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Muscular and Hydrostatic Action in the Sea-Anemone Metridium Senile (L.)

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Muscular and Hydrostatic Action in the Sea-Anemone Metridium Senile (L.) [264] MUSCULAR AND HYDROSTATIC ACTION IN THE SEA-ANEMONE METRIDIUM SENILE (L.) BY E. J. BATHAM AND C. F. A. PANTIN Zoological Laboratory, University of Cambridge (Received i March 1950) (With Plate 7 and Nine Text-figures) The majority of sea-anemones present to the eye an almost constant shape which only changes as the result of activities of a very simple kind. This is true, for instance, of Calliactis parasitica, the subject of an earlier study (Pantin, 1935). The unstimu- lated animal rarely departs from the form of a short cylinder surmounted by a disk bearing the tentacles round its edge and the mouth at its centre. In contrast with Calliactis, Metridium senile is notable for the variety of shapes which it assumes (PL 7). It may be fully expanded with a narrow or with a wide column (e, /). The same anemone may next day be found contracted down to a mere button-like object (d). It may indulge in a variety of contortions in which it is laterally bent, or possesses deep transverse constrictions locally or completely embracing the column (g, i). The whole anemone may on occasion temporarily assume a shrivelled and contracted condition (h), in which it might easily be discarded from the aquarium as dying; but which, in fact, is a normal temporary state from which there is com- paratively rapid recovery (i, j). At other times, particularly during locomotion, a part of the body may become greatly distended (c). Prolonged observation of Calliactis shows that even here there is some variation of shape, though less evident than with Metridium. Casual observation of an unstimulated Metridium rarely shows visible movement. But close and prolonged inspection shows that the shape of the animal at any moment is brought about by extremely slow but continual activity. We shall discuss the nature of this activity in subsequent papers. It is not simply the immediate conse- quence of external stimuli. But before we discuss the origin of this slow but active change of shape, it is necessary to consider the relation of the shape of the animal to the action of its peculiar system of musculature. This is the object of the present paper. THE MUSCULAR SYSTEM The muscular organization of Metridium will be discussed in another paper (Batham & Pantin, 1951). The body consists essentially of a cylindrical sac, the wall of which is composed of a layer of mesogloea, each side of which is covered with epithelium. The cavity of the sac is traversed by vertical mesenteries arising from its wall and attached to the oral and pedal disks, which close the top and bottom of the cylindrical column (Text-fig. 1). Like the body wall, the mesenteries consist of a layer of mesogloea covered on each side by epithelium. Muscular action in Metridium 265 Muscle sheets consisting of a single layer of muscle fibres are developed on one or both surfaces of the mesogloea where the epithelial layers rest upon it. The sheets consist of fields of muscle fibres which reach 1-2 mm. in length when fully extended, but which can contract down to 200-300/i. in free retraction. This great extensibility of the muscle fibres gives the body wall very great power of deformation. Within wide limits of extension the muscles have no fixed resting length. Unlike its effect on vertebrate skeletal muscles, a light load can cause their steady extension to a new length which they retain on removal of the load with very little elastic recovery (Jordan, 1934; Kipp, 1939; the 'plastic' extension of these authors). Under natural conditions, the animal in maximal contraction becomes a flattened rugose cone in which the body wall is dense and firm. At maximal elongation, the parietal muscles of the body wall are about 400 % longer than at maximal contraction (PI. 7). The comparison of the maximum and minimum diameters of the column shows that this is also about the normal range of extensi- bility of the circular muscle. Isolated pieces of tissue can be extended beyond this, but very great extension is partly irreversible and ends in the tissue gently parting. In life, threads sewn into an animal for experimental purposes and subjected to the light tension of a recording lever will during many days gradually pull themselves through the extended tissue which heals up behind the wound. The ability of the body to undergo deformation depends not only on the extensive contractility of the muscle fibres but also on their relation to the mesogloea. We shall show elsewhere (Batham & Pantin, 1951) that contraction and extension enforces great changes in thickness of the mesogloea as a whole, but that the extensibility of that layer of it to which the muscle fibres are attached is much less than that of the body of the mesogloea. Thus the limit of extension of the column is reached when the sheet of its circular muscle is extended to form a single unfolded layer of muscle fibres. When, on the other hand, the body wall contracts and the mass of the meso- gloea shortens and thickens, the circular muscle layer and the layer of mesogloea to which it is immediately attached does not shorten. It buckles; thereby throwing the circular muscle layer into folds at right angles to its fibres. When contraction approaches its limit, as in PI. 7 d, buckling of the muscle layer has already reached its limit and a second order of buckling, of the whole body wall, occurs. Beyond this, further shortening cannot take place. Complete unfolding and complete buckling set the limits of extension and contraction (Batham & Pantin, 1951). This phenomenon of buckling seems to be characteristic of all the muscle sheets of Metridium. Where specific effectors such as the marginal sphincter or the mesen- teric retractors are developed, over-growth of the muscle layer leads to the very great folding so familiar in sections of sea-anemones. Much of this folding is permanent and cannot be smoothed out during expansion of the animal but it is essentially only a special case of the buckling phenomenon. The body wall of Metridium may be divided into three functional regions which are to some extent independent of each other. The region of the tentacles, capitulum and disk is primarily concerned with feeding. The region of the pedal disk is concerned with adhesion and locomotion. It is the region of the column which is 266 E. J. BATHAM AND C. F. A. PANTIN responsible for the more obvious changes of shape in the animal, and it is with this region that we are primarily concerned in this paper. In the resting state the column is a cylinder closed at the base by the foot and at the upper surface by the oral disk. Constriction of the circular muscle fibres-narrows the column. When this happens the compressed coelenteric fluid enforces an ex- tension of the longitudinal muscles of the column system (Text-fig, i). Sphincter muscle Circular muscle Perfect Ciliated mesentery siphonoglyph Retractor Parietal muscle muscle Parietal—*\ muscle Text-fig, i. Diagram of chief musculature of column of Metridium. a, section through whole specimen, showing endocoelic faces of a non-directive perfect mesentery on left, of a young imperfect mesentery on right; 6, exocoelic face of a non-directive perfect mesentery, showing direction of radial muscles. Broken lines indicate ppsition of retractor muscle on opposite face. The longitudinal musculature of the column is provided by the ' parietal' muscles. These are longitudinal bands developed most greatly in the youngest cycles of imperfect mesenteries just at the base where they are attached to the column. In the older imperfect mesenteries and in the perfect ones, strong retractor muscles become developed attaching the oral disk to the foot (Text-fig, i); but the parietal muscle immediately in contact with the column wall is less developed. The origin and relations of the parietal system are considered elsewhere (Batham & Pantin, The tension set up by the coelenteric pressure on the thin-walled oral disk is carried by the retractors and other longitudinal fibres of the mesenteries. In the uppermost and lowermost parts of the mesenteries these muscle fibres spread out in a fan-like manner on to the oral and pedal disks, so that they have insertions of considerable length along a radius in the two disks. The retractors have two distinct functions. Their slow tonic contraction plays its part in maintaining the shape of the column in concert with the parietals. But on sudden stimulation their rapid contraction leads to withdrawal of the disk and closure of the anemone. In this fast response, the powerful retractors act antagonis- Muscular action in Metridium 267 tically not only to the circular muscle of the column but also to the parietals. The sudden withdrawal of the disk may cause a general bulging outwards of the whole body wall. The concurrent contraction of the marginal sphincter results in the animal passing from a cylindrical to a hemispherical shape (PI. 7 a). The only other mesenteric muscles which concern our present study are the radial muscles of the perfect mesenteries (Text-fig. 1 b). The contraction of these opens the pharynx. Their contraction causes vertical lines to appear at five or six places in the body wall (PI. 76). These lines correspond to the insertion of the pairs of perfect mesenteries, the number of which varies in different individuals in Metri- dium (Parker, 1897). We shall see that this reflex is important in the control of the coelenteric volume. THE SPEED OF MUSCULAR CONTRACTION We will now consider the mechanics of muscular action in the column system.
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