12 BULLETIN OF THE BUREAU OF FISHERIES. 



clings to the silk with which its burrow is lined by means of the hooked claws on the 

 anterior and posterior prolegs. 



The current of water which is driven through the burrow by the undulating motion 

 of the body of the larva serves the double function of bathing the branchial gills, thus 

 renewing the oxygen supply, and of bringing in whatever particles may be floating in 

 the adjacent water that are of use to the larva as food. The normal undulations move 

 from the head backward, and the larva always turns about after spinning its net, so 

 that the current is driven into the open end of the conical net. The position assumed 

 by the larva places the caudal filaments in such proximity to the net that they are able 

 to serve a more or less important tactile function. When the larva has maintained 

 this current for about 10 minutes (the time element appearing more uniform than the 

 amount of food actually present in the net at any one time), it turns about in its 

 burrow quickly and gathers in and swallows the catch, net and all. 



The net is "hauled" in a very characteristic way. The larva seizes that portion 

 of the rim with which it first comes in contact. The mandibles, the labrum, and prob- 

 ably the lateral arm of the epipharynx are brought into use, and the flimsy net is torn 

 away from the silk of the burrow and crowded down the throat of the larva by the 

 labrum. Then the larva rotates its body and seizes the other side, which is swallowed 

 at once. Then the remainder of the net is swallowed while the larva rotates its body 

 first to one side and then to the other as if to wring out or twist up the net, so that it 

 can be more easily swallowed. The conical tip of the net usually contains a consider- 

 able variety of plankton organisms ranging from bacteria, which are either stuck to 

 the net or caught in its meshes, to crustaceans and various rotifers, which sometimes 

 succeed in escaping but are nevertheless often captured. The entire process of "haul- 

 ing" the net and eating it takes only about six seconds. 



The most striking and fundamental use made of silk by Chirotxotrms lobiferus is in 

 the construction of a net by means of which the larva obtains its entire food supply. 

 Silk has, however, other uses of very great adaptive importance even in this unusual 

 habitat. Many burrows are found where old openings have been entirely sealed up 

 by its use. The regular openings through the epidermis are usually made round and 

 smaller in size by the addition of a silk margin, and the burrow itself is lined with silk 

 which is uniformly made of such a diameter that the movements of the larva are espe- 

 cially effective. This ability to spin a thin, flexible, and at the same time practically 

 water-tight lining enables the larva to adapt itself to cavities of varying sizes. 



The small size of the openings at the ends of the burrow seems to be a special 

 adaptation, for when the larvae live under the very different conditions afforded by 

 glass tubes they retain this same habit. It seems probable that the narrow openings 

 increase the speed of the current and so prevent Protozoa, Crustacea, and other small 

 organisms from swimming against it. Large particles are also prevented from entering 

 the burrow. In case these small openings are plugged by an accumulation of particles 

 the larva stops its rhythmic undulatory movements and suddenly throws its body into 

 several much shorter waves which move in the reverse direction. This sets up a strong 

 countercurrent which usually dislodges the obstruction, although the contents of the 

 net are usually lost. In case an obstruction is not readily dislodged the larva creeps 

 forward and brings its mandibles and labrum into play. 



