THE NEUROMUSCULAR STRUCTURE 87 



but so distributed that they show at once that they have 

 been derived from an entodermio source. In fact, the 

 sphincter is merely a differentiated part of the circular 

 muscle of the column. 



8. The circular muscle of the pedal disc is a well-de- 

 veloped, vigorous organ composed of a system of fibers 

 concentric with the disc and more or less imbedded as 

 circular bundles in the inner face of the supporting la- 

 mella of the disc. 



9. The basilar muscles are radial strands that extend 

 along the mesenteries at the junction of these organs with 

 the pedal disc. There is a pair of these muscles for each 

 mesentery and they vary in length in accordance with 

 the size of the mesenteries to which they are attached. 

 These muscles cross the fibers of the circular muscle of 

 the pedal disc at right angles and lie only a very short 

 distance orally from them. 



10. The longitudinal muscles of the mesenteries are 

 sheets of muscle fibers on the exocele faces of the direc- 

 tive mesenteries and on the endocele faces of the non- 

 directives and of most of the incomplete mesenteries. 

 They extend from the oral to the pedal disc. 



11. The transverse muscles of the mesenteries are 

 very thin uniform sheets of muscle that cover the endo- 

 cele faces of the directives and the exocele faces of the 

 other larger mesenteries. They are thus on faces op- 

 posite those on which the longitudinal muscles are lo- 

 cated. They are better developed on the complete mesen- 

 teries than on the incomplete ones, from the smaller of 

 which they may be entirely absent. 



12. The parietal muscles of the mesenteries consist 

 of longitudinal ridges on the exocele and endocele faces 

 of almost all the mesenteries at their line of junction with 



