191''] Barrows: Ship worm in San Francisco Bay 37 



during the year 1913 than it did during the year 1912, for which we 

 have observations. 



While it is not definitely known whether shipworms began their 

 disastrous operations in San Pablo Bay in 1912 or in 1913, there are 

 certain considerations which indicate that the primary infection by 

 the borers in this attack may have occurred in the summer and fall of 

 1912, with probably a second infection following upon the breeding 

 season of the following year. The breeding habits of certain species 

 of shipworms have been observed by Sigerfoos in Chesapeake Bay, 

 including Xylotrya gouldi Jeffreys and Teredo dilatata Spengler, 

 which are very abundant there, and Teredo navalis Linnaeus, which 

 occurs in that locality but rarely. Sigerfoos (1908, pp. 195-198) 

 writes as follows concerning the spawning habits of these species : 



T. navalis retains its eggs in the gills during their embryonic development. . . . 

 On the other hand . . . the eggs of the other two species are laid free into and 

 fertilized in the water. . . . 



In association with their character of free development in the water, the 

 eggs of the shipworm are very small and very numerous. While they vary 

 somewhat in size, they have an average diameter of somewhat less than 1/20 mm. 

 (1/500 inch). ... In one case I estimated the number laid by a large female of 

 T. dilatata to be one hundred millions. . . . Development is very rapid and on 

 warm days the embryos become free-swimming within three hours after the 

 eggs are laid. Within a day the shell has been formed and the typical lamelli- 

 branch veliger stage has been reached. . . . 



. . . What becomes of the larvae after hatching from the eggs, how and 

 where they live, it is difficult to surmise. Though the developed larvae are 

 settling on wooden structures constantly, I have not taken them and the inter- 

 mediate stages in the tow-net, and where they develop I do not know. The 

 rate of growth of larvae of the marine lamellibranehs, however, is slow, and I 

 think the larvae of shipworms when they attach themselves must be at least a 

 month old. Thej' may be more, for at this time their development is quite 

 advanced and their organization complex. 



The breeding season of X. gouldi and T. dilatata seems to extend throughout 

 the warm season. I have found ripe sexual products of both species from early 

 in May till the middle of August. At the latter time there seemed no abatement 

 in their development. . . . Individuals become sexually mature in a month after 

 they have attached, and those which attach in August must bear ripe sexual 

 products later in the season, so that the breeding period would seem to extend 

 throughout the warmer months. . . . 



The shipworm in its larval stages develops slowly, but once in the wood it 

 grows with remarkable rapidity. . . . The newly attached larva is somewhat 

 less than 0.25 mm. long. In 12 days it has attained a length of about 3 mm.; 

 16 days, 6 mm.; 20 days, 11 mm.; 30 days, 63 mm.; and 36 days, 100 mm. In a 

 month specimens may contain ripe sexual products, though normally these seem 

 to be retained till larger quantities of spermatozoa and eggs are stored for 

 extrusion at one time. 



