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Muscular and Hydrostatic Action in the Sea-Anemone Metridium Senile (L.)
1 Zoological Laboratory, University of Cambridge
1. In contrast with most other Actinians, Metridium senile exhibits a great variety of shapes of the body. These are brought about by continual slow muscular activity.
The mechanics of muscular action are discussed. The action of most of the muscles is extremely slow. An isotonic contraction of the parietal muscles requires 40-60 sec. to reach its maximum and many minutes to relax. The body wall is capable of extension by about 400%. There are limits to extensibility in the normal animal. The mechanisms by which the animal itself increases or reduces extension by controlling its coelenteric volume are described. Fluid is gained chiefly through the siphonoglyph, though under certain conditions there may be suction into the coelenteron. Fluid is lost chiefly through reflex opening of the mouth. From time to time Metridium empties itself of fluid, and then refills in a few hours. A rate of refilling of 14 c.c./hr. has been measured.
2. Pressure changes in the coelenteron which occur during activity show that both retraction and extension of the column are active processes involving a rise in pressure which enforces reciprocal extension of the opposing musculature.
3. The relation of normal activity and shape to the coelenteric pressure is shown. This average pressure is extremely low; about 2-3 mm. of water. In a moderately filled unstimulated animal the natural muscular contractions are accompanied by a rise in pressure not generally exceeding 6-7 mm. of water. In such animals the natural contractions are of considerable extent, reaching over 30% of the body length.
4. By experimental inflation of the coelenteron with sea water, the system can be made to work more isometrically. The extent of movement is reduced and the animal may appear inactive. The presence of considerable though ineffective muscular activity is shown by the fact that large pressure changes (up to about 12 mm. of water) now take place. By raising the coelenteric pressure increased contractile activity in the body wall may actually reduce the extent of movement.
5. The isometric pressure which the body wall can develop in the coelenteron has been estimated. Pressures developed during natural contractions of a moderately filled animal demand muscular tensions in the body wall ranging between 20 and 50% of the isometric tension. The range of tension corresponds to that which would be most mechanically efficient if Metridium muscle resembles that of other animals.
6. An estimate is deduced from the coelenteric pressure of the isometric tension developed by the circular muscle of the column of Metridium. It is about 3-5 g./cm. of body wall transverse to the muscle. This is in agreement with direct observation of the isometric tension developed by strips of circular muscle. This tension in the column may correspond to a tension of 40 kg./sq.cm. of the individual muscle fibres and is very much greater than the values obtained from the frog's sartorius.
7. The extensive responses of the powerful retractor muscles involve much greater pressures (40-100 mm.) than those against which the column muscles can operate. The development of these muscles is related to the necessity of speed of action in a system undergoing great deformation.
8. Muscular action in a hydrostatic skeletal system is contrasted with that in the jointed skeletal system of Vertebrates and Arthropods. The former system is characterized by slowness of action and great change of length. In contrast with the Vertebrate skeletal system, in the hydrostatic system reciprocal muscular action is not localized. The movement of every muscle influences the mechanical conditions of every other in the system. Each muscle has two actions, a local direct action, and an indirect action, as in the elongation of Metridium on contraction of the circular muscles. The consequences of this are discussed.
Submitted on March 1, 1950
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