484 PROCEEDINGS OF THE NATIONAL jVIXJSEUM vol.99 



LISTRIOLOBUS HEXAMYOTUS, new species 



Plate 29 



Diagnosis. — Differing from all known species in having only six 

 longitudinal muscle bands and one pair of nephridia ; neurointestinal 

 blood vessel does not form a loop surrounding interbasal muscle, but 

 it is undivided from ventral blood vessel to gut where there is an 

 inconspicuous ring vessel ; f oregut very short ; body wall translucent ; 

 proboscis unknown. 



Description. — Length 40-63 mm.; thickness 7-10 mm.; proboscis 

 lost; form subcylindrical, sometimes slightly produced at anterior 

 end. The thoroughly relaxed type has a thin translucent body wall 

 and the six longitudinal muscle bands are very inconspicuous. The 

 other 3 specimens are contracted and have an opaque body wall marked 

 by six shallow longitudinal furrows representing the muscle bands. 

 Only under strong magnification are the papillae visible — largest at 

 anterior end. These are low, conical, with a central dark spot, and 

 are in spaced transverse series. They can be best seen only in the 

 relaxed type. When the skin is contracted they may be more or 

 less submerged in the folds, though here and there on limited areas 

 they are definite enough. Color gray or brownish gray. Coelomic 

 surface smooth, the inner oblique layer not interrupted, as in 

 OchetostoTna, by the longitudinal bands. 



Setae two, relatively long, close together, with a slender interbasal 

 muscle which does not pass through a loop of the neurointestinal 

 blood vessel. There are a large number of very slender and long basi- 

 lateral muscles, only a few of which are indicated in the figure. The 

 seta sheaths are connected at body wall by a strong transverse muscle ; 

 and extending laterally, at right angles to body wall like a low 

 dissepiment, is a muscular apparatus the purpose of which appar- 

 ently is to separate the distal ends of setae (pi. 29, fig. 2, SM). 



Nephridia only two, very small, with a conspicuous nephrostome 

 having an obvious neck and the lips produced as in other members 

 of the genus. There are no genital products in the nephridia. 



Anal vesicles relatively very small, only 8 or 9 mm. long in the 

 type (length 63 mm.), thin walled and without special features. 

 The cloaca is also very small (about 2 mm. long), but very muscular. 

 The end of the nerve cord passes on to its ventral surface and divides 

 into two trunks, which appear to end at the narrow constriction be- 

 tween intestine and cloaca. 



The alimentary canal is of moderate length, very delicate, and the 

 thin-walled intestine is crowded with mud pellets. The foregut is 

 short, much shorter than in L. pelodes Fisher, and its parts are not 

 readily recognizable. Both gizzard and stomach are reduced almost 

 to a minimum (pi. 29, fig. 2). The presiphonal intestine is also of 



