1906.] NATURAL SCIENCES OF PHILADELPHIA. 229 



one or thirty-two somites, but the number is not correlated with the 

 size of the worm, the largest two having thirty-one and the smallest 

 thirty-two. 



The prostomium is a minute conical organ, thin-skinned, weak- 

 walled and hollow, and apparently capable of distention by internal 

 fluid. It is followed by a short, uniannular segment continuing the 

 general conical form of the anterior end. Dorsally this segment is 

 crenulated on its posterior margin; below it is flattened and slightly 

 grooved in the middle of the posterior part ; and on each side it is pro- 

 vided with a dorso-lateral groove which terminates anteriorly in a deep 

 sensory pit at the posterior margin of the prostomium. The next 

 segment is biannular, with the anterior ring distinctly larger. Below 

 it forms the upper lip and is thrown into a number of deep longitudinal 

 folds and furrows which pass into the mouth. On each side, above a 

 rather prominent swelling, is a quadrangular area opposite the groove 

 on the first segment and itself bounded by a longitudinal groove above 

 and another below, both of which cut the segment for its entire length. 

 This area bears the small tufts of notopodial and neuropodial setse and 

 between them an elliptical clear spot or pit. Dorsally each ring is 

 marked by longitudinal grooves which effect a peculiar lobed and 

 crenulated arrangement of the posterior margin. The third somite is 

 triannulate, but the anterior two rings are somewhat united and on the 

 ventral surface the first enters the sides of the mouth and the second 

 forms the longitudinally grooved lower lip. At the sides this segment 

 is constructed like the second, except that the posterior third of the 

 quadrate area is depressed and smooth and that a cirrus is borne on 

 each side immediately above and behind the notopodial setae. Above 

 the same longitudinal sulcation and posterior lobing appear. The 

 next twelve segments are formed of three equal rings completely 

 separated by continuous furrows, except for the short interruptions at 

 the setigerous areas on each side. Dorsal longitudinal sulcations are 

 wanting and a second sensory pit, which first appears on the middle 

 of the second ring below the setigerous area of somite IV, becomes 

 very conspicuous on the posterior segments of this region, and on every 

 specimen finally ceases on XV. Somites XVI, XVII and XVIII each 

 consist of a large posterior and a small anterior ring, the furrow separ- 

 ating which becomes successively more and more restricted to the 

 dorsal and ventral regions. Traces of a short anterior ring, differen- 

 tiated only dorsally and ventrally, still continue on XIX and XX, but 

 all remaining somites are strictly uniannulate with prominent over- 

 lapping posterior margins, which finally become telescopic. Somite 



