10 N. T- Agricultural Experiment Stations, Circular 139 



female. Curiously enough there is only one male to about every 500 

 females. 



Within the gills of the female many thousands and even millions 

 of eggs develop into very small larvae looking much like those 

 of the clam or of the oyster (fig. 7). (Compare with figures 5 to 11 

 in reference 5.) They have a pair of thin transparent shells of lime 

 joined together by a straight hinge, the whole animal resembling a 



Fig. 7. Larvae of Teredo navalis Taken From the Gills of the Parent 



These larvae are ready to be thrown out into the water. Note the swimming 

 organ, the velum, which projects from between the shells 



purse. Between the shells the ciliated swimming organ, the velum, is 

 pushed out. After the larvae are cast out into the water through the 

 excurrent siphon of the parent they swim actively about with the aid 

 of the velum while carried to and fro by currents. 



It is not known just how long the larva of Teredo remains free- 

 swimming in the water, but it is believed to be about one month in 

 San Francisco Bay. In the warmer waters of this state the time is 

 probably somewhat shorter. During the free-swimming period the 

 larva grows to be about 1/100 inch loner, or about five times the 

 length it was when it left the parent. It then settles down on the sur- 

 face of unprotected wood and attaches by means of a sticky thread, 

 the byssus. which it spins out of the base of its foot. Then occurs a 

 series of changes too complex to consider here, by which the velum 

 is lost, and after the shells are modified for boring, the little pileworm 

 enters the wood. When sexually mature it begins giving off larvae 

 as did its parent. Since one teredo liberates from 500,000 to 1,000,- 

 000 larvae in a season, it is easy to understand how from one 

 piece of infected wood an entire harbor might become infested with 



