256 Mr. C. P. Sigerfoos on the Pholadidse. . 



and old and in males and females, its function cannot be the 

 foimation of a secretion to nourish the embryos. 



The posterior part of the body, usually called the muscular 

 collar, is a very interesting region, containing a complex of 

 several well-defined muscles. The collar is attached to the 

 calcareous tube in an irregularly elliptical line. The outer- 

 most part is the line of origin of the protractor of the pallet^ 

 which radiates from the inner end of the handle of the pallet, 

 its insertion, to its line of attachment. To the inner line is 

 attached the retractor of the siphons, which is almost wholly 

 distributed to the respiratory siphon. A well-defined muscle, 

 the adductor of the pallets, connects the upper parts of the 

 handles of the pallets. From the upper end of the handle a 

 small muscle, the retractor of the pallet, passes to the body 

 forwards. The animal defends itself as follows : — While the 

 siphons are retracted the pallets are protracted to completely 

 close the tube at its outer end. The siphons are extended by 

 an inflow of blood, and the pallets are retracted by the action 

 of their retractors, accompanied by the adductor of the 

 pallets. 



In specimens 3 millim. long, in which the visceral ganglia 

 have taken up the position of the adult, the sexual organ is 

 present on either side as a rudiment of a small mass of cells 

 ventral to the ganglia. From this part branches grow out, 

 and the lumen appears later. That in the oldest part of the 

 gland becomes the main part of the genital duct. In addi- 

 tion, there is in the genital papilla a small ectodermal inva- 

 gination, which much later unites with the part which is of 

 mesodermal origin. The part which is of mesodermal origin 

 forms sexual cells, at least at first. The sexes are separate 

 from the first. Protandry is not present. Only rarely are 

 individuals hermaphrodite. 



The sliip-worm larva is already highly organized when it 

 becomes attached, A functional byssus apparatus is present. 

 The cells of the velum are cast off and eaten. The cerebral 

 and pleural ganglia are still separate. The crystalline style 

 arises from tlie posterior part of the stomach. While the 

 foot performs a cupping action to draw the shell close against 

 the surface of the wood, the contraction of the posterior 

 adductor muscle causes the two valves to swing upon each 

 other, and the teeth rasp away the wood. 



Contrary to the description of Quatrefages, the pedal 

 ganglia are not separate and not rudimentary; the cerebrals 

 are not fused, but separated by a long commissure. The 

 first-formed filaments of the adult gill have no ascending 

 limbs. They form a series of bars in the anterior region of 



