The eggs develop within the capsule until the 

 larvae have acquired three pairs of setiferous 

 segments; then they leave the tube. At a tem- 

 peratm-e of 21° to 23° C. the development of 

 P. ligni under laboratory conditions varied from 

 4 to 8 days. Larvae of P. websteri (fig. 383) also 

 have three setiferous segments. According to 

 Hopkins (1958), planktonic larvae, presumably 

 P. websteri, occm" m Louisiana waters throughout 

 the year; eggs were found in the tubes when 

 water temperature ranged between 12° and 18° C. 



The duration of the pelagic life of either species 

 of Polydora is not known. The planktonic larvae 

 grow and develop additional segments before they 

 settle on the substratum. Since the largest 

 P. websteri worm found in plankton had 17 seg- 

 ments and the smallest found on oysters also had 

 1 7 segments, it is probable that this species settles 

 at that age. The appearance of young P. ligni at 

 an early bottom stage is shown in fig. 384. 



The larvae of P. websteri settle on the rough 

 exterior surface of young oysters and make shoe- 

 shaped burrows near the extreme edge of the 

 valves. As the worm grows it enlarges its burrow. 

 The process of excavation is probably chemical, 

 apparently similar to that described by Wilson 

 (1928) for P. hoplura and by Hannerz (1956) for 

 P. ciliata. 



The tubes of P. ligni are made of mud particles 

 held together by mucus secreted by the antennae 



and the body surface. Ciliary motion along the 

 tentacle grooves serves as an efficient mud-gather- 

 ing device. Experimental evidence shows that if 

 the lumps of mud are too large or if particles con- 

 sist of the finest sand or foreign materials such as 

 corn starch or powdered glass, the ciliary motion 

 is reversed and the material is rejected. These 

 laboratory observati^ms prove that the worm is 

 capable of selecting the substances needed for the 

 building of a soft tube. 



The tube inhabited by the worm, whether U- 

 shaped or straight, is lengthened by the worm at 

 both ends. To accomplish this P. ligni reverses 

 its position in the tube by folding itself halfway 

 and sliding over its own ventral side. The 

 process, frequently observed in the Woods Hole 

 laboratory, is accomplished with great speed and 

 remarkable ease. 



The amount of mud which P. ligni can accumu- 

 late in the formation of their tubes is astonishing. 

 A sample collected on June 8, 1944 from the tidal 

 flats of Delaware Bay contained about 430 closely 

 packed worm tubes per square inch of mud area. 

 They all lay nearly perpendicular to the surface. 

 A cubic inch of the washed and dried sample 

 weighed 20 g., of which 12.8 g. consisted of mud 

 with the balance made up of sand, empty shells, 

 and organic matter. On this basis it is estimated 

 that the worms gathered 4.9 pounds of diy mud 



Figure 381. — Photograph of an adult C. virginica from Matagorda Bay, Tex., heavily infested with D. smilhii. The 



outer layer of the shell was chiseled off to expose the cavities. 



FACTORS AFFECTING OYSTER POPULATIONS 



423 



