VOL. III.] Balanoglossus. 189 



The abdomen is several times longer than the other two parts 

 combined and is very distensible. It is soft and frail and one rarely 

 sees it whole, so easily does it break as the animal is being extracted 

 from its tube in the soft mud or sand in which it lives. 



The surface of this region is much less regular than that of the 

 other two parts previously described, it being, particularly in its an- 

 terior, pharnygeal portion, somewhat quadrilateral with irregular 

 transverse folds affecting particularly the dorsal angles. As seen by 

 the figure the gills, gi., are arranged in a double series on the dorsal 

 side of the animal, each series, as seen from the surface, being 

 composed of a large number of crescentic openings. 



The sexual orifices are also found in this region, but are very 

 minute, s. or. The three portions of the animal differ from one 

 another in color. The proboscis is a uniform very light yellow, the 

 collar is also yellow but of a considerably more pronounced shade. 

 The abdomen is of a brownish tint marked with darker spots, and 

 for a portion of its length has a greenish shade from the presence of 

 the liver within showing through the body wall. In size the crea- 

 ture may reach a length of eight inches in some of the larger species. 

 It will be noticed from this description that the animal is entirely 

 without paired appendages, either for locomotion, prehension, or 

 sensation. Its only organ of movement is its proboscis. 



The animal is entirely marine, so far as known, and is confined to 

 shallow water near shore, and as already said, lives buried in mud 

 or sand. The species found on the New England coast can be col- 

 lected at low tide only, when the earth in which it lives is uncovered. 

 It is usually found about a foot or two below the surface, and one 

 readily determines where to dig for it by the very characteristic 

 spirally coiled cast of sand and mud at the opening of its tube that 

 has been ejected by the animal within. 



As it is in the creature's role as a candidate for a place among 

 the chordata that it has become chiefly distinguished in recent years, 

 the attention that we here give to its anatomy may profitably be 

 from the standpoint of a comparison with the fundamental chordate 

 structure. In this way the points of agreement may be brought 

 out with emphasis, and at the same time the points of disagreement 

 may be made equally emphatic. 



Bateson, who, as already said, was the first to carry out this com- 

 parison in detail, points out three primary and four secondary par- 



