EFFECTOR SYSTEMS OF ACTINIANS 475 



may issue through a single cinchs. If the stimulus is unilateral, 

 the acontia are discharged chiefly on the side stimulated, as 

 Torrey ('04, p. 208) has already noticed in Sagartia. The 

 acontia do not emerge in consequence of their own activity 

 but are carried outward by the streams of water that are escap- 

 ing under pressure through the mouth and the cinclides. After 

 the acontia have emerged they are in no sense directed toward 

 external objects, harmful or useful, but rest in long straightish 

 lines on the surface of the actinian or they are wafted about shght- 

 ly by the currents of water. They gradually disappear' by 

 being drawn back into the animal. They are ciliated and the 

 effective stroke of their cilia is vigorous enough to move them 

 bodily and is always toward their free ends, hence they themselves 

 are moved by this stroke back toward their attachments. The 

 discharge and the return of the acontia, therefore, are processes 

 in which their contained muscle plays no part. 



If an extended acontium is stimulated by having seawater 

 vigorously squirted on it, in the course of one or two minutes 

 it gradually draws itself up into a close snarl. If seawater con- 

 taining a little meat juice is used instead of pure seawater, the 

 snarl is more pronounced and remains for a longer time. Sooner 

 or later the acontium untwists and straightens out preparatory 

 to its withdrawal into the body of the actinian. The twisting 

 and contorting of the acontium is brought about through the 

 contractions of its longitudinal muscle the fibers of which, as 

 already mentioned, are closely applied to the mesogleal axis 

 of this organ. Evidence has already been advanced (Parker 

 and Titus, '16, p. 451) to show that the acontial muscle is an 

 independent effector and not under the influence of nerves and 

 that no nervous transixiission occurs through the acontia. The 

 straightening out of the coiled acontium is due in part to its 

 ciliary activity and in part probably to the elasticity of its 

 mesogleal axis against which its muscle probably acts. 



TJie longitudinal and circular muscles of the tentacles. In a 

 resting, expanded Metridium the tentacles are usually quiescent, 

 radially disposed, and directed in the main away from the 

 mouth. If a tentacle in such an animal is touched or otherwise 



