EFFECTOR SYSTEMS OF ACTINIANS 471 



THE MUSCULAR SYSTEM 



The muscular system in Metridium consists of at least thir- 

 teen muscles or groups of muscles (Parker and Titus, '16) which 

 show a rather unusual degree of differentiation when it is re- 

 called that this animal is one of the lower metazoans. Not- 

 withstanding this specialization, Metridium, like most other 

 actinians, goes into a state of such complete contraction when 

 \igorously stimulated and remains for so long a time entirely 

 closed that the general impression made by it is that it is a sira- 

 ple muscular sac capable of only one general form of response. 

 This condition is both misleading and instructive. 



The prolonged contraction of Metridium shows that one of 

 the most characteristic features of actinian muscle, like that 

 of many other invertebrates, is its very high degree of tonicity 

 as compared with the corresponding tissue of vertebrates. This 

 contrast has been emphasized of late by von Uexkiill ('09 b) 

 and especially by Jordan ('07, '08, '12). 



This exhibition of excessive tonicity, however, has led inves- 

 tigators away from a finer scrutinizing of the muscular res- 

 ponses in actinians, responses which will be found to justify, 

 I believe, the high degree of anatomical differentiation already 

 pointed out. It is from this standpoint that I wish to give 

 an account of some of the muscular reactions of Metridium. 



The- longitudinal and parietal muscles of the mesenteries. These 

 muscles extend lengthwise of the mesenteries and for the most 

 part from the pedal disc to the oral disc. By their action the 

 oral disc with its crown of tentacles is drawn down toward the 

 attached pedal disc (Hertwig, '79, p. 527). In the non-directive 

 mesenteries, the longitudinal muscles are on the endocoel faces 

 of these organs, on which they show as a thickened median band 

 which spreads out as one approaches the oral or the pedal disc. 

 The arrangement and concentration of the fibers in these mus- 

 cles is especially favorable for a depression of the oral disc. 

 On the directive mesenteries the longitudinal muscles are on 

 the exocoel faces and each muscle is thickened as a longitudinal 

 ridge close to the accompanying siphonoglyph. As the wall 



