232 NATURAL HISTORY OF SOME TUBE-FORMING ANNELIDS. 



covered, whitish, tough, leather-like portion, which extends more or less directly 

 into the mud and sand. Near the outer end of the tube the pieces of shell appear 

 much less worn than those toward the other end. When discovered the shelly por- 

 tion of the tube is usually lying more or less flat in the sand, with only one or two 

 inches of the upper end uncovered. Undoubtedly all the shelly portion is free at 

 one time, but is subsequently covered by the drifting sand. Judging from the 

 manner in which Diopatra constructs the shelly part of the tube that portion must 

 be uncovered at the time of formation. In all cases examined the lower end of the 

 tube was open. Whether it is necessarily so is a question I am unable to decide. The 

 animal can turn within its tube, and in the construction of the lower end it very 

 probably dug the sand aside or carried it out. It may be that the tube is left open 

 incidentally, as the annelid ceases to build at the lower end of its tube from time 

 to time. The lower part of the tube may be considered both as a protection from 

 other boring animals and also as an anchor while it is living in the outer portion. 



The free end of the tube is extended on the surface into a sort of hood, which 

 may act as an additional protection while the animal is resting at the opening. The 

 edge below this hood is worn smooth and round by the animal while moving out and 

 in during food-getting. The end of the tube is kept open permanently for conve- 

 nience in feeding and breathing. I have never witnessed the process of obtaining 

 food, although I have left pieces of other annelids in the aquaria within its reach, and 

 even in the tube-opening, but except for an occasional bite at the piece with its man- 

 dibles and a push that sent it from the tube-opening, the animal gave no response to 

 the presence of food. 



In the aquarium, as it rests with the tips of the longer tentacles extending be- 

 yond the edge of the tube, Diopatra is constantly engaged in moving the anterior 

 end of the body up and down. This movement proceeds at the rate of forty-four 

 times per minute with great regularity. The movement is undoubtedly for the 

 purpose of creating currents in the water to carry oxygen to the gills, which he on 

 the dorsal surface. Tests with waste particles show that currents of water are pass- 

 ing in and out at the mouth of the tube constantly. When the animal is removed 

 from its tube, and is not moving about, the breathing process is carried on by the 

 periodic waving outward and inward toward the mid-dorsal line of the series of 

 gills which extend upward from the dorso-lateral region. In each of the two series 

 there are about thirty of these miniature fir-tree-like gills, with a central slender stem, 

 and, more or less distinctly, eight vertical rows of minute branches. Adjacent to, 

 and extending alongside each gill on its outer surface is an elongated dorsal cirrus 

 from the parapodia. The function of these cirri is probably to protect the gills 



