314 U.S. NATIONAL MUSEUM BULLETIN 22 7 



constantly; it has been suggested by Thulin (1921) that they function 

 in tube construction. 



In the rest of the thoracic region, the parapodia are modified 

 gradually, the setae becoming fewer in number, tlie notopodial 

 postsetal lobes become smaller with fewer lobules, finally becoming a 

 single digitiform lobe. The neuropodial postsetal lobes with fewer 

 lobules, finally with a single digitiform postsetal lobe. 



Middle or anterior abdominal region (fig. 82d) uniramous, lacldng 

 notopodia entirely. Neuropodia cylindroconical, with few slender, 

 slightly hairy capillary neurosetae and 1-2 projecting acicular neuro- 

 setae, usually one of which is more exposed, with tip blunt, the other 

 one with just the fine tip exposed (in younger specimens, both taper 

 to long fine tips), with single digitiform postsetal lobes and, developing 

 gradually, with subpodal thin, slightly undulate lamellar membranes 

 or flanges extending posteriorly to the next septa. 



Posterior abdominal region (fig. 82,e,f) differing from anterior 

 abdominal region by the addition of notopodia which develop gradually 

 in the form of small rounded, slightly conical low mounds equipped 

 with few (4-7), acicular dark yellow spines. Notopodial spines short, 

 wider basally, tapering to sharp tips; when withdrawn, only the dark 

 tips are exposed; when extended, they form a characteristic stellate 

 cluster (it has been suggested by Hartman that they may function for 

 traction in the tube). Beginning on about setigers 20-40, with small, 

 cylindrical and retractile ventral papillae on either side of the mid- 

 ventral line, situated on a slightly raised area; they are more conspicu- 

 ous in the posterior region, 1-4 papillae per area (fig. 82/; they have 

 been referred to as branchiae; Hartman has suggested that they may 

 be sensory). Posterior end with thick collarlike pygidium with 

 border slightly lobulate or with 4 distinct lobes in younger specimens. 

 Young specimen transparent, w^hite. 



Tube long, cylindrical to somewhat flattened, flaccid, fragile, easily 

 broken. It may be up to 165 mm. long, 6 mm. in diameter, compli- 

 cated with anastomoses and free branches (Thulin). Wall formed of 

 fine mud particles cemented together by longitudinally arranged fibers 

 secreted by the worm. Without membranous lining. Thulin has 

 suggested that the tube building is aided by the penicillate acicular 

 neuropodial setae of the thoracic region. 



Biology, — They are dredged on bottoms of fine sand, mud, includ- 

 ing blue mud, black foul-smefling loose mud, mud with algae, Zostera, 

 and stones. They live in mud tubes of thek own making and are 

 probably continually in the process of tube building. Sexually mature 

 adults have been found in April m Swedish waters (Hannerz). The 

 eggs are discoid, whitish, with thick egg membranes and membrane 

 vesicles (21-32 at the edge, Hannerz, Hartman). The early develop- 



