Mr. C. P. Sigerfoos on the Pholadidee. 251 



freely to boxes hung in the water, and in this way 1 was 

 able to secure all stages of development and to observe the 

 rate of growth. I have observed as many as thirty to 1 square 

 centini. of surface. Coming in contact with the wood, the 

 larva throws out a single long byssus-thread for attachment, 

 and never again leaves its place. The velum is lost within 

 a few hours, and the transformation of the small bivalve into 

 the ship- worm is begun. Tlie newly attached larva is some- 

 what less than 2*5 milHm. long. In 12 days it has attained 

 a length of 3 millim. ; 16 days, 6 millim. ; 20 days, 11 millira. ; 

 30 days, 63 millim. ; and 36 days, about 100 millim., when 

 it bears ripe eggs or sperm. In four to five weeks small 

 timbers may be completely ruined in the warm water at 

 Beaufort. 



Organization of the Larva. 



In striking contrast with the adult, the larva (shown in 

 longitudinal vertical section in tig. 1) is perhaps more fore- 

 shortened than that of any other Lamellibranch. The valves 

 of the shell are elliptical in outline, somewhat wider than 

 long. Seen dorsally they are circular in outline. The left 

 valve bears two teeth, the right three. The long internal 

 process of later stages (apophysis) is present as a rudiment. 



The velum still occupies a large space (VC) in the anterior 

 dorsal region. I have never seen it extended after the larva 

 had become attached. Both siphons {A and R) are already 

 formed, and from their point of union the septum has grown 

 forwards and become attached to the foot. The gills of this 

 stage have developed but little beyond the last stage tigured 

 by Hatschek. There are on either side two gill-slits, with a 

 rudiment of a third. The " filaments " are true gill-bars, for 

 they are attached to the foot by their ventral end. 



The foot is present as a very long tongue-shaped structure, 

 with a byssus apparatus at its posterior ventral angle. In 

 fig. 1 the foot is shown considerably contracted, occupied 

 mostly by mucus-cells and the byssus apparatus. 



The retractors of the foot are attached to the umbonal 

 region of the valves. Both adductor muscles are present, the 

 posterior already the larger of the two {Aa and Ap). 



The nervous system is already highly developed (tigs. 1 

 and 2). All of the ganglia are well formed, and the com- 

 missures form well-detined nerves. The cerebral and pleural 

 ganglia are still separated hy a short commissure. The 

 pleural {PI) is lateral to the cerebral (C), and from its poste- 

 rior angle the pleuro-visceral commissure passes. The pedals 



17* 



