NEUROMUSCULAR MECHANISM IN METRIDIUM 439 



7. The basilar muscles are radial strands which 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 mesen- 

 tery to which the}'' are attached. On the larger mesenteries 

 each muscle reaches from the periphery of the disc to about 

 its center; on the small mesenteries from the periphery only a 

 short way toward the center. 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. The morpho- 

 logical and systematic significance of the basilar muscles has 

 been emphasized by Carlgren ('93, p. 107; '05). 



The most complex group of muscles in Metridium is that 

 found on the mesenteries: it consists of the longitudinal, the 

 transverse, and the parietal muscles. 



8. The longitudinal muscles of the mesenteries (figs. 1, 2, 3, mu. 

 Ig. ) are sheets of muscle fibers on the exocoel faces of the direc- 

 tives, on the endocoel faces of the non-directives and of most 

 of the incomplete mesenteries. These muscles extend from 

 the oral to the pedal disc. 



On a directive mesentery the longitudinal muscle (fig. 1) 

 is thickened into a very marked longitudinal ridge, which in 

 the middle of its course runs close to the siphonoglyph and nearly 

 parallel wdth it. Orally this thickened portion turns some- 

 what away from the chief axis of the animal and ends near the 

 bases of the tentacles; aborally it runs nearly parallel with 

 the free edge of the mesentery spreading out somewhat fan- 

 like as it approaches its termination near the central portion 

 of the pedal disc. Toward the oesophagus from this ridge the 

 longitudinal muscle is represented by a very thin layer of longi- 

 tudinal fibers. In the opposite direction the ridge gradually 

 gro-vVs thinner and thinner till finally it, too, becomes a very thin 

 sheet of longitudinal fibers. 



On the non-directive and larger incomplete mesenteries 

 (figs. 2, 3) the longitudinal muscle (mu. Ig.) shows a band-hke 

 thickening in place of the ridge just described. This band takes 

 much the same course on these mesenteries that the ridge does 



