ABSTRACT
Feeding can be initiated in Anemonia sulcata by mechanical, chemical or electrical stimulation of the tentacles provided the stimulus sets up sufficiently prolonged excitation. Owing to rapid adaptation, mechanical stimuli rarely set up enduring excitation and inert objects are therefore usually rejected. Chemical stimuli set up prolonged excitation and food objects are therefore usually accepted. A series of electrical stimuli can produce rejection or feeding according to whether it is brief or prolonged.
The sensitivity of the tentacles varies greatly to different foods. It is greatest to animal foods. There is great sensitivity to certain kinds of mucus.
The active substances of natural foods are closely associated with protein. They fail to pass through a membrane which retains colloids. A feeding reaction can be obtained to food substances which appear to be insoluble in water.
Though the active substances of many natural foods are not in free solution, soluble derivatives of proteins, such as peptones and amino acids cause a feeding reaction. With pure proteins, the response of the cnidae is diminished. With amino acids, there is no cnida response. From this it follows that increased mechanical contact due to cnida discharge is not essential for excitation of the feeding response.
Fat, such as tristearin, and ethereal extracts of food are without effect. Alcoholic Soxhlet extraction of food yields a substance.which causes the food reaction. Carbohydrates are without effect except in the case of glycogen, the action of which may be due to impurities.
The lower fatty acids, quinine and bile salts produce a withering contraction of the tentacles which differs from the response of the latter to food. The effect can be produced by prolonged electrical excitation at a higher frequency than that required for the feeding response and is therefore probably due to excessive stimulation.
The mouth responds to a greater variety of chemical stimuli than the tentacles. The relative sensitivity of - these organs to different chemical stimuli is not the same, but for most agents the mouth is the more sensitive. Quinine, histamine and 10% bile salts excite the musculature of the mouth directly.
The range of chemical sensitivity in different coelenterates is discussed.
THE FEEDING RESPONSE
Like all sea anemones, Anemonia sulcata possesses a well-marked feeding reaction. A lump of meat placed on the tentacles is instantly seized and rapidly conveyed to the mouth which then ingests it. It is our object to determine the stimuli which cause this response.
The feeding reaction is complex and orderly. There is first a discharge of the cnidae from the tentacles into the food. This discharge is apparently independent of the nervous system (Pantin, 1942), though the increased mechanical contact caused by it helps to stimulate the tentacles. Immediately after contact, the tentacles clasp the food and bend strongly towards the mouth. The edge of the disk carrying these tentacles contracts, so that they bunch together round the food, and then rises up and turns inwards thereby folding tentacles and food towards the mouth. After a few seconds, the part of the oral disk between the food and the mouth slowly contracts and sinks downwards, so that the mouth turns towards the food. The mouth then protrudes and begins to open, the food is gradually thrust in, and muscular action of the pharynx pulls it into the gastral cavity. An essentially similar feeding reaction is found in other anemones.
Normally, a mechanical stimulus alone fails to produce a feeding response. A piece of clean cotton-wool soaked in sea water is handled for a little while by the tentacles as at the beginning of feeding, but these soon fall back and let the wool roll off the disk. However, this rejection is simply an unfinished feeding reaction which fails in its initial stages, and all gradations can be found between the two. As many investigators (Nagel, 1892; Allabach, 1905; Jennings, 1905; Parker, 1917) have shown in other actinians, the response varies greatly with the individual. Starved anemones in good condition may give a complete feeding reaction even to inert objects. After such anemones are given food, the reactions of the mouth and disk and finally of the tentacles themselves towards inert objects become progressively reduced till the feeding passes into rejection. Later, the reaction becomes so depressed that well-fed anemones will even reject food.
A piece of food is always more effective than an inert object. This is due to a chemical stimulus. If about 2 ml. of an active food solution, such as 10 % human saliva in sea water (0·1 % dry weight), is allowed slowly to flow over one group of tentacles of Anemonia, a fairly complete feeding response follows, except that there is no material body for the open mouth to ingest. While therefore both local mechanical and chemical stimuli are together involved in normal feeding, the reaction can on occasion be brought about by localized stimuli that are either purely mechanical or purely chemical. Even local electrical excitation of the tentacles will induce the reaction, so that it is not fundamentally dependent upon the mode of excitation.
The failure of inert objects to cause feeding appears to be due to rapid sensory adaptation to mechanical stimuli. Isolated tentacles touched with a glass rod soon cease to respond, whereas if they are immersed in a food solution, they continue giving contractions for many minutes. This interpretation is borne out by experiments on Calliactis parasitica and Anemonia. If the tentacles of these anemones are locally stimulated electrically at a frequency of about 1 shock in 4 sec. with non-polarizable electrodes, a series of 3 or 4 shocks causes a typical rejection reaction. If the series of shocks continues, more and more of the feeding reaction is elicited, and after 10 or 20 stimuli movements of the mouth and disk may even lead to ingestion of the electrodes.
The above observations show that it does not matter whether the stimulus is mechanical, chemical or even electrical, but the excitation set up must continue for sufficient time if it is to result in a feeding reaction. Chemical stimuli by food usually result in the necessary prolonged excitation: mechanical stimuli, though sometimes effective, do not usually have this result.
Isolated tentacles and mouths go through their proper reactions in the presence of food. But co-ordination of the feeding response is not simply due to successive localized excitation of auto nomous effectors. Electric stimulation of the tentacles gradually causes the mouth to bend towards the stimulated region and to open. In the same way, food placed on the tentacles in Metridium will induce the mouth to open even if it is removed before it touches the mouth (Parker, 1917). The mouth can therefore be caused to open by transmission of excitation from the tentacles, Nevertheless, direct chemical stimulation of the mouth by food substances causes it to open and, if there is a solid food object, to ingest. The mouth is therefore also directly excitable chemically. Both indirect excitation from the tentacles and direct chemical excitation occur in the normal feeding reaction.
According to Nagel (1892, 1894), feeding in anemones is initiated by chemical stimulation of the tentacles. But Loeb (1895) showed that even after the tentacles were cut off, selective feeding could still take place through chemical excitation of the mouth. Fleure & Walton (1906) agreed with Loeb and suggested that with natural food the tentacles of Anemonia only respond to mechanical stimuli. It is true that the mouth of Anemonia is sensitive to chemical stimuli and that the tentacles are very sensitive to mechanical stimuli, but we have found in all normal specimens of Anemonia a clear distinction between the rejection by the tentacles of inert objects like plain cotton-wool and their acceptance of solid food or of cotton-wool-soaked in food solution. While chemical stimuli to the mouth are important, it is those to the tentacles that initiate the feeding reaction. What is this chemical stimulus?
METHODS OF STUDYING CHEMICAL SENSITIVITY
The response of Anemonia to food substances and their derivatives was tested by placing on the tentacles pieces of cotton-wool about 2 ml. in volume soaked with known concentrations of solutions or suspensions of the food substance in sea water. Controls were made with wads of cotton-wool soaked in sea water or in a standard food solution. In some experiments, the food solution was directly squirted on to the tentacles or mouth without cotton-wool.
Among freshly collected animals starved for 12 hr., the threshold concentration of food solution varied as much as tenfold. These animals were used for trials. Those that were in poor condition or that had been recently fed required abnormally high concentrations or even failed to respond to the most potent foods. Such animals were rejected. On the other hand, exceptional individuals responded to concentrations three or four times less than the average ; but such animals might give some feeding response even to inert objects such as plain cotton-wool. Usually six anemones were taken and tested simultaneously in each trial. Frequent testing of the same anemone at short intervals was avoided because rapid successive trials of a food substance in the same anemone caused the feeding reaction to fall off owing to adaptation.
Though the sensitivity of individual animals varies considerably, their relative sensitivity to different foods remains very constant. In the experiments extract of minced beef, human saliva, egg white, and Witte’s peptone were used as standards by which to compare other substances. Most of the anemones just reacted definitely to pieces of cotton-wool soaked in 0·01 % dry weight of saliva, 1·0 % dry weight egg white or between 1·0 and 0·1 % dry weight Witte’s peptone. In the tables given below, the limiting concentrations of other food substances relative to these are given. The limiting concentrations for animals with sensitivity somewhat above or below the average have been scaled down or up according to the values given for the standard substances. Except where otherwise stated, all solid substances were ground up to as fine a state of division as possible while fresh and suspended in sea water. Coarse particles that could not be dispersed were removed. Dry weights were determined for the fresh foods by drying in vacuo to constant weight in a desiccator.
THE RESPONSE TO FOODSTUFFS
With active food substances, contact with the food object is accompanied by strong clinging of the tentacles to it owing to discharge of cnidae. The part played by muscular activity of the tentacles and by the cnidae is not relatively the same for all foodstuffs. Table 1 shows the limiting concentration expressed as dry weight which will cause the tentacles to initiate a feeding response to cotton wads soaked in a variety of food substances. When all allowance has been made for individual variability, it is still evident that food substances differ enormously in their effectiveness. There is a high sensitivity to certain animal substances, particularly certain kinds of mucus. Distinct responses have sometimes been obtained even with concentrations of 0·001 % dry weight of human saliva or the mucus washings from Pecten. There is evidently some specificity in the kinds of substance to which the tentacles respond. The high sensitivity to certain kinds of mucus is of great interest. This substance is often the first indication of the presence of animal food among aquatic organisms. It will, however, be noticed that different kinds of mucus differ among themselves in the extent to which they can provoke a response. Anemones themselves produce considerable quantities of mucus, to the presence of which they are quite indifferent. Anemonia sulcata is indifferent to its own mucus, or indeed to freshly cut-off tentacles. But if a portion of Anemonia is boiled for some minutes and allowed to cool, the coagulated protein mass now becomes acceptable as food, though less so than beef, or human saliva on cotton-wool. In the same way, if living pieces of Anemonia are fed to Calliactis or vice versa, feeding frequently takes place; and cases were noticed in which Anemonia ingested-fragments previously torn off from its own body if these had decomposed.
THE CAUSE OF THE FOOD RESPONSE IN THE TENTACLES
In view of the greater sensitivity to some food substances than to others, it is not surprising that a number of localized stimuli which accompany the presence of almost any food produce no feeding reaction. Simple increase of osmotic pressure with sea water of double concentration has no effect on the tentacles ; nor does a solution of high colloidal osmotic pressure such as is obtained with 10% gum arabic in sea water. Many tissues are relatively rich in potassium but even isotonic (0·6 M) KC1 on cotton-wool wads produces no feeding reaction. The response is not brought about by hydrogen ions. Sea water at pH 4 on wads produces no effect, while even sea water at pH 1 only produces a general withering contraction of the tentacles which is quite unlike the feeding reaction. The-food response to beef is equally effective whether the solution is raised to pH 9 or brought down to pH 6. Acidity does not therefore have even an indirect influence on feeding. Sea water saturated with carbon dioxide, or saturated solutions of Na2HPO4 also produce no effect. From this it appears that the inorganic accompaniments of food are not causing the reaction.
THE SUBSTANCES STIMULATING THE TENTACLES
When soaked into cotton wads, a suspension of beef in sea water was found to show a limit for the feeding reaction at 0·2% dry weight. The suspension was centrifuged and the supernatant fluid poured off and the flocculum resuspended in the same volume of sea water. Both supernatant fluid and resuspended flocculum produced an active feeding reaction at about half the limiting dilution of the original suspension. The high activity of the supernatant fluid shows that the active substances are not necessarily connected with the grosser solid parts of the food.
Though soluble or suspended substances from the food can produce the feeding response, these appear to be colloidal. Undiluted saliva and also beef suspension (e.g. 4% dry weight) were subjected to ultrafiltration through collodion membranes under 40 cm. of mercury. The clear ultrafiltrates produced no feeding reaction. No protein could be detected in them by the usual qualitative tests. It is noteworthy that human blood produces a strong feeding reaction (2 – 0·2% dry weight) but that the active substances failed to pass through the kidneys and urine is ineffective. These experiments indicate that the active food substances are either colloids or substances combined with or strongly adsorbed upon such colloids.
The obvious substances to suspect are the proteins. A beef suspension was prepared (3·5 % dry weight) and made acid to pH 4·4 by the addition of acetic acid. The coagulum when washed gave a strong feeding reaction while the supernatant fluid after filtration and neutralization to pH 7·6 gave no response. The active substance had thus gone with the protein. Similarly, after prolonged boiling of a beef suspension (2% dry weight), the coagulum produced a strong feeding reaction though the clear filtrate showed none. It seems therefore that the active substance not only goes with the protein but that normally little, if any, is in free solution. It is especially remarkable that though the feeding reaction seems to involve a chemical stimulus, yet it is readily produced by apparently insoluble coagula, or even by insoluble natural products such as human skin, while washings from these produce no effect.
PROTEINS AND THEIR DERIVATIVES
When necessary, proteins and amino acids were dissolved with the aid of a little dilute sodium hydroxide and the solutions brought within the range pH 6 – 9, which pH range is without influence on the response to ordinary foodstuffs. Table 2 shows the limiting concentrations for various proteins and their derivatives. Proteins have a high activity which in many cases reaches the same order as that of natural food substances. As in the latter, the activities of the proteins vary. It is noteworthy that fat-free proteins appear to activate only the tentacles and the mouth, while the cnidae are not discharged. Indeed, the distinctive feature of the pure proteins is the responsiveness of the muscular reactions of the tentacles to these substances while the cnida response is almost always below normal. Substances which fail to activate the cnidae necessarily produce much less contact between the food object and the tentacles. Their higher limiting concentration is probably partly due to this.
At similar concentrations, peptones are slightly more effective than the proteins from which they are derived. Amino acids are also effective (Table 3). The low solubility of many amino acids makes their comparison difficult. Certain of them are especially active, notably tyrosine. The feeding reaction to cotton-wool soaked in amino acids is, however, strikingly deficient in one respect. The cnidae are not discharged at all. This tends to raise the apparent limiting concentration. Biuret has no effect; and di-and tripeptides are not especially potent in causing the feeding response. The experiments show that proteins themselves and their derivatives may excite the response, but that the mere presence of a peptide linkage is not enough.
The experiments raise the question of the relation of excitation to solubility. The effects of amino acids show that substances in true solution can excite the response. But it has also been found that the active substances of natural food cannot pass through a collodion membrane and they are therefore either colloidal solutions, or in some cases actually insoluble coagula. How insoluble food substances can excite the tentacles is a matter of interest. It is possible that they exert their effect by direct contact with the sensory cells in a manner analogous to the sensitization of cnidoblasts by certain insoluble substances (Pantin; 1942).
FATS
The fact that the active agent in foods is associated with proteins and that these are themselves active does not necessarily mean that they are the only active agents. There may be others adsorbed on them, particularly lipoids. The fats obtained from dried beef, Pecten mantle or other foods by Soxhlet extraction with ether do not cause a feeding reaction. The same is true of fats extracted in the cold for 12 hr. by alcohol (2 parts) with ether (1 part). This is true both of the solid fat and of suspensions in cotton-wool made with the aid of NaHCO3 solution. Pure fats such as tristearin and tripalmitin are also ineffective. Triacetin also causes no feeding response, though at concentrations of 3 % or above, there is a local withering contraction of the tentacles where these have come in contact with the solution. Emulsions of fatty acids, such as 10% oleic acid, have no effect. Lower fatty acids, like butyric, do not cause a feeding response, but bring about a strong withering contraction like that of triacetin. A 10% solution of glycerol has no effect on the tentacles. The simpler fats and their constituents do not cause the feeding response.
Of other lipoids, cholesterol smeared on cottonwool is without effect. Lecithin is certainly active, whether smeared on wads of cotton-wool or dispersed as an emulsion in NaHCO3 solution. A true feeding response is obtained, with some clinging of the tentacles. The response of the tentacles, however, is relatively slight compared with the enormous response of the mouth to this substance. In view of the fact that two different samples of lecithin consistently showed limits of sensitivity at concentrations of about 10 and 1 % respectively, it is possible that the reaction here may be due not to lecithin itself but to impurities.
When studying the chemical activation of the cnidae (Pantin, 1942) it was found that, while fats extracted with ether were ineffective, there were other highly active lipoid substances. These remained in the food after ethereal extraction but could be obtained from the residue by Soxhlet extraction with ethyl alcohol or acetone. Pecten gill and mantle was dried and extracted with ethyl ether in a Soxhlet for 3 hr. The residue was similarly extracted with acetone, and finally with ethyl alcohol. The dried alcoholic extract easily forms a suspension in sea water with the aid of a little NaHCO3. The resulting solution strongly sensitized the cnidoblasts to contact stimuli in concentrations at and above 0·3%. The same suspension on cotton wads caused a marked feeding response, with clinging of the tentacles to the wads, at concentrations of 1·0% or above. The extract is less active weight for weight than the gill and mantle from which it is extracted. At all concentrations it evokes a less thorough muscular response than solutions of natural foods. But if a 1 % solution of the extract is squirted locally on the tentacles, these bunch and contract as in the first stages of the feeding reaction. It seems therefore that the alcohol extract truly excites the tentacles and that the feeding reaction it produces with cotton wads is not simply due to the increased mechanical stimulus resulting from discharge of the cnidae.
CARBOHYDRATES
Maltose, saccharose, lactose, glucose, laevulose and starch produce no feeding reaction on cotton wads even in concentrations above 10 %. If the concentration of the sugar is sufficient for the osmotic pressure to be more than about twice that of natural sea water, the mouth may open if the solutions are squirted on it. This is probably due to osmotic stimulation, since high concentrations of sea water of the same order of osmotic pressure act similarly. In any case, neither the tentacles nor the cnidae show any reaction. There is, however, one carbohydrates glycogen, which is certainly effective. Parker (1928) found that ciliary reversal of the lips of Metridium was caused by glycogen as well as by food. Starch and glucose, on the other hand, had no effect. In Anemonia a suspension of glycogen on wads was found to give a characteristic feeding reaction though with little clinging of the tentacles to the wads. The limiting concentration required to produce an effect is, however, about 10%, so that it is far less effective than the majority of foods containing animal proteins. Further, of two separate samples of glycogen tested, one appeared to be three or four times as active as the other. It is possible that, as in the case of lecithin, the activity may be due to traces of adsorbed impurities.
OTHER ACTIVE SUBSTANCES
The derivatives of proteins and certain alcohol-soluble lipoids in food seem to be the only substances which normally evoke the feeding response in the tentacles. The fact that a substance has a high or low olfactory or gustatory activity in man gives no indication of its activity in the Actinians. We may note the high sensitivity of Anemonia to some kinds of mucus and its insensitivity to sugars. On the other hand, substances of high olfactory activity in man frequently have no effect. Thus M/10 NaSH, brought to pH 8 ·2, has no effect upon the tentacles, nor do the powerful substances indol and skatol. Henschel (1935) records that pieces of the manubrium of many medusae are very sensitive to the latter. But in the tentacles of Anemonia there is no such sensitivity. Saturated solutions, or even crystals of these substances, placed upon them have no effect at all, though squirted on the mouth they cause dilation.
There is however another response produced in the tentacles by a number of substances. It is a ‘withering’ retraction of the stimulated tentacle with excessive secretion of mucus. This occurs in sea water rendered acid below pH 4 with HC1, and with the lower fatty acids, acetic, butyric, valerianic, at 0 ·1 –1%. Triacetin at 3% acts in the same way. Certain substances which taste bitter to man produce withering. Quinine chloride (pH 7) causes it in concentrations of 0 ·3 %. A similar action of quinine is described by Nagel (1892) on the tentacles of Calliactis and by Henschel (1935) on the tentacles and manubrium of the hydromedusan Sarsia. Torrey (1904) records a like effect for picric acid in Sagartia davisi, and in Anemonia. Picric acid acts in Anemonia at the same concentration as quinine. But the most active substances for Anemonia appear to be bile salts and saponin. Fresh ox-gall, pure sodium taurocholate, pure sodium glycocholate, or saponin produce withering down to a concentration of 0 ·1 % :
While normally this withering response does not resemble the feeding reaction, in a few cases, both with quinine and with bile salts, a weak feeding response was obtained to cotton wads soaked in these substances just at the lowest limit of effective concentration, although controls of cotton-wool with pure sea water were rejected. But with quinine and bile salts a slight increase in concentration was sufficient to produce complete withering with no attempt at the feeding reaction. On the other hand, with true food substances the concentration can be raised 100-or even 1000-fold with only an increase in the speed and effectiveness of the feeding reaction. Nevertheless, the fact that substances in the bile salt group can under certain conditions cause a feeding reaction indicates that the mechanisms of feeding and withering are not altogether separate. Certain very soluble food derivatives which can produce an unquestioned feeding response cause withering in high concentration. This is the case with 10% solutions of glycine or of Witte’s peptone. This suggests that withering is simply the result of excessive general stimulation of the tentacle. This is the more probable because it can be produced by prolonged electrical stimulation of the tentacles at a frequency (e.g. 1 shock per sec.) well above that which induces a feeding response. If the feeding reaction is due to a prolonged low frequency train of nervous impulse, and rejection to a similar shorter train, withering may be due to a prolonged train of higher frequency.
MOUTH RESPONSE
The feeding response begins with the tentacles and is completed by opening of the mouth and ingestion of food. The sensitivity of the mouth was tested by squirting about 2 ml. of a known solution on it and comparing the effect with that on the tentacles. As Loeb (1895) said, the mouth is far more sensitive than any other part of the animal including the tentacles. But its response is less specific and a great variety of chemical stimuli cause it to open. Indeed, sea water concentrated to a 50 % increase in osmotic pressure causes some opening, and consequently all substances in concentrations sufficient seriously to increase the osmotic pressure exert such an effect. This is the case with the sugars, saccharose, maltose, laevulose, glucose and lactose, which are effective in the region of 10% concentration.
It is evident from Tables 2, 3 and 4 that the mouth is not only more sensitive but that its sensitivity differs qualitatively from that of the tentacles. There are substances like the amines and skatol to which the mouth is very sensitive which have no obvious effect on the tentacles. On the other hand, the sensitivity to saponin and taurocholate is about equal. A similar relative variation of sensitivity is evident in food derivatives. Most of these are about ten times as effective on the mouth as on the tentacles. But those amino acids which were without effect on the tentacles had no action on the mouth either. On the other hand, lecithin smeared on wads or 10% egg yolk cause great opening of the mouth but have little action on the tentacles. The mouth may normally respond to substances in the food which are not those that initiate the feeding response in the tentacles. The sensitivity is greatest however to protein derivatives, just as it is in the tentacles.
The mouth responds to food solutions by swelling and protruding as it opens just as it does to solid food. But to some substances in Table 4 the mouth opens by a contraction of the disk and mesenteries which causes the pharynx to gape in a manner not seen with food. Quinine and histamine produce this effect. This is to some extent true of the action of all the amines and physiologically related substances. For this reason and because they have no effect on the tentacles it is unlikely that they are effective agents in causing the natural food response.
Most of the mouth responses are rapidly paralysed by excess magnesium, indicating that they take place through the medium of sensory structures. But quinine, histamine, strong taurocholate (10%) and sea water at pH 2 cause gaping opening of the mouth even in the presence of excess magnesium (50% isotonic MgCl2), though the effect is then somewhat reduced. Magnesium inhibits the responses to all food substances.
CHEMICAL SENSE IN COELENTERATES
Nagel (1892) showed marked selective chemical sense in the tentacles of Calliactis parasitica and Actinia equina. Sardine meat was readily seized, while inert substances such as filter paper were rejected unless soaked in food juice. He also showed that ‘sugar’ failed to sensitize filter paper, while to quinine the tentacles responded by a retraction quite different from the feeding reaction. Our conclusions clearly agree with his. They are, however, contradicted by Fleure & Walton (1906). According to these authors, the feeding response of Anemonia tentacles is solely due to mechanical stimuli. Their evidence for this is that sometimes such stimuli alone will cause feeding; while the tentacles were only found to respond to chemical stimuli of comparatively high intensity. The latter observation is only founded, however, on responses to certain commercial food extracts, and we have shown not only that there is an unquestionable difference between the response to solid food and inert substances such as cottonwool, but also that the latter can be activated by immersion in low concentrations of certain food solutions, notably mucus.
The power of food solutions to activate inert objects so that they are accepted as food is found not only in actinians, but also in Hydrozoa and various medusae. Beutler (1924, 1926) showed in Hydra and various Hydrozoa that food masses such as pieces of gelatine which produced no feeding reaction were rapidly ingested in the presence of juices from crushed Daphnia or the body fluids of various animals. Similarly, Henschel (1935) showed that feeding reactions could be obtained in Aurelia aurita, Cyanea capillata and Sarsia tubulosa to filter-paper or clay soaked in suitable food solutions.
We have shown that, in Anemonia, the substances which call forth the feeding response are particularly proteins and their derivatives, including amino acids. Fats are ineffective, though certain lipoids which can be extracted from food by means of alcohol, though not by ether, are also fairly active. Carbohydrates are generally without action, except in the case of glycogen, the activity of which may perhaps be due rather to adsorbed impurities than to glycogen itself. This ineffectiveness of carbohydrates was also found not only by Nagel in Calliactis and Actinia, but also by Torrey (1904) in Sagartia davisi.
The marked effect of proteins, as opposed to other food derivatives, is particularly characteristic of the tentacles. The mouth has both a higher chemical sensitivity and responds to a wider range of substances. Nevertheless even here, as we have shown, the reaction to proteins predominates, while there is no apparent sensitivity to sugars. In different coelenterates, a variety of different structures are involved in feeding, and as with the mouth and tentacles of Anemonia these do not necessarily have the same range of chemical sensitivity. However, the strong sensitivity to proteins and lack of response to carbohydrates other than glycogen appears to be fairly general. Parker (1905, 1928) found that ciliary reversal on the lips of Metridium took place to such substances as natural foods, Witte’s peptone, and as paraginic acid. Cane sugar, glucose and other sugars, together with starch, had no effect. Glycogen on the other hand, was found to cause reversal Whatever the nature of this phenomenon, it is evidently controlled by a somewhat similar range of sensitivity to that controlling the responses of the tentacles in actinians.
The experiments of Henschel on the manubria of various medusae show a parallel range of sensitivity. Here again local feeding responses are obtained to proteins, peptones and various amino acids, just as they are to food solutions. Sugars and starch are remarkable for their ineffectiveness. In these animals, glycogen also is without action. On the other hand, Henschel found that the isolated mouth of Aurelia gave a strong positive reaction to triolein, oleic and palmitic acids, which substances are without effect on the tentacles of Anemonia.
Skatol, which we have seen affects the mouth though not the tentacles in Anemonia, has a variable effect in the medusae studied by Henschel. It has little action in Cyanea, more in Aurelia, while the manubrium of Sarsia is extraordinarily sensitive to its presence.
It seems therefore that though there is some individual variation, both with different species and’ with different parts of the animal, a number of very different coelenterates appear to be characterized by a high sensitivity to proteins and a low sensitivity to carbohydrates. On the one hand, there is an obvious correlation between this and the carnivorous mode of life of these animals. On the other, the restricted range of substances to which these animals are sensitive as compared with man should not necessarily be supposed to be due to a condition-of primitive simplicity. Not only do we see cases of individual sensitivity to fats and such substances as skatol, but there seems to be some mechanism by which there is a high degree of discrimination between related chemical substances. This is notably true of mucus of prey as opposed to that of animals of the same species. There is a related phenomenon among animals commensal with anemones. Thus Thomson (1923) has shown that the crab Stenorhyncus phalangium normally associates with Anemonia sulcata and during life is untouched by the anemone. When it dies, however, its body is readily taken up and eaten.
ACKNOWLEGMENTS
Most of this work was done at the Marine Biological Laboratory, Plymouth. Our thanks are due to the Director and staff of the laboratory for the many facilities they gave us.