COE: NEMEKTEANS OF WEST AND NORTHWEST COASTS. 147 



Wliere this layer is most massively developed (PI. 13, fig. 82) the 

 'dorsal and ventral median nerves become greatly increased in size, 

 and lie immediately on its periphery above and below. This inner 

 circular layer belongs more pro])erly to the viscera (alimentary canal 

 and proboscis sheath) than to the body walls. It is not composed 

 exclusively of circular muscular fibers, but contains an abundance of 

 longitudinal ones arranged in minute bundles (of a few fibers each) 

 between the circular fibers. 



Througliout its length the inner circular layer is connected vdih 

 the outer both dorsally and ventrally by means of fibrous decussa- 

 tions (PI. 13, fig. 81). 



In the head (PL 12, fig. 78) an outer circular muscular layer lies 

 immediately beneath the cephalic glands, while the blood lacunae 

 are mainly surrounded with longitudinal muscles. A few circular 

 fibers surround the rhynchodaeum, but the layers are all more or 

 less interrupted by irregular oblique or dorso-ventral fibers. 



A thin band of longitudinal muscles lies between the proboscis 

 sheath and alimentary canal, as described below. 



Proboscis sheath. — The proboscis pore is minute and situated 

 subterminally (PI. 12, fig. 76). 



The proboscis sheath is strongly developed. There is a single 

 muscular layer composed of circular fibers interspersed with longitu- 

 dinal fibers in small groups. The muscular layer is throughout 

 much thicker than in either of the other species of the genus. In 

 the esophageal region this musculature is continuous "wdtli the 

 internal circular muscular layer, which becomes so enormously 

 thickened in the nephridial region (near the posterior end of the 

 esophagus). The rhynchocoel is thus bound up in the same muscu- 

 lature as the esophagus (PI. 13, fig. 81), 



Burger ( '95) jjrefers to consider the muscular layers of the pro- 

 boscis sheath as wanting in the region where the internal circular 

 muscular layer is developed (in the whole of the esophageal region), 

 and that its walls consist only of a flattened epithelium supported 

 by a thin basement membrane. The musculature of the proboscis 

 sheath, however, is encountered in front of the mouth, and extends 

 without a break as far backward as the anterior end of the intestinal 

 region. In the region of the mouth it is clearly evident that these 

 same muscular fibers which compose the muscular wall of the pro- 

 boscis sheath extend ventrally and surround the esophagus. At 



