1906.] NATURAL SCIENCES OF PHILADELPHIA, 249 



mm. long and 4.5 mm. wide. Dorsally it is completely closed, over- 

 arching and concealing the prostomium and mouth. Together with 

 the palpi which it bears below it forms a broad flat plate, of which the 

 palps form the margins and are rolled inward and nearly touch ven- 

 trally. At its distal end the peristomium bears the operculum — a 

 circular or somewhat elliptical disk directed slightly dorsad and com- 

 posed of three whorls of stout, stiff, hard spines or peristomial setse, 

 so fitted together that they form a flexible and at the same time close- 

 fitting and impenetrable plug to the tube. There are some indications 

 in one specimen that this region may be regenerated when lost. The 

 paleoli of the three rows differ in form and number as indicated below. 

 Just external to and below the operculum is a circle of rather promi- 

 nent conical opercular papillse, which probably represent the much sub- 

 divided dorsal cirri of the peristomium. They are clearly divided in 

 two symmetrical halves like the opercular paleoli, and number 16 to 20 

 on each side. 



The palps may be opened from the ventral side and spread, together 

 with the peristomium, as a flattened plate deeply pigmented below 

 and bearing the branchial folds on the lateral thirds. Anteriorly the 

 two palps are conjoined in the operculum, the branchise also meeting 

 in the middle line below and behind the operculum. From 12 to 18 

 of these l^ranchial folds or ridges occur on each side, the usual number 

 in full-grown specimens being 18 pairs. They are prominent ridges 

 running transversely across the free ventral margin of the palps and 

 diminishing in size from behind forward. Owing to the medial bend- 

 ing of the anterior end of the palps, the anterior six pairs of branchiae 

 are arranged in the form of an arch and several of them lie in a nearly 

 antero-posterior plane. Each gill consists of a stiff plate or ridge, 

 with a serrate free margin behind which the filaments are borne. 

 Except the very last, which usually bears but 10 filaments, the pos- 

 terior gill ridges have about 20, the number decreasing regularly to the 

 most anterior. All gill filaments are very slender and thread-like 

 and the longest posterior ones equal the width of the peristomium. 



At the base of the branchial region is a pair of short rounded lobes 

 having much the aspect of the branchial plates, but shorter and thicker 

 than they and coming in contact across the mouth slit. Continuing 

 from this dorsally is a sloping ridge bearing a pointed conical cirrus 

 longer than the rounded lobe, and above this again a small tubercle 

 carrying a fan-shaped tuft of slender setse. 



Somite II is a short, simple ring partly fused with I and III ventrally 

 to form the first ventral gland plate. Laterally it bears three conical 



