86 THE ELEMENTARY NERVOUS SYSTEM 



show no special grouping, being uniformly distributed 

 around the whole tentacle. 



2. The radial muscle of the oral disc is made up of ir- 

 regular dense bundles of ectodermic fibers more or less 

 imbedded in the supporting lamella of the disc. They 

 radiate from the region of the mouth outward toward the 

 periphery of the disc, making their way between the bases 

 of the tentacles. 



3. The circular muscle of the oesophagus ensheathes 

 the oesophagus on its entodermic face. It is not very 

 strongly developed and its fibers, which take a circular 

 course, are more or less interrupted where the complete 

 mesenteries are attached to the cesophageal wall. 



4. The circular muscle of the oral disc is a flat cir- 

 cular ring, whose fibers take a course concentric with the 

 mouth and are often much involved in the supporting la- 

 mella of the disc on its entodermic side. 



5. The circular muscles of the tentacles are conical 

 sheets of muscle on the entodermic side of the supporting 

 lamella of those organs. In each muscle the fibers take 

 a circular course and are fewer in number and finer than 

 in the longitudinal muscle of the tentacle. They show no 

 special differentiation except at the base of the tentacle, 

 where there is a slight tendency to form a sphincter. 



6. The circular muscle of the column is a well-devel- 

 oped sheet of cylindrical fibers covering the entodermic 

 face of the supporting lamella of the column from its 

 attachment to the pedal disc to the region of its transition 

 to the oral disc. The fibers in their circular course pass 

 the lines of attachment for the mesenteries at right angles, 

 but are not to any great extent interrupted at these lines. 



7. The sphincter is a firm circular band of muscle 

 fibers embedded in the supporting lamella of the column 



