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stratum. The simple IJ-form is often modified in tubes tliat occur in slioals 

 of sand and siiells. The arms may here be so constructed that they turn 

 abruptly aside from large shells that are in their way. Tubes with three 

 arms are frequently found (Fig. 4). These are tubes that have been en- 

 larged by the extension of the horizontal portion and the formation of 

 a new (vertical) arm. A septum at the base of the intermediate arm sep- 

 arates its cavity from that of the horizontal portion. I have found inter- 

 mediate arms with little or no sand, some completely filled, while many 

 have begun to macerate. Every large tube bears the shreds of one or more 

 of these macerated intermediate arms, or the crescentic scars that mark 

 their former union with the newly formed extension. The annulations 

 near the orifices and the longitudinal strips of thinner, sand-covered, parch- 

 ment alternating with the thicker portion of the tubes represent successive 

 steps in the formation and enlargement of the tubes. 



There is great diversity in the size of the tubes. A very young worm 

 formed a characteristic U-shaped tube three millimeters in diameter at its 

 wider portion, and one and three-fourths millimeters at its orifices. The 

 distance between the orifices measured fourteen and one-half millimeters, 

 and the length of the arms (measured from the lower side of the horizontal 

 portion to its base) was sixteen millimeters. I have collected tubes which 

 ranged in length from six to fifty centimeters and with arms six to twenty- 

 two centimeters long. 



The formation of the first tube and the subsequent enlargements was 

 observed on larvae of Vliactoptcrua vayiopcdatus which I was fortunate 

 enough to collect in the tow-net. These larvae, which were transforming 

 rom the free-swimming mesatrochae into the creeping individuals, were 

 kept in aquaria of sea water well stocked with diatoms. When the larvae 

 move among the diatoms they leave a trail of mucus that cements the 

 sand and diatoms together. Later they make short, horizontal, mucus- 

 coated tunnels into the mass of diatoms and sand. One of these tunnels 

 may be extended to several times the length, of the body and from this 

 simple tunnel of agglutinated sand and diatoms the larvae may build the 

 tube within which it subsequently remains confined. 



The first tube in which the larvae lives and feeds for several days 

 is nearly a millimeter in diameter and from eighteen to twenty-two milli- 

 meters long. Tt is either a straight tube or a shallow U whose curved 

 portion is downward. 



[9—18192] 



