Gatljj }[ai'ine LaJioratory, St. Andrews. 125 



general form beinp; soniewliat ovoid, with the ))osterior edge 

 more rounded and elevated into a collar, which curves in- 

 ward at the notch on the border about a third forward. 

 Bending outward from the front of the notch it passes for- 

 waid, then flattens outward as it approaches the nuchal 

 organs, tlie furrow from each of which runs to a slight 

 notch in the margin, the thin plate being continued forward 

 in the median line as a bluntly conical process above the 

 mouth. A little within (i. e., in front of the collar posteriorly) 

 a prominent median ridge, with a converging oblique muscle 

 on each side, curves upward and passes forward to the thin 

 conical anterior plate, upon which it is lost. In some the 

 ridge is distinctly notched in lateral view. On each side of 

 its base anteriorly are the nuchal grooves, a line from which 

 passes to the notch on the margin. In the majority of 

 specimens slight grooves, which run obli([uely forward and 

 outward from the median ridge, give a resemblance to a leaf 

 with its midrib and veins. The mouth opens a short 

 distance behind, the anterior border of the cephalic plate, 

 and appears as a puckered dimple, often with a groove in 

 front in ordinary preparations, which often have the snout 

 bent at an angle to the trunk, the mouth being thus carried 

 outward. The peristoraial segment is covered dorsally by 

 the cephalic plate and is thus pushed ventrally. It bears no 

 bristles or hooks. 



The body is continued behind the cephalic plate as a more 

 or less cylindrical region, though much depends on its pre- 

 paration. Thus some removed from their tubes are quite 

 cylindrical, whilst others, killed in the free condition, show 

 numerous segmental contractions and dilatations. Slight 

 diminution occurs just in front of the caudal disc when 

 viewed from above downward, but, on the other hand, a 

 distinct dorsal increase, terminating at the vent, is evident 

 in lateral view. The dorsum is generally convex throughout, 

 but the ventral surface is more or less flattened, especially 

 after the anterior fifth, where it is*ringed and marked by a 

 median line (nerve-cord), which toward the terminal fourth 

 is sunk in a groove, rising, however, toward the caudal 

 plate. The aniis terminates dorsally at the end of a ridge, 

 and the margin is slightly crenate. Moreover, a separate 

 papilla just in front of the caudal plate aids in its closure. 

 Occasionally a prolapse of the gut occurs as a flask-shaped 

 hernia, with the narrow neck at the vent. In this the 

 circular and longitudinal fibres of the gut, as well as a thin 

 ehitinous layer, are conspicuous. The caudal plate is slightly 

 oblique with regard to the axis of the body, sloping from 



