PEDAL LOCOMOTION IN ACTINIANS 119 



as is well known (compare Parker and Titus, '16), all the muscles 

 of the pedal-disc region are entodermic, there being no ectoder- 

 mic muscles in this part of the animal. Next the supporting 

 lamella of the disc and on its entodermic face is an extensive sys- 

 tem of circular muscle fibers, the circular muscle of the pedal disc, 

 covering the whole inner surface of the disc and arranged con- 

 centrically with respect to it. To the inner face of the pedal disc 

 are attached in radial arrangement all the mesenteries; the larger 

 ones extending from the outer edge of the disc to its center, the 

 smaller ones reaching from the edge only part way to the center, 

 and the smallest ones being limited almost to the edge itself. 

 Each mesentery carries on its two sides and next its region of 

 attachment to the disc slight muscle-bands, which extend from 

 near the edge of the disc to or at least towards the center of this 

 structure. These muscle bands, which are known as the basilar 

 muscles, have therefore radial courses and cross the circular 

 fibers of the disc at right angles. Beside the circular fibers and 

 the radial fibers just mentioned, the longitudinal fibers of the 

 mesenteries terminate in the disc, many of them passing almost 

 perpendicularly from the disc into the mesenteries. It there- 

 fore follows that the pedal disc at most points, but especially 

 towards its edge, may be said to be provided with three systems of 

 fibers mutually at right angles to one another, the whole arrange- 

 ment being strictly on a radial plan. The supporting lamella of 

 the pedal disc like that of the wall of the column contains a rich 

 supply of interlacing nerve fibers (Parker and Titus, '16). 



The creeping movements of the pedal disc are carried out by 

 the neuromuscular mechanism just described. As the loco- 

 motor wave begins at the hind edge of the disc, the initial con- 

 traction probably includes all three sets of muscles but especially 

 the radial fibers and the mesenteric fibers. These muscles work 

 on parts whose fluid contents are under some pressure from the 

 general muscle tonus of the body as a whole. 



As the wave passes over the pedal disc and approaches the 

 front edge of that organ, this edge can be seen to be much dis- 

 tended by the pressure of its fluid contents. The walls of the 

 edge are thin and bulge out between the mesenteries which serve 



