120 G. H. PARKER 



as internal resisting supports and are marked by depressed lines 

 on the surface. If one of these bulging faces is punctured by a 

 needle, water will flow freely from it and the chamber will 

 collapse for a time. It is thus clear that the projection of the 

 front edge of the disc as the locomotor wave reaches it, is de- 

 pendent upon the pressure of the contained fluid acting on the 

 relaxed walls of that region. How considerable this pressure is 

 has not been determined, but it is without doubt the chief factor 

 in forcing the anterior edge of the disc forward. A small glass 

 tube carefully inserted into the body of a large creeping Metri- 

 dium showed an internal pressure in one instance of 5.3 cm. of 

 water and in another of 6 cm. Both these pressures must be 

 decidedly above the normal, for in both instances the insertion of 

 the tube caused the animals to contract somewhat and thereby 

 to increase their internal pressure. The mechanism of creeping, 

 then, depends upon the action of the muscles of the pedal disc 

 and its immediate vicinity on the fluid which is contained in 

 the animal under a pressure of less than 5 or 6 cm. of water. 

 This method of producing locomotion, without the participa- 

 tion of skeletal parts, has long been recognized in many inverte- 

 brates and is generally admitted for such organs as the foot of 

 the gastropod. 



Ever since the work of the Hertwigs ('79-80) it has been 

 customary to assume that the normal neuromuscular activities 

 of the actinian body were more or less under the control of at 

 least slightly centralized nervous organs, which were believed to 

 be located in the oral disc. To what extent are the creeping 

 movements of the pedal disc dependent upon such oral centers? 

 To answer this question experiments were carried out on Sagar- 

 tia luciae. Fully expanded, attached specimens of this actinian 

 were suddenly cut transversely in two with a pair of sharp scis- 

 sors. The oral pieces thus cut off and carrying with them in 

 each case the whole of the oral disc, tentacles, etc., were dis- 

 carded. The attached pedal discs and remaining portions of 

 the columns were held under careful observation. These con- 

 tracted at the level at which they had been cut so as to look 

 like an actinian normally withdrawn. They soon filled them- 



