The Circulatory Physiology of Helix Pomatia: I. Observations on the Mechanism by Which Helix Emerges from its Shell and on the Effects of Body Movement on Cardiac Function

Previously reported work on the cardiac physiology of Helix pomatia has been concerned with partly dissected animals or isolated hearts. The present work has been concerned with observing the living animal carrying out normal movements under the minimal amount of experimental interference. The way in which the animal emerges from its shell has also been investigated.

During the breathing cycle the muscular floor of the mantle cavity (roof of cephalopedal haemocoel) is depressed, thereby increasing the volume of the cavity, while the pneumostome opens and air enters. The pneumostome then closes and the mantlecavity floor rises, reducing the volume of the cavity (Meisenheimer, 1912). These movements are accompanied by characteristic changes in heart rate which have been shown to be under nervous control in the partially dissected animal (Rywosch, 1905; Carlson, 1905; Zubkov, 1935; Jullien & Ripplinger, 1953; Jullien et al 1960; Meng, 1958). In the present experiments the cardiac nerve has been severed, leaving the animal otherwise intact so that the hydrodynamic effects on heart rate could be investigated.

The course and structure of the anterior (cephalic) aorta has been investigated in Helix pomatia, Helix aspersa, Arion ater and Archachatina purpurea.

The main experimental animals used were specimens of Helix pomatia weighing 15–20 g.

The only preparation carried out before taking cinematograph recordings of the snails was to make a small hole in the shell over the heart region and to break away about 1 cm off the lip of the shell so that the mantle edge showed clearly.

In order to measure the frequency of heart beat the shell over the heart region was decalcified by dropping on small amounts of concentrated hydrochloric acid until the shell became transparent. The procedure was carried out over 10–15 min in order to minimize the heating of the animal. Each systolic contraction of the ventricle and any body movements were recorded by two manually operated markers registering on a revolving smoked drum.

In order to observe the movement of the floor of the mantle cavity, pericardium and heart, a piece of shell of approximately 1·5 cm square was sawn out over the posterior part of the mantle cavity. A similar hole was made in the roof of the mantle cavity by burning through the tissue with a hot needle avoiding any of the larger pulmonary veins and sealing off the smaller. A loop of thread was passed through the mantle at each corner of the opening and anchored to the sawn edge of the shell in order to retain the intact mantle in its normal position against the shell. The opening in the shell was covered by a piece of heavy-duty Polythene sheet stretched to match the curvature of the shell. The adhesive used to glue the Polythene in place was ‘U.H.U.’ (Werk. H.u.M. Fischer, Buhl (Boden)).

In order to destroy the nerve connexions to the heart a piece of shell 1·5 cm square over the upper surface of the kidney was removed and a hot needle was used to make an incision in the mantle between the rectum and the floor of the mantle cavity. When the rectum was drawn out, the auriculo-renal nerve could be seen passing across the border of the visceral haemocoel and the kidney. A section of the nerve was pulled out from this point, usually breaking both the auriculo-renal and ventricular branches of the visceral nerve. The window in the shell was covered by Polythene sheet as described above. After the operation 3000 i.u. of penicillin and 3000 i.u. streptomycin were applied to the site, and experiments were carried out 3–4 days later when the condition of the surviving animals appeared normal. At the end of the experiment each animal was dissected to ascertain the extent of denervation; a representative specimen is shown in Plate 2. The pericardial pressure changes were measured using an S 19 hypodermic needle (2 cm long, bore 0·3 mm) connected by 80 cm of rigid Polythene tubing (bore 1 mm) to a Davson & Purvis closed water manometer (Davson & Purvis, 1959). A continuous photographic record of the pressure changes was made, and from this the heart rate could be measured. The apparatus is described more fully elsewhere (Chadwick (Sommerville), 1962).

The diagrammatic cross-section of the snail (Text-fig. 1) shows the relationship between the body cavities.

When the shell over the mantle cavity was removed it was noted that the muscular floor of the cavity was depressed each time the pneumostome opened and that the depression was particularly marked when the animal was emerging.

As the snail emerges there is a clearly defined sequence of events. First the rate of opening and closing of the pneumostome increases for a few seconds and then the pneumostome opens wide and the posterior part of the foot protrudes from the centre of the mantle collar, followed by the rest of the foot and head. Often the pneumostome closes during this period and protrusion then stops, the animal sinking back into the shell a little. The pneumostome opens again and protrusion continues. Once the foot and head are well out complete extension takes place irrespective of the pneumostome, which is usually opening and closing rapidly. These points are illustrated in Plate I(a)-(m), which is composed of consecutive frames from a cine film taken at 12 frames/sec.

Frames (Pl. 1) illustrate a further point. Frame (f) differs from frame (e) in that the mantle is clearly visible pressed against the small hole in the shell and the extended part of the body is curved upwards, its volume appearing smaller than in frame (e). These signs suggest that the mantle cavity floor was raised between the two frames, so increasing the pressure within the cavity and causing blood to be displaced from the anterior to the posterior part of the cephalopedal haemocoel. The upward curve of the head suggests that the flow of blood was assisted by contraction of the anterior musculature. Frames (g) and (h) illustrate the reverse sequence of events.

If the tentacles and head of an extended animal are pinched, the pneumostome closes (if open) and rapid retraction occurs. The air displaced from the mantle cavity is directed medially by a flap of mantle collar to escape through a slit in the collar immediately above the head (Pl. I n-p). The mantle collar seems to release mucus so that the displaced air reaches the outside as a mass of bubbles (frame p). A modification of this activity produces a large bubble screen if the animal is disturbed. Usually no bubbles are produced upon spontaneous retraction, although the air follows the same outlet. This suggests that the mucous secretion is under independent nervous control while the pneumostone movements are linked with those of the mantle cavity floor.

The heart rate was measured by direct observation through the decalcified shell.

The heart rate was found to increase with temperature and to be higher at any particular temperature during the summer than during the winter.

The heart rate of the retracted, inactive snail was found to be about 60% of that of the extended, active animal. The heart rate also varied during the breathing cycle, being faster and more regular when the pneumostome was open than when it was closed (see Table 1, part I).

In order to discover how the nervous system influenced the rate of heart beat, the rate was recorded after the cardiac nerves had been broken. On completion of the experiment the animals were dissected to check that denervation was complete.

Recordings were obtained from four animals on the 3rd and 4th days after denervation. Their condition seemed normal. Animal H is shown after dissection (Pl. 2). The broken cardiac nerve can be seen with the original course of the nerve dotted in. The cardiac nerve of animal J was in the same condition, but in animal G only the auricular branch had been severed although the ventricular branch was damaged and probably non-functional, and in animal L the whole visceral nerve had been broken just proximal to the origin of the cardiac nerve. Table 1 shows an analysis of the traces obtained from animals G, H, J and L and from four intact animals, A-D. It will be noted that the direction of change in heart rate was inconsistent in the denervated animals whilst the heart rate was lowest when the pneumostome was closed in the intact animals. There is no obvious correlation between the variation in pericardial pressure associated with breathing activity and the heart rate.

By shining a torch light through the plastic-covered opening made in the shell and mantle it could be observed that :

• (1) There was always some part of the pericardial wall in contact with the air in the mantle cavity. When the pneumostome was open, the floor of the cavity was depressed, exposing the whole of the ventral surface of the pericardial wall.

• (2) Ventricular diastole accounted for a smaller part of the cardiac cycle than did systole.

• (3) The total volume of the heart and pericardial cavity appeared greater when the animal was active than when it was inactive.

• (4) The total volume of the heart and pericardial cavity was greater at ventricular diastole than at ventricular systole.

• (5) The pulmonary vein showed rhythmic expansion and contraction, reaching its greatest diameter just before maximal diastole of the ventricle. This pulsation was more marked when the pneumostome was open.

• (6) The overall diameter of the pulmonary vein was greater when the pneumostome was open than when it was closed.

• (7) Pulsation was clearly visible in the anterior (cephalic) aorta but not in the posterior (visceral) aorta, indicating that the first aortic valve (see below) collapsed to occlude the posterior aorta so directing the main flow of blood into the anterior aorta at each ventricular systole.

The anatomy of the anterior (cephalic) aorta of Helix pomatia shows several points of physiological importance. There is a well-developed semilunar valve (the first aortic valve) on the common aorta at the point where this vessel passes through the pericardium (Text-fig. 1) This valve was also observed in a similar position in Helix aspersa, Archachatina purpurea, Monodonta (Gibbula) cineraria and Arion ater. The position of the valve indicates that when it collapses it will occlude the posterior aorta so that blood cannot enter that vessel during ventricular systole but can only enter as backflow from the anterior aorta during ventricular diastole.

In Helix a second semilunar valve was found in the anterior aorta just after the point where it crosses the spermathecal duct in the visceral haemocoel. Just anterior to the second valve the aorta passes through the thin septum which separates the visceral from the cephalopedal haemocoel (Text-fig. 2). This second aortic valve could not be found in the other species mentioned above. In Archachatina purpurea the anterior aorta runs along the floor of the mantle cavity, above the sheet of muscle which forms the roof of the cephalopedal haemocoel, until it reaches the septum separating visceral from cephalopedal haemocoels; there it passes through a muscular constriction in the sheet to enter the cephalopedal haemocoel. No valve or muscular constriction at this point could be found in Arion.

These points are summarized in Text-fig. 2 as well as the arrangement found in Monodonta (Nisbet, 1953).

The ciné film and direct observations suggested that the extension of the foot is functionally linked with the depression of the floor of the mantle cavity, which explains Meng’s observation (1958) that both movements result in an accelerated heart-beat Meng also noted a corresponding fall in rate of heart-beat when the mantle cavity floor was raised or the foot was retracted.

Rywosch (1905), Carlson (1905), Zubkov (1934), Jullien & Ripplinger (1953), Jullien et al. (1960) and Meng (1958) have all demonstrated that nerves control changes in heart activity. The results of the experiments carried out here on snails in which the cardiac nerves had been severed indicated that a definite correlation between respiratory and cardiac activity still existed in the absence of the visceral nerve connexion, although the direction of the change in heart rate accompanying particular parts of the respiratory cycle was not consistent. Ripplinger (1953) stated that an additional nerve supply to the heart is provided by the left pallial nerve, which carries impulses increasing the tone and amplitude of beat but not affecting the frequency. This nerve would have been intact in the ‘denervated’ snails used here, but as the analysis of the results was confined to the frequency of beat the conclusion that mechanical as well as nervous elements are involved in the control of heart rate is not affected.

Ramsay (1952) and Krijgsman & Divaris (1955) put forward a compensation-chamber theory involving the pericardial cavity to account for the filling of the molluscan heart chambers in the absence of a veno-auricular valve. This theory requires that the volume of the pericardial cavity is constant so that the combined volume of the auricle and ventricle is always constant. The results of the present investigation show that the total volume of the heart varies with the state of activity of the animal and also that part of the pericardium is always in contact with the air in the mantle cavity. Jones (1971) states that the pericardium is kept rigid by the slight pressure (1–3 cm H₂O) of the pericardial fluid in excess of atmospheric pressure. However, this situation could not be maintained when the pneumostome closes since the mantle-cavity pressure then rises from 1 to 8 cm H₂O above atmospheric pressure (Sommerville, 1973 b).

Civil & Thompson (1972) have shown that the isolated heart will only pump fluid if it is surrounded by a rigid, sealed chamber, but Schwartzkopff (1954) and Sommerville (1973 a) have shown that efficient pumping occurs without a pericardium providing the first aortic valve is included in the preparation.

Although the present investigations indicate that the ‘compensation chamber’ theory of heart filling is not tenable in its strictest interpretation, there may be sufficient lag in the deformation of the pericardial wall to contribute some suction effect tending to distend the relaxed heart chamber.

Noll (1929) and Schwartzkopff (1954) mentioned a valve on the aorta of Helix pomatia which is the first aortic valve of the present report. Since it occurs in the other species investigated, this valve may be common in gastropods. The existence of a second aortic valve near the junction of the visceral and cephalopedal haemocoels in H. pomatia and H. aspersa has not been reported before. Although this valve does not exist in Monodonta, Nisbet (1953) has described a muscular constriction surrounding the aorta at the same point in M. linneata. The arrangement of the second aortic valve of Helix is such that it would prevent backflow of blood if the haemocoelic pressure rises above the aortic pressure. In Archachatina and Monodonta the muscular constriction surrounding the aorta would presumably contract with the floor of the mantle cavity so restricting the backflow of blood in the aorta at a time when the haemocoelic pressure was high. Since no equivalent structure could be found in Arion it seems likely that the second aortic valve or muscular constriction helps to buffer the heart of gastropods with a well-developed shell from the wide pressure fluctuations which occur in the cephalopedal haemocoel (see Sommerville, 1973 b).

I wish to thank Professor A. Graham for his advice, encouragement and for the facilities of the University of Reading where most of this work was carried out; Dr H. Dawson and Mr C. Purvis for their advice and gift of manometers; Dr R. H. Nisbet and Professor R. J. Linden for their help; my husband, Dr A. Chadwick, for his great help with the work and the preparation of this paper; and Professor J. M. Dodd and the Department of Pure and Applied Zoology, University of Leeds, where this work was finished.

(a)-(m) Ciné film of snail emerging taken at 12 frames/sec. t, Time (sec) from beginning of sequence. (n)-(p) Ciné film of snail producing a bubble screen, taken at 12 frames/sec.

Dissection of snail H to show degree of denervation.