REALAKICABLE SPECIES OF ECIITUROIDS. 2/) 



the pharynx, the œsophagiis, the crop, the midgut and the in- 

 testine. The names here employed to designate tlie different parts 

 are in a measure provisional and may not be exactly identical 

 with the parts called by the same names in other Echiuroids. 

 I have found it not alwa3's an easy matter to draw homology 

 between the different regions of the digestive tract in the present 

 species and those in other Echiuroids. 



The mouth leads into the pharynx which is a sac-like, 

 relatively short, muscular tube (figs. 18, 19 and 23, ph.). In 

 preserved specimens it is about 45 mm. long and 7-10 mm. wide. 

 It is the widest and the thickest-walled part of the entire 

 digestive canal. When filled with sand it may be distended into 

 a large thin-walled sac. The outer surface of the wall is smooth ; 

 under circumstances the circular folds of the inner surface may 

 be seen on the outside through the semitransparent wall. For 

 its entire length the pharynx is fixed to the body- wall by a pair 

 of tolerably wide and thick suspensory membranes of muscular 

 nature, the wing muscles or lateral mesenteries (fig. 23, w.m.). 

 These arise right and left from along the lateral sides of the 

 pharynx and insert themselves on the body-wall along two 

 parallel lines, each running 7-10 mm. apart from, and on either 

 side of, the ventral nerve-cord. Tiie two mesenteries are asym- 

 metrical in that the one on the left side terminates behind with 

 a free edge which slants down ventro-posteriorly from the hind 

 part of the pharynx, while the other on the right side is more 

 prolonged and extends farther backwards in connection with 

 the œsophagus, at the same time the line of its parietal insertion 

 gradually approaching the ventral median line. The right lateral 

 mesentery just referred to terminates posteriorly by becoming 

 confluent with the suspensory membrane {d.m.) of the dorsal 



