14 BULLETIN OF THE BUREAU OF FISHERIES. 



of silk spread by the prolegs. During this retraction the prolegs are held pointing 

 forward at an angle of about 45 degrees with the body, and their exact use can only be 

 surmised, but from their position and the speed of the movements it seems possible 

 that the semifluid silk is spread either by the short spines in front or what seems more 

 probable by the carding effect of variously hooked and serrate spines located farther 

 back on the prolegs. When the larva reaches the end of its backward movement, 

 the prolegs are spread and rapidly touched to the silk lining of the burrow at two 

 nearby points. Then the forward movement is carried out. In this movement the 

 prolegs are extended slightly forward and are more or less spread out. When the 

 end of this movement is reached, the thread is attached either by the contact of the 

 head or the prolegs to this central point of attachment and the process repeated. It 

 is impossible to tell whether the head takes part in the process of attachment or not, 

 because both the head and the prolegs are so close together at this point. It would 

 seem probable from the small size of the apex of the finished structure that at some 

 point in its construction the head occupying such an advanced position would be the 

 only possible part of the body that could accomplish the attachment of the fibers. It 

 is obvious, however, that the head does not touch the wall in the process of attaching 

 the silk at the rim of the net, for the head is held projecting straight out and the move- 

 ments of the prolegs are unmistakable. 



The forward and backward movements of the body are accomplished largely through 

 the instrumentality of the posterior prolegs. These are held attached to the silk, and 

 the last three or four segments of the body are flexed on them as axes. On the forward 

 movement the body is straightened and the prolegs extended forward ; on the backward 

 stroke the prolegs point backward according to the degree with which the body is flexed. 



The silk net (figs. 26 and 27) is too long to be spun from one place by the simple 

 flexing of the body. This means that it has to be spun in two sections. The over- 

 lapping of the sections gives the appearance of a continuous sheet of silk extending from 

 the apex to the base of the net, and the original posterior attachments of the first section 

 appear as radiating strands from the sides of the net. 



The entire process of constructing the net requires less than half a minute and in- 

 volves the spinning of 42 to 44 ribbons or sheets, as determined by counting the move- 

 ments. When this process is completed and the larva turns about and begins forcing 

 the water into the net, it can readily be made visible by adding a few drops of water con- 

 taining powdered carmine. 



The method by which the silk lining of a burrow is spun is not so easy to observe 

 as the process of spinning a net. It takes longer, and the number of movements is so 

 great that it is almost impossible to correlate them with any definite structure later 

 observed. But even here, if proof were lacking that the prolegs are the one necessary 

 factor to explain the entire process, there are structures that bear unmistakable evi- 

 dence of their use. The lining, as the silk net, is spun in sections which, while not of 

 uniform length all the way around the tube, are nevertheless approximately so. 



The exact way in which the first section is constructed is not so easily understood, 

 but from this on the process involves a considerable repetition of the method employed 

 in the construction of the net. The body is extended and retracted in the process of 

 attaching the sheets of silk to the first section, to each other, and to whatever support 



