254 Mr. C. P. Sigerfoos on the Pholadidfe. 



The burrow is formed as follows : — Almost as soon as the 

 larva has settled it begins to clear avA'ay a place by means of 

 the ventral edges of the valves of the sliell. In this way a 

 small pit is formed. But very soon rows of teeth are formed 

 in succession on the anterior edges of the valves ; the small 

 knobs are formed on their umbonal and ventral regions ; the 

 ligament becomes functionless, and the two adductors become 

 antagonistic to each other. The teeth are formed indepen- 

 dently, and afterwards are cemented to the valves, pointing 

 outward and backward. While the foot performs a cupping 

 action, the posterior adductor contracts, the two valves swing 

 on each other by means of the two pivots formed by the 

 knobs, and the teeth are brought to bear on the wood, 

 rasping away its surface. 



The essential changes in the nervous system are the fusion 

 of cerebral and pleural ganglia and the concentration of the 

 viscerals and their shifting in position. The cerebrals and 

 pleurals have fused in specimens 5 millim. long, but their 

 double origin is still indicated. The visceral ganglia become 

 concentrated in specimens 1 millim. long, and shift from a 

 position anterior to the posterior adductor to one ventral to it, 

 and at last come to lie far behind it in the adult. It seems 

 interesting that the description of the nervous system given 

 by Quatrefages * is still accepted, except with slight modifica- 

 tions in the viscerals as given by Pelseneer. Quatrefages 

 described the cerebral ganglia as fused, the pedals as rudi- 

 mentary and separate. On this ground, with others, he 

 proposed to separate the Teredidse from the Pholadidaj. He 

 mistook the pedal for the cerebral, but I am unable to say 

 what he described as the pedal. The pedals are fused from 

 the first and never become rudimentary, though their develop- 

 ment is not nearly so great as that of the visceral. The 

 cerebro-pleurals remain separated by a long commissure and 

 lie almost at the sides of the mouth. 



The gills of the ship-worms are.more specialized and more 

 interesting than has heretofore been supposed. Beginning 

 with the two " filaments " present in the larva, new filaments 

 are added posteriorly by the appearance of slits in the gill- 

 fold. These filaments are in reality bars, for the slits are 

 perforations which do not penetrate to the ventral border of 

 the fold. The fold progressively fuses with the body at its 

 ventral border. In this way a series of bars is formed 

 attached at both ends. This mode of formation persists 

 through life, except that after the tenth (usually) a second 



* ' Annales des Sciences Naturelle*,' Zool. s4r. 3, t. xi. 



