DISCUSSION.It follows from the results given in this paper that, although temperature affects

the normal rate of movement of PL alpina, it is not the controlling factor. This isclearly demonstrated by the increase in rate which follows stimulation of the animal.If temperature were the limiting factor, this increase would not be possible. There-fore it would be unreasonable to attempt to make deductions regarding the chemicalreactions responsible for locomotion from the form of the temperature-rate graphof the unstimulated animal. Only in the fully stimulated animal may temperaturebe limiting the rate of movement.

284 R. S. A. BEAUCHAMP

Since gliding can continue without the C.N.S. one must presume that theventral nerve net co-ordinates the normal gliding motion. But since damage to theC.N.S. leads to a considerable reduction in the rate of movement it is evident thatit has an excitatory influence on the locomotor apparatus. It is unlikely that theC.N.S. affects the ventral musculature directly but only through the ventral nervenet. Since external stimuli can increase the rate of movement it is clear that theC.N.S. under normal circumstances does not exert its full excitatory effect. Atpresent it is impossible to explain how the C.N.S. exerts this stimulating effect on thelocomotor apparatus, which results in the animal moving faster than it would withoutcentral nervous stimulation, yet at a speed well below the speed which results fromfull stimulation by the C.N.S. With increasing temperature it seems that the effectof the C.N.S. on the rate of movement becomes relatively greater. At least this issuggested by Fig. 11 which expresses the normal speed of the animal as a per-centage of the maximal speed. But it will be seen from the shape of the curve that

•aVV

a

Fig. 11. Shows the normal speed of the animal between or5 and 12° C.as a percentage of the maximal speed.

with increase in temperature the increment by which the normal rate is increasinggradually becomes less. But above 120 C. the increase in the normal speed occurssuddenly. It seems, therefore, that some other process is responsible for the in-crease in speed above 120 C. At the higher temperatures one must suppose that thenormal mechanism is damaged. This is strongly suggested by the experiment,illustrated in Fig. 6, in which the animal which had been kept for 3 weeks at 15° C.was lowered to 90 C. and yet for several days behaved as though it were still at thehigher temperature. Since the effect produced is an increase in rate, one mustsuppose that the mechanism which is damaged is inhibitory in character.

At present there is no explanation of the increase in rate of movement whichoccurs below 2° C. It is possible, however, that this increase in speed is analogousto the increase in the rate of respiration of certain plant storage organs at tempera-ture towards the lower end of the temperature range of the various species. In thecase of the potato and the banana it has been shown to be due to a breakdown ofcarbohydrates.

The Rate of Movement of Planaria alpina 285

SUMMARY.

1. The rate of movement and the length of the periods of activity of PI. alpinaare increased hy raising the temperature, except at very low temperatures.

2. The normal rate of movement appears to be controlled by a balance betweentwo nervous processes, one being excitatory, the other being inhibitory.

3. The inhibitory process is broken down at the higher temperatures beforethe excitatory or muscular mechanisms are damaged.

4. Starvation may lead to a slight increase in the rate of movement.5. External stimuli affect the rate of movement.6. The chemo-receptors are situated on the extreme anterior border of the head.7. The effect of temperature on the normal rate of movement gives no informa-

tion regarding the chemical reactions responsible for muscular contraction.8. PL alpina cannot tolerate indefinitely temperatures higher than 120 C.9. Under natural conditions PI. alpina is never found at temperatures above

15° C

REFERENCES.

ARMITAOE ROBERTSON, J. (1930). J. txp. Biol. 7, 88.BEAUCHAMP, R. S. A. and ULLYOTT, P. (1932). J. Ecol. 20, 200.COLE, W. H. (1926). J. gen. Physiol. 9, 503.DOFLEIN, I. (1925). Z. wits. Biol. Abt. C, 3, 60.KOEHLER, O. (1932). Z. vergl. Physiol. 16, 605.PEARL, R. (1903). Quart. J. mkr. Sei. 40, 547.STRINGER, C. K. (1917). Proc. nat. Acad. Set., Wash., 3, 691.VOIGT, W. (1904). Verh. naturh. Ver. preuss. Rheinl. 61, 103.